Download A review of the epidemiology of scrapie in sheep

Rev. sci. tech. Off. int. Epiz., 1996, 15 (3), 827-852
A review of the epidemiology
of scrapie in sheep
L.J. HOINVILLE *
Summary: The aim of this review is to summarise and evaluate the data
available about the aetiology of scrapie in naturally affected sheep flocks,
particularly data concerning the possible transmission of infection between
related animals. The author examines data taken from various relevant studies
carried out over the last thirty years. The main conclusions are that scrapie
is an infectious disease with a genetic influence on the incubation period. The
increased risk of disease in the offspring of affected animals is thought to be
largely the result of increased genetic susceptibility, with a large proportion of
the cases occurring in high-incidence flocks being the result of horizontal
transmission of infection.
KEYWORDS: Epidemiology - Genetics - Scrapie - Transmission.
INTRODUCTION
Although scrapie was first reported in sheep over two hundred years ago (85), the
natural means of transmission within affected populations remains unclear. This is
largely due to the long incubation period between infection and the development of
clinical signs of disease, and the absence of an ante-mortem diagnostic test to detect
infected animals during this period. Even histopathological examination of brain tissue
may fail to give positive results in recently infected animals incubating the disease. In
addition, the genetic influence on the occurrence of clinical disease complicates the
interpretation of study results. These factors combine to make experimental studies of
the disease difficult, but such factors also have more serious consequences for the
interpretation of observational studies of the 'naturally occurring' disease. In these
studies the length of time for which potentially exposed animals are monitored cannot
be controlled and this may introduce problems of outcome misclassification.
The aim of this review is to evaluate and summarise the relevant, available
epidemiological data about the occurrence of scrapie in naturally affected sheep flocks,
particularly the possible transmission of infection between related animals. Some data
from experimental studies are presented, where such data provide information that
may aid in the understanding of the aetiology in naturally affected flocks.
The epidemiological data were taken from observational and experimental studies
carried out over the last thirty years. As knowledge of the disease has developed, it
has become clear that some of these studies did not take into account all confounding
* Epidemiology Department, Central Veterinary Laboratory, New Haw, Addlestone, Surrey KT15 3NB,
United Kingdom.
828
factors. Consequently, the results presented are more an indication of the likely effects
of scrapie, rather than an accurate quantitative assessment.
The occurrence of scrapie in different countries and within flocks is briefly
described. The evidence for a genetic aetiology and for transmission of an infectious
agent is discussed. Finally, some of the available information about the incidence of
scrapie in the offspring of affected animals is summarised, and used to assess the likely
roles of genetic susceptibility, maternal transmission and horizontal transmission of
infection in the occurrence of the disease.
OCCURRENCE OF DISEASE
The world-wide distribution of scrapie is difficult to determine accurately. There is
no pre-clinical diagnostic test; clinical signs are variable, and pathological examination
of brain tissue or experimental transmission studies provide the only means of
confirming infection. The stigma associated with the occurrence of disease means that
some farmers may be reluctant to report cases. This means that the disease may remain
undetected in some countries unless comprehensive surveillance systems are in
operation.
Scrapie is known to be endemic in many European countries, such as Iceland (69,
72), as it is in India and the United States of America (USA) (42, 96). The disease has
been reported in many other countries as well (48), often, initially, in imported
animals. However, in some regions, like Cyprus, the disease was first detected in
native sheep (although imported animals may have been the original source of
infection) (90). Australia and New Zealand did not, apparently, import the disease with
their original European breeding stock. These countries have used vigorous
importation, quarantine and culling policies to prevent the subsequent entry of scrapie,
and to eliminate outbreaks in imported animals when necessary (4, 5), and have thus
remained free of disease.
Fluctuations in the incidence and regional distribution of scrapie have been reported
within affected countries (72). The exact prevalence of affected flocks within affected
countries is difficult to determine for the reasons given above. A team of researchers
has conducted several self-administered questionnaire surveys in Britain (67), and
found that between 17% and 34% of sheep farmers reported having seen at least one
case of scrapie in their flock at some time. A similar survey in the Netherlands found
that 6% of farmers reported ever having seen a case in their flock (80). These studies
relied on the opinions of the farmers as to whether the animals were affected with
scrapie, and may also suffer from selection bias as response rates for postal
questionnaires were poor and, in Britain, a non-random sample was used. Although the
exact figures may be inaccurate they do show that a considerable number of flocks are
affected.
The annual within-flock incidence found in these two studies was between 0.3% and
1.8%. The within-flock incidence has been reported as 2 % in Britain (95), 3 % to 5%
in Iceland (69) and 1% to 10% in India (56). A within-flock incidence of 2 0 % to 30%
has been reported in some flocks in Iceland (82), as well as in animals born in certain
years in some British flocks (71). Scrapie occurs in both sexes and in the majority of
sheep breeds, although the incidence may be higher in some breeds than in others (13,
95). This may reflect a variation in the genetic susceptibility between breeds.
829
AGE OF AFFECTED ANIMALS
The majority of clinical cases occur in sheep between two and five years of age (15).
However, cases have been reported in animals as young as twelve months (56), ten
months (55), and seven months (82) and in animals as old as eleven years (70).
Although only 10% to 15% of cases reported in Britain were found in sheep of more
than four-and-a-half years old (70, 71), it is possible that many older animals in the
pre-clinical stage of infection were culled before the disease was detected (16).
Histopathological changes have been reported in lambs as young as eleven months
(17), and the infectious agent is thought to have been present in the brains of
eight-month-old lambs which were used to prepare a vaccine for louping-ill (ovine
encephalomyelitis). These lambs were the offspring of scrapie-infected ewes, and the
use of the vaccine resulted in the transmission of scrapie to vaccinated animals in
flocks with no history of clinical disease (34, 41).
AETIOLOGICAL CONTROVERSY
Early observations on the occurrence of scrapie in flocks that had previously
been free of disease, after the introduction of affected animals, suggested that scrapie
could be a contagious disease (64). A contagious disease is defined as a condition
caused by an infectious agent, which can be transmitted from infected to uninfected
animals.
This theory was supported by the experimental transmission of infection to sheep
in 1936 (10) and to goats in 1939 (11). In the 1950s it was shown that scrapie was
caused by a filterable, self-replicating agent (86, 93). In the early 1960s the disease
was transmitted to various experimental animals (6, 97).
But the study of scrapie has been complicated by the fact that an infectious agent
has never been isolated, by the long incubation period between infection and the onset
of clinical signs of disease, and by the problem of detecting animals which are
pre-clinically infected. As a result the aetiology of scrapie has been a subject of
controversy for many years.
Despite the experimental evidence that scrapie was an infectious condition, some of
those working with the disease in the 1960s believed that scrapie was an inherited
condition. The variation in the incidence of scrapie between different breeds of sheep,
and especially between different families, suggested a genetic influence on the
occurence of the disease.
The natural disease appeared to favour flocks of certain breeds (95), although
experimental studies to investigate this variation may have overestimated the
difference in susceptibility between breeds (15). It was later shown that there is
considerable variation in the incidence of scrapie between different flocks of the same
breed (68). There was also, however, experimental evidence that some breeds were
more susceptible to inoculation with sheep scrapie brain pool (SSBP/1) (37).
Stronger evidence for a genetic influence was provided by the variation in the
incidence of disease among different families within affected flocks (71). This was
further supported by experimental evidence for a variation in the incidence of scrapie
among families of sheep after inoculation with SSBP/1 (39).
830
These findings led some of those investigating the disease in the 1960s to believe
that scrapie was an autosomally recessive inherited condition, which could be
transmitted experimentally but which could not be transmitted from an affected animal
to an unaffected animal, except by inoculation (71). This belief was based on the
following three pieces of evidence. First, the pattern of disease occurrence in naturally
affected flocks and in experimentally inoculated animals was shown to be similar to
that of an autosomally recessive condition (39, 7 1 , 72). Secondly, it was shown that
epidemics of a genetically transmitted disease could, theoretically, occur after the
introduction of infected animals (25). Finally, some of the early attempts to introduce
the disease to unaffected flocks, by transferring animals from affected flocks or
exposing healthy animals to infected stock, gave negative results (71, 74, 86).
However, Parry demonstrated the autosomal recessive pattern of inheritance by
investigating the occurrence of scrapie in commercial flocks (72). It has been
suggested that there may have been considerable misclassification of animals in these
flocks, due to the culling of infected animals before the onset of clinical signs of
disease. It was also suggested that the allocation of a presumed genotype to animals
on the basis of the presence or absence of scrapie in their progeny gave rise to a
circular argument. Dickinson et al. presented further evidence on the occurrence of
scrapie in affected flocks which was not consistent with either recessive or dominant
genetic transmission (17). Both Parry and Draper assumed that animals carrying the
scrapie-susceptible allele were likely to be of superior conformation and so would be
given preference in the selection of flock replacements (25, 72). They suggested that
this would account for the deviance from the expected pattern for an autosomal
recessive condition, but this assumption has been questioned (95).
In the experimental work of Gordon, the autosomal recessive pattern of
susceptibility to inoculation with SSBP/1 did not occur in all breeds. He suggested that
this may be due to errors in the recording of relationships or incomplete penetration
or dominance of the genetic effect (39).
The transmission of infection from affected to unaffected animals provides the
strongest evidence that scrapie is a contagious disease and not an inherited condition.
The studies referred to earlier, which failed to demonstrate the spread of disease, did
not take into account the need for long observation periods.
The studies that have shown that scrapie can be transmitted from affected sheep to
unaffected sheep, with no inoculation of infected tissue, are listed in Table I. The
studies conducted by Brotherston et al. in 1968 (1), Dickinson et al. in 1974 (21) and
Hourrigan et al. in 1979 (48) are the most reliable, as these researchers ensured that
exposed animals were very unlikely to be incubating the disease before exposure, and
examined all exposed animals pathologically to confirm the occurrence of scrapie. For
example, the study of Dickinson et al. exposed sheep from a large isolated flock that
had been free of scrapie for at least twelve years (21). The remaining studies listed in
Table I provide some evidence for the transmission of infection but could be criticised,
as it is possible that the animals involved were pre-clinically infected before exposure
to infected animals.
The occurrence of scrapie in a goat born into a flock of sheep with a high prevalence
of scrapie in the 1940s provided strong evidence for the spread of the disease, as
scrapie is rarely reported in goats (8). There have been several subsequent reports of
the occurrence of the disease in goats kept in contact with affected sheep ( 1 , 48, 84,
85, 87, 91).
I
ally
animals were
followed
(months)
t
ced
bs
3.2
13.5
3.2
18.8
3.0
17.4
10.7
35.7
28.0
42.8
35.0
16.7
22.0
3.3
57.1
25.0
37.5
25.0
18.5
13.0
25.0
gg 5 S 4 S ? ä ^ ^' ^ P c£ S ^ ^ ^ ^ ^
I IIIIIIII~ ~~ ~~ ~~ ~
IIIIIIIIIIIIllIIIIlIl
M»
ti
* Animals not in direct contact with infected animals were grazed on pasture previously used by infected animals
? Information unknown
3 months
From birth
6 months
From birth
8 months
From birth
7-9 months
From birth
From birth
1-3 days
Girnrncr
Adult
Birth
?
From birth
Adult
Weaning
From birth
?
Birth only
2.5 years
infected
Time for
itact
Oi
1 l!
31
111
31
101
33
23
28
28
75
11
20
6
9
30
14
6
8
20
27
23
4
P
contact with :
aturally or w
contacts
s
Rambouillet
Rambouillet
Targhee
Targhee
Hampshire
Hampshire
Suffolk
Suffolk
Blackface
Blackface
Halfbred
Cheviot
Halfbred x Che\not
Cheviot
Suffolk
Suffolk
Suffolk
Suffolk
Suffolk
Suffolk
Suffolk
Age when
first
exposed
Í
Number
TABLE I
Transmission of infection from infected sheep to scrapie-free
is)
it
Reference
831
^ ^'
á
832
Most experts now recognise scrapie as an infectious condition which can be
transmitted from affected to unaffected animals.
DEMONSTRATION OF A DOSE-RESPONSE RELATIONSHIP
The evidence for the transmission of infection from affected to unaffected animals
would be strengthened if a dose-response relationship could be demonstrated. If
animals which were exposed to a greater dose of infection had either a higher
incidence of disease, or were younger when clinical signs of disease developed, this
would indicate the existence of a dose-response relationship. In experimental
transmission studies the incidence of disease is lower and the incubation period longer
when the inoculum is diluted (38, 58). Variations in the incidence of disease, or the
age at the onset of disease, could be the result of differences in the level of exposure
to scrapie, the duration of exposure or the age when first exposed.
The variations in the incidence of scrapie among animals that are naturally exposed
to different levels of infection provide evidence of a dose-response relationship. These
variations are demonstrated by the high within-flock incidence seen in some Icelandic
flocks. This high incidence has been attributed to the high level of exposure to infected
animals, and possibly an infected environment, during the winter housing period, with
a higher incidence of scrapie in animals born during a long winter housing period (69,
82). An increase in the incidence of the disease and a decline in the age at which the
disease begins have been reported within affected flocks, and been attributed to
increasing levels of environmental contamination (7, 30, 57, 82).
The variation in the incidence of disease in sheep which are exposed to an infected
environment for different lengths of time also provides evidence of a dose-response
relationship. This is demonstrated by studies in which lambs born into an affected
flock are removed from exposure at various ages (Table II).
The final observation that provides evidence for a relationship between the dose of
exposure and the occurrence of disease is the variation in the incidence of disease with
the age at first exposure. The data in Table I show that there was a higher incidence
of disease in sheep which were first exposed to scrapie at birth, than in sheep which
were first exposed when between three and nine months of age (48). Sheep which were
first exposed at birth were also affected at younger ages. This may be partly explained
by the confounding effect of environmental contamination. Assuming that the
environmental contamination at the field study site increased over time, then those
animals born at the study site were exposed to a greater level of environmental
contamination than sheep that had been introduced to the site at the start of the study.
A short incubation period in sheep exposed to the disease at an early age is also
suggested in experimental studies by the occurrence of scrapie in the offspring of
inoculated ewes at between seven and nine months of age (18, 37).
TRANSMISSION FROM A CONTAMINATED ENVIRONMENT
Evidence for the transmission of infection from a contaminated environment was
provided when scrapie-free sheep developed the disease after grazing pasture used by
clinically affected sheep approximately twice a week over a three-year period with no
833
TABLE II
The relationship between the incidence of scrapie and the duration of exposure
Exposure
Infected flock
Infected flock
Offspring of naturally
affected ewes
Offspring of inoculated
ewes
Age when
removed from
exposure
(months)
Number
exposed
Incidence (%)
Reference
0
63
10
(48)
4
32
16
9
17
29
20
32
41
0
23
13
-
20
25
0
7
8
29
75
(18)
109
115
4
15
(41)
-0
-
(17)
direct contact between the scrapie-free and affected sheep (40). However, the
possibility that the introduced animals may already have been infected could not be
ruled out in this study.
Stronger evidence of environmental contamination was provided by the attempts to
eradicate maedi-visna and sheep pulmonary adenomatosis in Iceland, by slaughtering
affected flocks and re-stocking with lambs from scrapie-free areas. Scrapie recurred on
some sites that had previously had a high incidence of scrapie, but not on sites where
scrapie had not previously occurred. This recurrence only on heavily affected sites
makes contamination of the environment a more likely explanation than pre-clinical
infection of the introduced stock (69, 83). The subsequent, more vigorous control
policy, which included thorough disinfection of affected premises and a longer rest
period, followed by re-stocking with a more reliable source of scrapie-free stock, has
been more successful (82).
The resistance of the scrapie agent to various disinfection procedures and its
survival in soil for three years make contamination of the environment biologically
plausible (2, 89). However, the survival of the scrapie agent in soil was demonstrated
in an experimental study using a high titre of infectious agent, and the titre in the
material that was recovered at the end of three years was much reduced.
TRANSMISSION OF INFECTION FROM INOCULATED
ANIMALS
It has been observed that animals that are inoculated with the scrapie agent are less
able to transmit the infection to goats or sheep than naturally infected animals (1, 73,
76, 77). However, scrapie has occurred in animals that have been in contact with
inoculated sheep (35), as well as in the non-inoculated offspring of inoculated animals
(18, 37).
834
Pattison has suggested that experimentally inoculated animals were unable to
transmit scrapie because they were usually males or non-breeding females and did not,
therefore, excrete the agent in placental tissue (78). The placenta has been identified
as one possible source of the infectious agent (see 'Source of infection').
TRANSMISSION FROM INFECTED ANIMALS
IN THE PRECLINICAL PHASE
Early reports from the United Kingdom suggested that scrapie occurred in
unaffected flocks after the purchase of pre-clinically infected sheep (64, 88). This is
now believed to be the usual means of introduction to unaffected flocks in Britain (73),
the USA (48) and Iceland (82). The theory of transmission of infection from preclinically infected animals is supported by the pattern of disease occurrence in the
offspring of infected ewes. There is no evidence that the incidence of scrapie in such
offspring depends on whether or not the ewe had developed clinical signs of the
disease when these offspring were born. The risk of scrapie developing in offspring
that were born the year before the dam had developed clinical signs was the same
as for offspring that were born in the year of the onset of the disease in the dam (15,
7 1 , 72).
SOURCE OF INFECTION
For infected animals to transmit infection to healthy animals, they must first excrete
the agent, which must then enter the body of the healthy animal. The attempts to
isolate the infectious agent of scrapie in various peripheral tissues and excretions taken
from infected animals are summarised in Table III.
The isolation of the agent from the placenta of infected sheep, and possibly from
foetal cotyledon and amniotic fluid, suggests that the placenta is one source of
infection. A study by Pattison of the transmission of infection using placental tissue
(78, 79) has been criticised as no control animals were used (92), but the high
incidence of disease in the exposed animals makes pre-clinical infection of those
exposed animals an unlikely explanation of the results. Dickinson suggests that ewes
may eat the placenta and then excrete the infectious agent over pastures with little or
no reduction of infectivity after passage through the alimentary tract. This was based
on evidence from unpublished experimental work in mice, which showed that the
scrapie agent could pass through the alimentary tract with no loss of infectivity (15).
In contrast, there has been no successful isolation of the scrapie agent from the
faeces, urine or saliva of infected sheep or goats (Table III). Only a limited number
of experiments using clinically infected donor sheep have been carried out and the
results are crucially dependent on the sensitivity of the bioassay used to detect
infection. The infectious agent may be present in these excretions at a lower titre and
the assays used may have been unable to detect this level of infection. If these
excretions were infected, they may represent a more important source of infection than
the placenta. Even a low concentration of the agent continuously excreted in faeces or
urine could result in a greater level of environmental contamination than a single
excretion of a higher concentration of the agent in the placenta once a year. However,
835
TABLE I I I
Isolation of scrapie agent from peripheral tissues and excretions
Donor animals
Tissue or excretion
. . Natural or
.
. .
Species Clinical
rasps experimental
infection
c a s e s
Number of
donors in
which
infection
was
detected
Assay used
Reference
R o u t e
SDecies
F
o f
inoculation
Intestine
Intestine
Intestine
Faeces
Faeces
Faeces
Faeces
Faeces
Interna] parasite
Haemonchus
contortus
Haemonchus
contortus
Retropharyngeal
lymph node
Retropharyngeal
lymph node
Retropharyngeal
lymph node
Nasal mucosa
Nasal mucosa
Salivary gland
Salivary gland
Salivary gland
Sheep
Sheep
Goats
Sheep
Sheep
Goats
Goats
Goats
Sheep
Sheep
Yes
No
Yes
No
?
Yes
Yes
Yes
?
Yes
Natural
Natural
Natural
Natural
Natural
Natural
Experimental
?
Natural
Experimental
'9/9
5/8
3/3
0/8
0/4
0/3
0/1
0/?
1/5
0/6
Mice
Mice
Mice
Mice
Mice
Mice
Goats
Goats
Mice
Sheep
Cerebral
Cerebral
Cerebral
Cerebral
Cerebral
Cerebral
Cerebral
Oral
Cerebral
Cerebral
(44)
(44)
(43)
(44)
(47)
(43)
(75)
(73)
(47)
(26)
Goats
Yes
Experimental
0/6
Goats
Cerebral
(26)
Sheep
Yes
Natural
9/9
Mice
Cerebral
(44)
Sheep
No
Natural
7/8
Mice
Cerebral
(44)
Goats
Yes
Natural
3/3
Mice
Cerebral
(43)
Sheep
Goats
Sheep
Goats
Goats
Natural
Natural
Natural
Natural
Experimental
3/9
3/3
0/9
0/3
5/14
Mice
Mice
Mice
Mice
Goats
Cerebral
Cerebral
Cerebral
Cerebral
Cerebral
(44)
(43)
(44)
(43)
(77)
Saliva
Saliva
Saliva
Saliva
Kidney
Kidney
Urine
Urine
Urine
Urine
Sheep
Sheep
Goats
Goats
Sheep
Goats
Sheep
Goats
Goats
Goats
Yes
Yes
Yes
Yes
Yes (3)
No (11)
Yes
?
Yes
Yes
Yes
Yes
?
Yes
Yes
Yes
Natural
Natural
Experimental
?
Natural
Natural
Natural
Natural
Experimental
Experimental
0/9
0/2
0/1
0/?
0/9
0/3
0/4
0/3
0/1
0/14
Mice
Mice
Goats
Goats
Mice
Mice
Mice
Mice
Goats
Goats
Cerebral
Cerebral
Cerebral
Oral
Cerebral
Cerebral
Cerebral
Cerebral
Cerebral
Cerebral
(44)
(47)
(75)
(73)
(44)
(43)
(47)
(43)
(75)
(77)
Mice
Mice
Sheep
and goats
Mice
Mice
Mice
Cerebral
Cerebral
Cerebral
(44)
(47)
(78, 79)
Cerebral
Cerebral
Cerebral
(47)
(47)
(47)
M
INO
N
Foetal cotyledon
Foetal cotyledon
Placenta
Sheep
Sheep
Sheep
Yes
?
Yes
Natural
Natural
Natural
0/2
2/10
6/6
Amniotic fluid
Pasture
Trough water
Sheep
Sheep
Sheep
?
?
?
Natural
Natural
Natural
1/1
0/2
0/2
Positive isolation from salivary gland mentioned (41)
Positive isolation from goat foetal cotyledon mentioned (48)
Negative attempt at isolation from contaminated building manure mentioned (48)
836
the small number of attempts to isolate the agent from potentially contaminated
environmental sources have all been negative.
ROUTE OF ENTRY TO RECIPIENT ANIMALS
The oral route is a likely means of entry to the bodies of recipient animals.
Experimental studies have shown that the intra-gastric route of scrapie infection is
effective in mice (61). Oral administration of infected brain tissue has been shown to
transmit infection in both sheep and goats (75). Infected placental tissues have also
transmitted scrapie to sheep by the oral route (78, 79).
The isolation of the scrapie agent from the alimentary tract of pre-clinically,
naturally infected lambs also suggests that this may be the natural route of entry
(44).
Scarification of the skin and conjunctival inoculation are other suggested routes of
entry for infection (15, 81).
GENETIC CONTROL OF SUSCEPTIBILITY
There is now considerable evidence that scrapie can be transmitted between
unrelated animals and the suggestion that the disease is an autosomally recessive
inherited condition seems untenable. But there is strong evidence that genetic factors
in the host influence susceptibility to the disease. This idea was first suggested by the
familial pattern of occurrence of natural scrapie (71), and confirmed during
experimental inoculation studies, in which exposure to the scrapie agent could be
controlled.
These experimental studies were designed to create 'susceptible' and 'resistant' lines
of sheep, by selective breeding of a foundation flock according to the response of
related animals to inoculation with SSBP/1. The incidence of experimentally induced
scrapie in selected lines of Cheviot and Herdwick sheep suggested that susceptibility
to inoculation with SSBP/1 was controlled by a single gene known as Sip (a sheep
gene exerting control over Scrapie incubation period) (15, 20, 68). The Sip gene has
two alleles, sA and pA, and the sA allele is dominant for susceptibility to disease after
inoculation with SSBP/1.
A further study using Swaledale sheep examined the incidence of experimentally
induced scrapie in the offspring of sheep which had not developed clinical disease after
inoculation with pooled field strains of the scrapie agent. The study compared this
incidence to the incidence in control animals. This study also concluded that
susceptibility to scrapie was a dominant inherited trait (46). It is believed that the same
gene is responsible for genetic control in all three breeds (49).
Further work showed first, that the sA allele may be only partially dominant, and
secondly, that experimentally challenged, apparently resistant, homozygous pA sheep
may develop scrapie after a long incubation period.
Partial dominance was indicated by the division of susceptible Swaledale sheep into
two distinct groups according to the length of the incubation period of the disease (13),
suggesting that homozygous susceptibles had a shorter incubation period.
837
The absolute resistance of homozygous pA animals was brought into question by the
occurrence of scrapie (after a very long incubation period) in a few of the Herdwick
sheep that had been selectively bred for resistance to subcutaneous inoculation with
SSBP/1 (67). Scrapie also occurred in some of the homozygous pA Cheviot sheep that
were resistant to subcutaneous inoculation with SSBP/1, after these sheep had been
inoculated intra-cerebrally (20).
These results suggest that the Sip gene controls the incubation' period of scrapie in
experimentally inoculated sheep. This could give the appearance that the Sip gene
influences the susceptibility of sheep to inoculation with the scrapie agent, because
those animals with an incubation period longer than the observation period, or longer
than their natural lifespan, will appear to be resistant to inoculation.
The mechanism by which the Sip gene controls the incubation period of scrapie in
sheep may be similar to the way in which the homologous gene in mice, known as Sinc
(Scrapie incubation period), controls the incubation period of experimental murine
scrapie. The Sinc gene has two alleles, p7 and s7, so called because homozygous s7
mice inoculated with the ME7 strain of the scrapie agent develop scrapie with a short
incubation period, whereas the incubation period is prolonged in homozygous p7 mice
(19).
When mice are inoculated with scrapie, using the intra-peritoneal route, the initial
replication of the disease takes place in the lymphoreticular system (LRS), before
replication in the central nervous system (CNS) produces the lesions that result in
clinical disease (60). Variations in the Sinc genotype have only a small effect on the
onset of detectable replication of the ME7 strain of scrapie in the LRS after intra­
peritoneal inoculation. But there is a profound effect on the time of onset of replication
in the brain. Such replication occurs earlier in homozygous Sinc s7 mice than in
homozygous p7 mice (59). Other studies, using the 87V strain of the scrapie agent,
showed that it is possible to produce a life-time persistence of infection in the LRS
of homozygous Sinc p7 mice without the occurrence of clinical disease, by using
intra-peritoneal inoculation. The scrapie agent was able to replicate itself in the brain,
but neuroinvasion was severely impaired (3, 9). Similar results were obtained when
C57BL mice were inoculated by the intra-peritoneal route with a low dose of the 22A
strain of the scrapie agent (22). These results show that the specific interactions
between different strains of the agent and the alleles of the Sinc gene greatly influence
neuroinvasion and the subsequent replication of the scrapie agent in the CNS.
The growing evidence that Sip and Sinc genes are the same as the ovine and
murine PrP genes, respectively (50), justifies extrapolating these findings to sheep
scrapie.
GENETIC CONTROL OF THE NATURAL DISEASE IN SHEEP
The role of Sip in the control of natural scrapie was suggested by the occurrence
of major outbreaks of natural scrapie in those Cheviot and Herdwick flocks that had
been selected for susceptibility to experimental inoculation with the scrapie agent (14,
57), but not in those flocks selected for resistance.
This was confirmed in an experiment, using some of the unaffected female offspring
of affected rams from a Suffolk flock which had been bred to maximise the occurrence
of natural scrapie. These ewes were assumed to be heterozygous sApA. They were
838
mated with Cheviot rams, which had been bred for resistance to experimental
inoculation and were assumed to be homozygous pA. The incidence of disease in the
offspring of these matings, following subcutaneous inoculation with either the
Cheviot-derived SSBP/1 or a Suffolk strain of the scrapie agent, known as" SUF81, was
as one would expect, assuming that a single gene from these Suffolk ewes and Cheviot
rams conferred dominant susceptibility to scrapie in the cross-bred offspring (28).
The pattern of inheritance in naturally affected flocks suggests that susceptibility to
scrapie is a recessive trait (28, 71), not a dominant trait as has been indicated in
experimental studies (20, 46, 68). This could be explained by a difference between the
effective exposure of naturally exposed sheep, and the effective exposure of
experimentally exposed animals. The effectiveness of the exposure is likely to depend
on the dose of the agent received and the route of infection. The route of experimental
inoculation in mice is known to determine the effective dose of the agent received (58),
in that the oral route of exposure is less efficient than intra-cerebral inoculation and
other parenteral routes (61). Heterozygous susceptible sheep, exposed to a low dose
of the scrapie agent by the oral route, may not develop clinical disease within their
lifetimes and the natural disease may appear to occur in an autosomal recessive
pattern.
BIOLOGICAL MARKERS FOR GENETIC SUSCEPTIBILITY
It may now be possible to determine whether sheep are genetically susceptible to
scrapie without the need for prolonged breeding experiments and inoculation of the
scrapie agent. It has recently been shown that specific variations in the nucleotide
sequence in and around the PrP gene are associated with alleles of the Sip gene. This
was first demonstrated in experiments using Cheviot sheep bred for susceptibility or
resistance to inoculation with SSBP/1 (51). This work has provided evidence that Sip
and PrP may be the same gene, and that polymorphisms in the PrP gene may provide
biological markers for Sip alleles that could predict susceptibility to scrapie. In
addition, these markers were used to confirm the partial dominance of the Sip allele
by dividing susceptible animals into those that were homozygous and those that were
heterozygous for the PrP polymorphism associated with the Sip susceptibility allele.
Homozygous susceptible Cheviot and Cheviot crosses, when inoculated
subcutaneously with SSBP/1, develop the disease after a shorter incubation period than
heterozygous susceptible animals (31, 65).
Several studies have now shown that these PrP polymorphisms are associated with
the occurrence of scrapie in naturally affected flocks (52, 53, 54, 62).
The pattern of genetic susceptibility may be more complex than has been suggested
so far, with different strains of the scrapie agent producing different patterns of
susceptibility. In experimental studies mice that are apparently resistant to inoculation
with one strain may develop clinical disease when inoculated with another strain (23).
Differences between the genetic susceptibilities of sheep that have been inoculated
with different strains of the scrapie agent have also been demonstrated (29, 33). These
results suggest that disease occurrence is determined by an interaction between the
host genotype and the strain of the scrapie agent. This interaction may explain why
different PrP polymorphisms are not consistently associated with the occurrence of
natural scrapie in different breeds of sheep (53, 63).
839
INCIDENCE OF SCRAPIE IN THE OFFSPRING OF
AFFECTED ANIMALS
An increase in the incidence of scrapie in the offspring of affected animals may
simply reflect the genetic influence on the occurrence of the disease, although it could
also indicate that the infectious agent is transmitted from parent to offspring (17).
Several studies have investigated the incidence of scrapie in the offspring of affected
and unaffected sheep. These data have been summarised in Table IV.
These results are presented as an indication of likely effects rather than as an
accurate quantitative assessment. The most important problems in interpreting the data
from these studies are, first, the possible misclassification of infected animals which
showed no clinical signs of the disease as unaffected and, secondly, the confounding
effects introduced by the management of the flocks. The misclassification of infected
animals is likely to be a problem in the work of Parry, as not all brains were examined
histopathologically to determine whether culled animals were pre-clinically infected.
The confounding effects of flock management are likely to be important in the early
work of Dickinson et al. in which the offspring of affected and unaffected animals
were separated (17).
The data from the 1974 paper of Dickinson et al. (21) and the 1979 paper of
Hourrigan et al. (48) are included because they provide probably the least biased
estimates of the effects of these risk factors. The data from the 1965 paper of
Dickinson et al. (17) and Table 5 of the 1962 paper of Parry (71) are included as they
are often quoted as evidence that the dam has a greater influence on the risk of disease
than the sire (92, 94). Further examples of the data of Parry, in addition to Table 2 of
the 1962 paper, have been included to show the variable dam and sire effects obtained
in analysing the data from different groups of animals (71, 72).
The data presented in Table IV have been analysed to provide answers to two
important questions. First, relative risks (RR) have been calculated to determine
whether the risk of disease is greater in the offspring of affected animals than in the
offspring of unaffected animals born into the same flock. Furthermore, if the risk of
disease is greater in the offspring of affected animals, then are the offspring of affected
dams and the offspring of affected sires at equal risk? Secondly, population attributable
risk percentages (PAR%) have been calculated to determine the proportion of all cases
of scrapie occurring in these populations that occur because these sheep are the
offspring of affected animals.
The relative risks (RR) were calculated by dividing the incidence of disease in the
offspring of affected animals by the incidence of disease in the offspring of
asymptomatic animals. An RR significantly greater than one indicates that the
offspring of affected animals are at increased risk of developing scrapie. In all of the
studies presented, the offspring of affected dams have a significantly increased risk of
developing scrapie, with the RR varying from 1.85 to 4.80.
The effect of the sire on the risk of disease is not so clear. In only some of the studies
did the offspring of affected sires have a significantly increased risk of developing
scrapie. However, in all of the studies, the offspring of an infected dam and an infected
sire are at greater risk than the offspring of an infected dam and an uninfected sire.
The data from the early study of Dickinson suggest that the offspring of affected sires
are not at increased risk of developing scrapie (15); however, the confounding effect
RR: relative risk
Suffolk
Suffolk
S3
60.5
S Ä
vi
cu
50.0
(4/8)
8.0
(26/325)
27.1
(23/85)
6.6
(18/273)
85.7
(12/14)
14.7
(25/31)
(15/102)
80.6
18.5
(5/27)
(46/76)
13.5
8.9
19.8
11.9
4.4
38.8
(24/75)
32.0
(10/22)
46.0
(7/32)
21.9
(2/11)
18.2
(24/65)
36.9
(50/129)
1.64
(2.13-4.72)
3.17
(1.01-2.17)
4.61
(0.84-1.70)
1.19
(0.96-1.31)
1.15
(0.93-1.52)
1.19
(1.19-2.25)
PAR%: population atlributable risk percentage
(2.34-4.38)
3.20
(1.85-2.85)
2.30
(3.12-7.40)
4.8
(2.30-9.56)
4.69
(1.49-2.43)
1.86
35
Suffolk
cu
E
Suffolk
CU'
35.3
&
(54/153)
«
«¡£
>n
X Blackface
5.2
21.5
35.5
17.9
3.14
(16/18)
88.9
(74/79)
93.9
(18/21)
85.7
(14/18)
93.8
7.5
68.5
27.1
31.6
(8.07-17.23)
11.79
(8.18-20.5)
30.4
(3.68-9.92)
6.04
(2.30-11.20)
5.07
(1.84-3.20)
2.43
(2.07-4.76)
8.8
CI: confidence intervals
(31/32)
(48/56)
86.0 13.00
77.8
eu
(1.37-2.51)
PÄ
35
1.85
i#
41.9
Risk in offspring
of two affected parents
IT)
Suffolk
-Ç3
•S
CU
(13/31)
RR
(95% CI)
HS
24.8
Incidence (%)
(cases/at risk)
Risk in offspring
of affected sire
is
(26/105)
Incidence (%)
(cases/at risk)
Risk in offspring
of affected dam
u
Suffolk
Breed
Incidence in
offspring of
two unaffected
parents
The incidence of scrapie in the offspring of affected and unaffected sheep
TABLE IV
(Assumes
parental
outcome from
genotype)
(71: Table 2)
(Assumes
parental
outcome from
genotype
(72: Table Al-A)
management
information
from (41)
(71: Table 5)
(15: Table 10.2)
(21: Table 1)
(48: Table 2)
Reference
840
35
S Ä
841
of flock management in this study has already been mentioned. The affected and
unaffected flocks were initially separated, so that the offspring of affected ewes were
kept in an affected flock and the offspring of unaffected ewes were kept in an
unaffected flock (17). This means that horizontal transmission could be an important
confounding factor, which would exaggerate the effect of the disease status of the dam,
and underestimate the effect of the status of the sire on the risk of disease. In the later
study of Dickinson et al. (21), the affected and unaffected flocks were not separated,
but data from three generations of matings were combined to produce the results given
in Table IV. Although there was no significant increase in the risk of developing
scrapie for the offspring of affected sires using the combined data from all generations,
in the final F generation the offspring of affected sires were at significantly increased
risk of developing disease (RR = 1.48, 9 5 % confidence interval ± 1.01-2.17). In
summary, these results suggest that both the dam and the sire have an effect on the risk
of the offspring developing scrapie and, although the dam may have more effect than
the sire, the extent of that difference is not as great as has been suggested in the
past.
2
Any difference between the effect of the dam on the risk of disease and the effect
of the sire could be used to indicate if the increased risk in the offspring of affected
animals is the result of genetic susceptibility, transmission of infection from the dam
or the sire to the embryo, or transmission of infection from the dam during gestation
or post-partum. Lambs do not usually have any contact with the sire after conception.
An increase in the risk of disease occurrence in the offspring of affected sires would
suggest that at least part of the increased risk for the offspring of affected animals is
due to a genetic susceptibility to the disease or transmission of infection to the embryo.
Given the previous evidence for a genetic influence on susceptibility to scrapie, and
the experimental evidence that the scrapie agent has never been isolated from semen
or male reproductive tissues (Table V), any effect of the sire on the incidence of
disease is most likely to be a genetic effect.
Assuming that the increased risk in the offspring of affected sires is because these
animals are genetically susceptible to scrapie, it is likely that the risk of disease in the
offspring of affected dams is increased by a similar amount as a result of increased
genetic susceptibility.
A greater risk of disease in the offspring of affected dams than in the offspring of
affected sires could be due to transmission of infection from the mother to the embryo
or foetus during gestation, or to the lamb post-partum. There is some evidence that
infection may be transmitted to the embryo. An embryo transfer experiment has
recently been performed. Unwashed embryos from inoculated donor ewes (which
subsequently developed clinical disease) were transferred to genetically resistant
recipent ewes. Six of the twenty lambs produced in this experiment developed scrapie,
which suggests that transmission to the embryo may occur (32). This conflicts with the
results of a previous study in which washed embryos were transferred from inoculated
ewes (which subsequently developed clinical disease) to recipient ewes from scrapiefree flocks. None of these twenty-seven lambs developed scrapie (27). These results
are difficult to reconcile but the genetic susceptibility of the lambs born, the origins
of the scrapie-free embryo recipients, the interval of time between the inoculation of
embryo donors and the collection of the embryos, and the washing of collected
embryos are all important variables which differed in these studies. The positive results
of the most recent study suggest that infection of the embryo is a possibility that
requires further investigation. In addition, the scrapie agent has been isolated from the
842
TABLE V
Isolation of scrapie agent from reproductive and mammary tissues
Number of
donors
Assay used
, Natural or
in which
experimental infection Species Route
infection
detected
Reference
Donor animals
Tissue or
excretion
Species
C
h
m
c
a
c a s e s
l
Sheep
Yes
Natural
2/7
Mice
Cerebral
(44)
Mammary gland
Sheep
Yes
Natural
0/7
Mice
Cerebral
(44)
Mammary gland
Goats
Yes
Natural
0/3
Mice
Cerebral
(43)
Colostrum
Sheep
0/1
Mice
Cerebral
Sheep
No
?
Natural
Colostrum
Natural
0/3
Mice
Cerebral
(44)
(47)
Milk
Sheep
?
Natural
0/6
Mice
Cerebral
(47)
Milk
Goats
Yes
Natural
0/3
Mice
Cerebral
(43)
Milk
Goats
Yes
Experimental
0/1
Goats
Cerebral
(75)
Ovary
Sheep
0/9
Mice
Cerebral
(44)
Sheep
Yes
?
Natural
Ovary
Natural
4/14
Mice
Cerebral
(47)
Ovary
Goats
Yes
Natural
0/3
Mice
Cerebral
(43)
Uterus
Sheep
0/3
Mice
Cerebral
(44)
Sheep
Yes
?
Natural
Uterus
Natural
4/13
Mice
Cerebral
(47)
Uterus
Goats
Yes
Natural
0/3
Mice
Cerebral
(43)
Testis/seminal vesicles Sheep
Yes
?
Natural
0/1
Mice
Cerebral
(44)
Natural
0/6
Mice
Cerebral
(47)
0/21
Mice
Cerebral
(47)
(44)
(47)
Supra-mammary
lymph node
«
Testis/seminal vesicles Sheep
Semen
Sheep
?
Natural
Foetal tissue
Sheep
0/4
Mice
Cerebral
Sheep
Yes
?
Natural
Foetal tissue
Natural
1/13
Mice
Cerebral
.
? Information unknown
ovary, uterus and foetal tissues from infected animals in some studies (Table V). This
suggests that infection may be transmitted in the female reproductive tract. The
possibility that infection is transmitted from mother to offspring in utero is also
supported by the occurrence of scrapie, at a young age, in the non-inoculated offspring
of inoculated ewes (18, 36, 37).
843
The increased risk of disease in the offspring of affected dams could also be due to
the post-partum transmission of infection, possibly as a result of contact with placental
tissue. Post-partum transmission of infection in milk is also a possibility but the
scrapie agent has never been isolated from the milk of affected ewes in experimental
studies (Table V). The theory of post-partum transmission of infection is supported by
the increased risk of disease in those naturally exposed animals that remain in a
potentially contaminated environment for longer periods (Table II).
The second question that was addressed using the data in Table IV was the
proportion of the cases in these populations that occurred because these animals were
the offspring of infected sheep. The population attributable risks percentages (PAR%)
were calculated using the equation given in the Appendix. Such percentages provide
an estimate of the proportion of cases that occurred within the study population
because they were the offspring of affected animals. The remaining cases in these
populations cannot be attributed to parental transmission of disease and would have
occurred whether or not the parents of these sheep were affected. If infected animals
are misclassified as uninfected animals, then the RR and the proportion of the
population exposed will be underestimated. This, in turn, means that the PAR% will
also be underestimated. Such a misclassification will occur if animals incubating
scrapie are culled before they develop clinical signs, or if animals can carry and
transmit infection without developing clinical signs. As both these sources of
misclassification are likely to occur in scrapie-affected flocks, the PAR% have been
calculated to give an indication of the proportion of cases that may be due to increased
genetic susceptibility or transmission of infection from infected animals to their
offspring, but cannot be used to provide an accurate quantitative assessment. The
results from the two most reliable studies at the top of Table IV suggest that a total
of between 2 1 % and 3 2 % of cases can be attributed to parental status. This figure was
obtained by combining the proportion of cases attributable to affected sires, to affected
dams and to two affected parents. For example, using the data from the first study
listed, 17.9% of cases occurred because the sire was affected, 5.2% because the dam
was affected and 8.8% because both parents were affected, giving a total of 31.9% of
cases that could be attributed to parental status. However, as part of the increased risk
in the offspring of affected animals is thought to be due to an increased genetic
susceptibility of these animals, only a proportion of these cases could be attributed to
maternal transmission of infection. These results suggest that a considerable
proportion of the cases occurring in these flocks are the result of horizontal
transmission of infection between unrelated animals.
These estimates apply only to these populations. The proportion of cases attributable
to maternal and horizontal transmission in other flocks will depend on the incidence
of disease in the flock and the amount of contact between unrelated animals. It is
possible that in low-incidence flocks, and in flocks where there is not much contact
between unrelated animals, maternal transmission of infection may account for a much
greater proportion of cases. It has been suggested that maternal status is important in
determining whether lambs develop scrapie when the disease is first introduced to a
flock, but that, as levels of environmental contamination increase, horizontal
transmission becomes more important (24). This could be the reason why the offspring
of affected sires appear to be at increased risk only in the later generations in the 1974
study of Dickinson et al. (21), when the level of environmental contamination had
increased.
844
CONTROL OF DISEASE
The evidence for an increased risk of disease in the offspring of affected animals
has been used as the basis of selective culling policies to control the occurrence of
scrapie in affected flocks. This approach has been effective in some flocks, especially
if combined with lambing management practices to reduce contact between animals,
and thus reduce the horizontal spread of the disease (66). But the culling of maternal
bloodlines has not been completely effective in controlling the disease in the USA
(12), which, as is consistent with the evidence presented here, suggests that horizontal
transmission of infection is important. Culling animals that are born at about the time
that an infected ewe has lambed has been suggested as an additional control procedure.
A research project is in progress at the Central Veterinary Laboratory (CVL) to
determine whether there is any evidence to support the hypothesis that these animals
are at increased risk of developing scrapie. However, any such culling policy could be
undermined by the possible transmission of infection from pre-clinically infected
animals and from sources other than the placenta. The use of careful lambing
management and disinfection procedures may aid in controlling the spread of infection
from pre-clinically infected animals, but environmental contamination from other
sources would be very difficult to control. A more effective method of controlling
clinical disease may become available with the identification of genetically resistant
rams, which could be used to increase the proportion of genetically resistant breeding
females in a flock. However, it will be important to determine whether these
genetically resistant animals are capable of carrying the infection in peripheral tissue,
as has been suggested by the experimental studies in mice (3, 9, 22), before this
becomes a widely used method of controlling scrapie. Research projects are also in
progress at CVL to determine the effect of using genetically resistant rams on the
incidence of clinical disease in affected flocks, and to discover whether genetically
resistant animals from affected flocks are infected with the scrapie agent.
CONCLUSIONS
This review of the available data on the occurrence of scrapie suggests that scrapie
is not an inherited condition but an infectious disease with a genetic influence on the
incubation period. The route by which the infectious agent is transmitted is still
unknown. While there is experimental evidence to suggest that placental tissues are a
possible source of infection, it is impossible to be certain whether other tissues,
discharges or excretions contain lower concentrations of the agent that may lead to
accumulation on pasture.
The data presented in Table IV show that the offspring of affected animals are
definitely at increased risk of developing scrapie. This increase is thought to be largely
the result of increased genetic susceptibility in these animals. The data also show that
a considerable proportion of the cases occurring in these high-incidence flocks cannot
be attributed to parental status and must, therefore, be the result of horizontal
transmission of infection.
Further epidemiological investigations of the independent effects of maternal
infection and genetic susceptibility on the incidence of disease are being conducted,
using recently discovered biological markers for genetic susceptibility. This would
845
allow the proportion of cases that could be attributed to maternal transmission of
infection, and those that could be attributed to horizontal transmission, to be
determined. Management factors which increase the likelihood of the infection being
transmitted between animals, both from mother to offspring and horizontally, are also
being investigated. The effect of using genetically resistant rams on the incidence of
clinical disease in these flocks will also be determined, as will the possibility that
genetically resistant animals are carrying the infection. These studies should help to
clarify the means of natural transmission of scrapie and prompt new and more effective
recommendations to aid in controlling the disease.
ACKNOWLEDGEMENTS
The author is very grateful to R.H. Kimberlin, M. Dawson and J.W. Wilesmith for
their constructive comments during the preparation of this review.
* *
ÉTUDE SUR L'ÉPIDÉMIOLOGIE DE LA TREMBLANTE DU MOUTON.
L.J. Hoinville.
-
Résumé : L'objet de cette étude est de résumer et d'évaluer les informations
disponibles sur l'étiologie de la tremblante du mouton dans les troupeaux
d'ovins atteints d'une infection naturelle, notamment celles concernant les
possibilités de transmission entre animaux apparentés. L'auteur examine les
résultats de plusieurs études majeures consacrées à la question au cours des
trente dernières années. Les principales conclusions sont que la tremblante du
mouton est une maladie infectieuse et qu'il existe une influence génétique sur la
période d'incubation. Le risque accru de maladie chez les descendants
d'animaux atteints s'expliquerait par une plus grande sensibilité génétique, les
cas survenant dans les troupeaux à forte incidence étant en grande partie dus à
une transmission horizontale de l'infection.
MOTS-CLÉS : Epidémiologie - Génétique - Transmission - Tremblante du
mouton.
*
#
*
REVISION DE LA EPIDEMIOLOGÍA DEL PRURIGO LUMBAR EN LA OVEJA. L.J. Hoinville.
Resumen: El objetivo de esta revisión es el de sintetizar y evaluar los datos
disponibles sobre la etiología del prurigo lumbar en rebaños de ovejas
infectados de forma natural. Se presta una especial atención a los datos
relativos a una posible transmisión entre animales emparentados. El autor
examina datos procedentes de diversos estudios realizados sobre el particular
durante los últimos treinta años. Las principales conclusiones establecen que el
846
prurigo lumbar es una enfermedad infecciosa, y que existe una influencia
genética sobre el período de incubación. Se piensa que el mayor riesgo de
enfermedad entre la descendencia de animales infectados obedece en gran
medida a una mayor susceptibilidad genética; en rebaños con una elevada tasa
de incidencia, una gran proporción de los casos son resultado de la transmisión
horizontal de la infección.
PALABRAS CLAVE: Epidemiología Transmisión.
Genética - Prurigo lumbar
-
* *
Appendix
Calculation of population attributable risk percentages
The population attributable risk percentages were calculated by combining the
following three formulae from Henekens and Burring (45).
1. PAR = AR
X
2. AR = I - I
e
P
e
0
3. PAR% = PAR H- I
T
To give
PAR% = ( [ I - I ] P ) ^ I
e
PAR:
AR:
P:
I:
I:
I:
PAR%:
e
e
0
x
0
e
T
Population attributable risk
Attributable risk
Proportion of cases in population exposed
Incidence in exposed group
Incidence in unexposed group
Overall incidence in the population
Population attributable risk percentage
*
* *
847
REFERENCES
1. BROTHERSTON J.G., RENWICK C.C., STAMP J.T., ZLOTNIK I. & PATTISON I.H. (1968). -
Spread of scrapie by contact to goats and sheep. J. comp. Pathol, 78, 9-17.
2. BROWN P. & GAJDUSEK D.C. (1991). - Survival of scrapie virus after 3 years' interment.
Lancet, 337, 269-270.
3. BRUCE M.E. (1985). - Agent replication dynamics in a long incubation period model of
mouse scrapie. J. gen. Virol, 66, 2517-2522.
4. BRUERE A.N. (1977). - Scrapie: a point of view. N.Z. vet. J., 25, 259-260.
5. BULL L.B. & MURNANE D. (1958). - An outbreak of scrapie in British sheep imported
into Victoria. Aust. vet. J., 34, 213-215.
6. CHANDLER R.L. (1961). - Encephalopathy in mice produced by inoculation with scrapie
brain material. Lancet, 1, 1378-1379.
7. CHATELAIN J., BARON H., BAILLE V., BOURDONNAIS A., DELASNERIE-LUAPRETRE N. &
CATHALA F. (1986). - Study of endemic scrapie in a flock of 'Ile de France' sheep. Eur.
J. Epidemiol, 2, 31-35.
8. CHELLE PL. (1942). - Un cas de tremblante chez la chèvre. Bull Acad. vét. Fr., 15,
294-295.
9. COLLIS S.C. & KIMBERLIN R . H . (1985). - Long-term persistence of scrapie infection in
mouse spleens in the absence of clinical disease. FEMS Microbiol. Letters, 29, 111-114.
10. CUILLE J. & CHELLE P L . (1936). - La maladie dite tremblante du mouton est-elle
inoculable? C.R. Acad. Sci, Paris, 203, 1552-1554.
11. CUILLE J. & CHELLE PL. (1939). - Transmission expérimentale de la tremblante à la
chèvre. C.R. Acad. Sci., Paris, 208, 1058-1060.
12. DAVIES D.C. & KIMBERLIN R.H. (1985). - Selection of Swaledale sheep of reduced
susceptibility to experimental scrapie. Vet. Rec, 116, 211-214.
13. DETWILER L.A. (1992). - Scrapie. In Transmissible spongiform encephalopathies of
animals (R. Bradley & D. Matthews, eds). Rev. sci. tech. Off. int. Epiz., 11 (2), 491-537.
14. DICKINSON A.G. (1974). - Scrapie. Nature, 252, 179-180.
15. DICKINSON A.G. (1976). - Scrapie in sheep and goats. In Slow virus diseases of animals
and man (R.H. Kimberlin, ed.). North Holland Publishing Company, Amsterdam, 209241.
16. DICKINSON A.G., YOUNG G.B., STAMP J.T. & RENWICK C.C. (1965). - A note on the
distribution of scrapie in sheep of different ages. Anim. Prod., 6, 375-377.
17. DICKINSON A.G., YOUNG G.B., STAMP J.T. & RENWICK C.C. (1965). - An analysis of
natural scrapie in Suffolk sheep. Heredity, 20, 485-503.
18. DICKINSON A.G., YOUNG G . B . & RENWICK C.C. (1966). - Scrapie:
experiments
involving maternal transmission in sheep. In Report of scrapie seminar, Washington,
D.C, 27-30 January 1964. Agricultural Research Service (ARS), United States
Department of Agriculture (USDA), 244-247.
19. DICKINSON A.G., MEIKLE M.H. & FRASER H. (1968). - Identification of a gene which
controls the incubation period of some strains of the scrapie agent in mice. /. comp.
Pathol, 78, 293-299.
848
2 0 . DICKINSON A.G., STAMP J.T., RENWICK C.C. & RENNIE J.C. ( 1 9 6 8 ) . - Some factors
controlling the incidence of scrapie in Cheviot sheep injected with a Cheviot-passaged
scrapie agent. J. comp. Pathol, 78, 3 1 3 - 3 2 1 .
2 1 . DICKINSON A.G.,
STAMP J.T. & RENWICK
C.C. ( 1 9 7 4 ) .
-
Maternal
and
lateral
transmission of scrapie in sheep. J. comp. Pathol, 84, 19-26.
2 2 . DICKINSON A.G., FRASER H. & OUTRAM G.W. ( 1 9 7 5 ) . - Scrapie incubation time can
exceed natural lifespan. Nature, 256, 7 3 2 - 7 3 3 .
2 3 . DICKINSON A.G. & FRASER H. ( 1 9 7 9 ) . - An assessment of the genetics of scrapie in
sheep and mice. In Slow transmissible diseases of the nervous system, Vol. 1 (S.B.
Prusiner & W.J. Hadlow, eds). Academic Press, New York, 3 6 7 - 3 8 5 .
2 4 . DICKINSON A.G. & OUTRAM G.W. ( 1 9 8 2 ) . - Genetic aspects of unconventional virus
infections: the basis of the virino hypothesis. In Novel infectious agents and the central
nervous system (G. Bock & J. Marsh, eds). Ciba Foundation Symposium 1 3 5 . John
Wiley, Chichester, 6 3 - 8 3 .
2 5 . DRAPER G.J. ( 1 9 6 3 ) . - 'Epidemics' caused by a late-manifesting gene: application to
scrapie. Heredity, 18, 1 6 5 - 1 7 1 .
2 6 . FITZSIMMONS W.M. & PATTISON IH. ( 1 9 6 8 ) . - Unsuccessful attempts to transmit scrapie
by nematode parasites. Res. vet. Sci., 9, 2 8 1 - 2 8 3 .
2 7 . FOOTE W.C., CLARK W., MACIULIS A., CALL J.W., HOURRIGANJ.,
EVANS R.C.,
MARSHALL M.R. & D E CAMP M. ( 1 9 9 3 ) . - Prevention of scrapie transmission in sheep,
using embryo transfer. Am. J. vet. Res., 54, 1 8 6 3 - 1 8 6 7 .
2 8 . FOSTER J.D. & DICKINSON A.G. ( 1 9 8 8 ) . - Genetic control of scrapie in Cheviot and
Suffolk sheep. Vet. Rec, 123 (6), 1 5 9 .
2 9 . FOSTER J.D. & DICKINSON A.G. ( 1 9 8 8 ) . - The unusual properties of CH 1 6 4 1 , a
sheep-passaged isolate of scrapie. Vet. Rec, 123, 5 - 8 .
3 0 . FOSTER J.D. & DICKINSON A.G. ( 1 9 8 9 ) . - Age at death from natural scrapie in a flock
of Suffolk sheep. Vet. Rec, 125, 4 1 5 - 4 1 7 .
3 1 . FOSTER J.D. & HUNTER N. ( 1 9 9 1 ) . - Partial dominance of the sA allele of the Sip gene
for controlling experimental scrapie. Vet. Rec, 128, 5 4 8 - 5 4 9 .
3 2 . FOSTER J.D., MCKELVEY W.A.C., MYLNE M.J.A., WILLIAMS A., HUNTER N., HOPE J. &
FRASER H. (1992). - Studies on maternal transmission of scrapie in sheep by embryo
transfer. Vet. Ree, 130, 3 4 1 - 3 4 3 .
3 3 . GOLDMANN W., HUNTER N., SMITH G., FOSTER J. & HOPE J. ( 1 9 9 4 ) . - PrP genotype and
agent effects in scrapie: change in allelic interaction with different isolates of agent in
sheep, a natural host of scrapie. J. gen. Virol, 75, 9 8 9 - 9 9 5 .
3 4 . GORDON W.S. ( 1 9 4 6 ) . - Louping-ill, tick-borne fever and scrapie. Vet. Rec, 58, 5 1 6 - 5 2 2 .
3 5 . GORDON W.S. ( 1 9 5 7 ) . - Scrapie. Vet. Ree, 69, 1 3 2 4 - 1 3 2 8 .
3 6 . GORDON W.S. ( 1 9 5 9 ) . - Scrapie Panel. Proc. U.S. Livestock Sanit. Assoc., 63, 2 8 6 - 2 9 4 .
37. GORDON W.S. ( 1 9 6 6 ) . - Review of work on scrapie at Compton, England, 1 9 5 2 - 1 9 6 4 .
In Report of scrapie seminar, Washington, D.C., 2 7 - 3 0 January 1 9 6 4 . Agricultural
Research Service (ARS), United States Department of Agriculture (USDA), 19-36.
3 8 . GORDON W.S. ( 1 9 6 6 ) . - Transmission of scrapie and evidence of spread of infection in
sheep at pasture. In Report of scrapie seminar, Washington, D.C., 2 7 - 3 0 January 1 9 6 4 .
Agricultural Research Service (ARS), United States Department of Agriculture (USDA),
8-13.
849
39. GORDON W . S . (1966). - Variation in susceptibility of sheep to scrapie and genetic
implications. In Report of scrapie seminar, Washington, D.C., 27-30 January 1964.
Agricultural Research Service (ARS), United States Department of Agriculture (USDA),
53-67.
40. GREIG J.R. (1940). - Scrapie: observations on the transmission of the disease by mediate
contact. Vet. J., 6, 203-206.
41. GREIG J.R. (1950). - Scrapie in sheep. J. comp. Pathol, 60, 263-266.
42. HADLOW W.J. (1990). - An overview of scrapie in the United States. JAVMA, 196 (10),
1676-1677.
43. HADLOW W.J., KENNEDY R.C., RACE R.E. & EKLUND C . M . (1980). - Virologic and
neurohistologic findings in dairy goats affected with natural scrapie. Vet. Pathol, 17,
187-199.
44. HADLOW W.J., KENNEDY R.C. & RACE R.E. (1982). - Natural infection of Suffolk sheep
with scrapie virus. J. infect. Dis., 146, 657-664.
45. HENEKENS C.H. & BURRING J.E. (1987). - Epidemiology in medicine. Little, Brown &
Co., Boston, 383 pp.
46. HOARE M., DAVIES D.C. & PATTISON I.H. (1977). - Experimental production of scrapie-
resistant Swaledale sheep. Vet. Rec, 101, 482-484.
47. HOURRIGAN J.L. (1990). - Experimentally induced bovine spongiform encephalopathy in
cattle in Mission, Tex, and the control of scrapie. JAVMA, 196, 1678-1679.
48. HOURRIGAN J., KLINGSPORN A., CLARK W.W. & D E CAMP M. (1979). - Epidemiology
of scrapie in the United States. In Slow transmissible diseases of the nervous system,
Vol. 1 (S.B. Prusiner & W.J. Hadlow, eds). Academic Press, New York, 331-356.
49. HUNTER N. (1991). - Natural transmission and genetic control of susceptibility of sheep
to scrapie. Curr. Topics Microbiol Immunol, 172, 165-180.
50. HUNTER N. (1992). - Scrapie in sheep and goats. In Progress in sheep and goat research
(A.W. Speedy, ed.). CAB International, Wallingford, Oxford, 131-151.
51. HUNTER N., FOSTER J.D., DICKINSON A.G. & HOPE J. (1989). - Linkage of the gene for
the scrapie-associated fibril protein (PrP) to the Sip gene in Cheviot sheep. Vet. Rec, 124,
364-366.
52. HUNTER N., FOSTER J.D., BENSON G . & HOPE J. (1991). - Restriction fragment length
polymorphisms of the scrapie-associated fibril protein (PrP) gene and their association
with susceptibility to natural scrapie in British sheep. J. gen. Virol, 72, 1287-1292.
53. HUNTER N., FOSTER J.D. & HOPE J. (1992). - Natural scrapie in British sheep: breeds,
ages and PrP gene polymorphisms. Vet. Rec, 130, 389-392.
54. HUNTER N., GOLDMANN W., BENSON G . , FOSTER J.D. & HOPE J. (1993). - Swaledale
sheep affected by natural scrapie differ significantly in PrP genotype frequencies from
healthy sheep and those selected for reduced incidence of scrapie. J. gen. Virol, 74,
1025-1031.
55. JOUBERT L., LAPRAS M., GASTELLU J., PRAVE M. & LAURENT D. (1972). - Un foyer de
tremblante du mouton en Provence. Bull Soc. Sci. vét. Méd. comp. Lyon, 74, 165-184.
56. KATIYAR R.D. (1962). - A preliminary report on the occurrence of scrapie in Indian
sheep. Ceylon vet. J., 10, 93-96.
57. KIMBERLIN R.H. (1979). - An assessment of genetical methods in the control of scrapie.
Livestock Prod. Sci., 6, 233-242.
850
58. KIMBERLIN R . H . & WALKER C.A. (1978). - Pathogenesis of mouse scrapie: effect of
route of inoculation on infectivity titres and dose-response curves. J. comp. Pathol, 88,
39-47.
59. KIMBERLIN R.H. & WALKER C.A. (1988). - Incubation periods in six models of
intra-peritoneally injected scrapie depend mainly on the dynamics of agent replication
within the nervous system and not the lymphoreticular system. J. gen. Virol, 69,
2953-2960.
60. KIMBERLIN R.H. & WALKER C.A. (1988). - Pathogenesis of experimental scrapie. In
Novel infectious agents and the central nervous system ( G . Bock & J. Marsh, eds). Wiley,
Chichester, 37-62.
61. KIMBERLIN R.H. & WALKER C.A. (1989). - Pathogenesis of scrapie in mice after
intra-gastric infection. Virus Res., 12, 213-220.
62. LAPLANCHE J.L., CHATELAIN J., THOMAS S., DUSSAUCY M., BRUGERE-PICOUX J. &
LAUNAY J.M. (1992). - PrP gene allelic variants and natural scrapie in French Ile-deFrance and Romanov sheep. In Prion diseases of humans and animals (S.B. Prusiner,
J. Collinge, J. Powell & B. Anderton, eds). Ellis Horwood, Chichester, 329-337.
63. LAPLANCHE J., CHATELAIN J., BEAUDRY P., DUSSAUCY M. & BOUNNEAU J.L. (1993). -
French autochthonous scrapied sheep without the 136Val PrP polymorphism.
Mammalian Genome, 4, 463-464.
64. MCFADYEAN J. (1918). - Scrapie. J. comp. Pathol. Ther, 31, 102-131.
65. MACIULIS A., HUNTER N . , W A N G S., GOLDMANN W., HOPE J. & FOOTE W.C (1992). -
Polymorphisms of a scrapie-associated fibril protein (PrP) gene and their association with
susceptibility to experimentally induced scrapie in Cheviot sheep in the United States.
Am. J. vet. Res., 53, 1957-1960.
66. MINISTRY OF AGRICULTURE, FISHERIES AND FOOD (MAFF) (1982). - Flock recording as
an aid to the control of scrapie. Leaflet 823. Her Majesty's Stationery Office (HMSO),
Edinburgh, 4 pp.
67. MORGAN K.L., NICHOLAS K . , GLOVER M.J. & HALL A.P. (1990). - A questionnaire
survey of the prevalence of scrapie in sheep in Britain. Vet. Rec, 127, 373-376.
68. NUSSBAUM R.E., HENDERSON W.M., PATTISON L.H., ELCOCK N . V . & DAVIES D . C .
(1975). - The establishment of sheep flocks of predictable susceptibility to experimental
scrapie. Res. vet. Sci., 18, 49-58.
69. PALSSON P.A. (1979). - Rida (scrapie) in Iceland and its epidemiology. In Slow
transmissible diseases of the nervous system, Vol. 1 (S.B. Prusiner & W.J. Hadlow, eds).
Academic Press, New York, 357-366.
70. PARRY H.B. (1960). - Scrapie: a transmissible hereditary disease of sheep. Nature, 185,
441-443.
71. PARRY H.B. (1962). - Scrapie: a transmissible and hereditary disease of sheep. Heredity,
17, 75-105.
72. PARRY H.B. (1983). - Scrapie disease in sheep. Historical, clinical, epidemiological,
pathological and practical aspects of the natural disease (D.R. Oppenheimer, ed.).
Academic Press, London, 192 pp.
73. PATTISON I.H. (1964). - The spread of scrapie by contact between affected and healthy
sheep, goats or mice. Vet. Rec, 76, 333-336.
74. PATTISON LH. & MILLSON G.C. (1960). - Further observations on the experimental
production of scrapie in goats and sheep. J. comp. Pathol, 70, 182-193.
851
7 5 . PATTISON LH. & MILLSON G.C. ( 1 9 6 1 ) . - Experimental transmission of scrapie to goats
and sheep by the oral route. J. comp. Pathol, 71, 1 7 1 - 1 7 6 .
76. PATTISON I.H. & MILLSON G.C. ( 1 9 6 1 ) . - Scrapie produced experimentally in goats with
special reference to the clinical syndrome. J. comp. Pathol., 71, 1 0 1 - 1 1 0 .
77. PATTISON I.H. & MILLSON G.C. ( 1 9 6 2 ) . - Distribution of the scrapie agent in the tissues
of experimentally inoculated goats. J. comp. Pathol, 72, 2 3 3 - 2 4 4 .
7 8 . PATTISON I.H., HOARE M.N., JEBBETT J.N. & WATSON W.A. ( 1 9 7 2 ) . - Spread of scrapie
to sheep and goats by oral dosing with foetal membranes from scrapie-affected sheep.
Vet. Rec, 90, 4 6 5 - 4 6 8 .
7 9 . PATTISON I.H., HOARE M.N., JEBBETT J.N. & WATSON W.A. ( 1 9 7 4 ) . -
Further
observations on the production of scrapie in sheep by oral dosing with foetal membranes
from scrapie-affected sheep. Br. vet. J., 130, 6 5 - 6 7 .
80. SCHREUDER B.E.C., DEJONG M.C.M., PEKELDER J.J., VELLEMA P., BROKER A.J.M. &
BETCKE H. ( 1 9 9 3 ) . - Prevalence and incidence of scrapie in the Netherlands: a
questionnaire survey. Vet. Rec, 133, 2 1 1 - 2 1 4 .
8 1 . SCOTT J.R., FOSTER J.D. & FRASER H. ( 1 9 9 3 ) . - Conjunctival instillation of scrapie in
mice can produce disease. Vet. Microbiol,
34, 3 0 5 - 3 0 9 .
82. SIGURDARSON S. ( 1 9 9 1 ) . - Epidemiology of scrapie in Iceland and experience with
control measures. In Sub-acute spongiform encephalopathies (R. Bradley, M. Savey &
B. Marchant, eds). Kluwer Academic Publishers, Dordrecht, 2 3 3 - 2 4 2 .
83. SlGURDSSON B. ( 1 9 5 4 ) . - RIDA, a chronic encephalitis of sheep (with general remarks
on infections which develop slowly and some of their special characteristics). Br. vet. J.,
110, 3 4 1 - 3 5 4 .
84. STAMP J.T. ( 1 9 6 2 ) . - Scrapie. Trans. Highlands agrie Soc. Scotland, 7, 1 9 - 3 6 .
85. STAMP J.T. ( 1 9 6 2 ) . - Scrapie: a transmissible disease of sheep. Vet. Ree, 74, 3 5 7 - 3 6 2 .
8 6 . STAMP J.T., BROTHERSTON J.G., ZLOTNIK I., MACKAY J.M.K. & SMITH W . ( 1 9 5 9 ) . -
Further studies on scrapie. J. comp. Pathol, 69, 2 6 8 - 2 8 0 .
87. STEMSHORN B.W. ( 1 9 7 5 ) . - Histoire de cas - un cas de tremblante naturelle chez une
chèvre. Can. vet. J., 16, 8 4 - 8 6 .
88. STOCKMAN S. ( 1 9 1 3 ) . - Scrapie: an obscure disease of sheep. J. comp. Pathol, 26,
317-327.
89. TAYLOR D.M. ( 1 9 8 9 ) . - Scrapie agent decontamination: implications for bovine
spongiform encephalopathy. Vet. Ree, 24, 2 9 1 - 2 9 2 .
90. TOUMAZOS P. ( 1 9 9 1 ) . - Scrapie in Cyprus. Br. vet. J., 147, 1 4 7 - 1 5 4 .
9 1 . TOUMAZOS P. & ALLEY M.R. ( 1 9 8 9 ) . - Scrapie in goats in Cyprus. N.Z. vet. J., 37,
160-162.
92. WESTAWAY D. & PRUSINER S.B. ( 1 9 9 0 ) . - Link between scrapie and BSE? Nature
(London), 346, 1 1 3 .
9 3 . WILSON D.R., ANDERSON R.D. & SMITH W . ( 1 9 5 0 ) . - Studies in scrapie. J. comp.
Pathol, 60, 2 6 7 - 2 8 2 .
94. WRATHALL A.E. & BROWN K.F.D. ( 1 9 9 1 ) . - Embryo transfer, semen, scrapie and BSE.
In Sub-acute spongiform encephalopathies (R. Bradley, M. Savey & B. Marchant, eds).
Kluwer Academic Publishers, Dordrecht, 2 4 3 - 2 5 3 .
852
9 5 . YOUNG G.B., STAMP J.T., RENWICK C.C. & DICKINSON A.G. ( 1 9 6 6 ) . - Field observations
of scrapie incidence. In Report of scrapie seminar, Washington, D.C., 2 7 - 3 0 January
1964. Agricultural Research Service (ARS), United States Department of Agriculture
(USDA), 1 9 9 - 2 0 6 .
9 6 . ZLOTNIK I. & KATIYAR R.D. ( 1 9 6 1 ) . - The occurrence of scrapie disease in sheep of the
remote Himalayan Foothills. Vet. Rec., 73, 5 4 3 - 5 4 5 .
9 7 . ZLOTNIK I. & RENNIE J.C. ( 1 9 6 3 ) . -
Further observations on the experimental
transmission of scrapie from sheep and goats to laboratory mice. J. comp. Pathol., 73,
150-162.