Download Risk assessment on the importation of milk and milk products

Rev. sci. tech. Off. int. Epiz., 1993,12 (4), 1135-1146
Risk assessment on the importation of milk
and milk products (excluding cheese) from
countries not free from foot and mouth disease
N.H. HENG and D.W. WILSON *
Summary: The authors discuss the risk assessment conducted by the Australian
Quarantine and Inspection Service (AQIS) on the importation of milk and milk
products (excluding cheese) from countries not free from foot and mouth
disease (FMD). This assessment was undertaken in response to requests from
countries wishing to export dairy products for sale on the Australian market.
AQIS conducted a public consultation on the proposal, in line with
Australian Government policy on transparency and accountability in the
quarantine decision-making process.
The authors examine the procedures involved in the investigation of the
likely presence of FMD virus in milk of vaccinated and non-vaccinated cows,
and of the heat treatment parameters effective in the inactivation of the virus.
The data provide a useful aid in the assessment of the risk factors associated
with the importation of milk and milk products, and in the development of
quarantine conditions for importation.
KEYWORDS: Foot and mouth disease virus - Heat treatment - Milk - Milk
products - Public consultation - Risk assessment - Risk factors.
BACKGROUND
Foot and mouth disease (FMD) is of significant quarantine concern to Australia, as
this is a major exotic disease which has t h e p o t e n t i a l to d e v a s t a t e t h e A u s t r a l i a n
livestock industry. The disease has not occurred in Australia since 1872.
Current Australian quarantine legislation permits the importation of dairy products
other than cheese and lactose only from countries considered to be officially free from
FMD. T h e s e countries include C a n a d a , D e n m a r k , Finland, J a p a n , New Z e a l a n d ,
Norway, the Republic of Ireland, Sweden, the United Kingdom and the United States of
America.
In response to requests to export dairy products to the Australian market from the
governments of several countries which are not considered to b e free from F M D ,
the Australian Government decided to review the relevant quarantine conditions. The
Australian Quarantine and Inspection Service (AQIS) conducted a risk assessment on
the importation of dairy products (excluding cheese) from countries not free from F M D .
* The Australian Quarantine and Inspection Service, G.P.O. Box 858, Canberra, A C T 2601,
Australia.
1136
A literature search was conducted on the epidemiology of the disease in vaccinated and
non-vaccinated herds, particularly in relation to virus secretion in milk. As the role of
milk and milk products in the spread of the disease is well established, information on the
heat inactivation of F M D virus (FMDV) was sought, in order to establish conditions for
the safe importation of dairy products.
Although F M D V was the main object of the risk assessment, other important viruses
and b a c t e r i a excreted in the milk of cows - including bovine virus d i a r r h o e a virus
( B V D V ) , enzootic bovine leukosis virus (EBLV), Mycobacterium tuberculosis (TB),
Listeria spp. and Brucella abortus - were also considered in the assessment. While a
review of the scientific literature revealed that F M D V is not inactivated in the
conventional pasteurisation of milk, other viruses and bacteria were found to be of
negligible significance, as heat treatment in pasteurisation regimes has been shown to be
effective in the inactivation of B V D V (2), EBLV (13), TB (3), Listeria spp. (9, 26) and
B. abortus (21).
A Q I S prepared and circulated the risk assessment discussion paper - for evaluation
and c o m m e n t - to the Australian States, industry, scientific organisations and all
interested parties, including the governments of the countries seeking market access in
Australia for their milk and milk products. T h e public consultative process is
in conformity with Australian Government policy on transparency and accountability in
the quarantine decision-making procedure. Relevant comments from respondents to the
risk assessment process were incorporated in a position paper which was circulated,
together with draft protocols for possible importation, for a further round of consultation.
ASSESSMENT OF DISEASE RISK
T h e four principal factors influencing the disease risk are as follows:
- zone/country factors (i.e. diseases p r e s e n t in the animal p o p u l a t i o n of the
exporting zone/country)
- commodity factors (i.e. diseases present in the animal or product)
- risk reduction factors (i.e. methods of reducing risk by product treatment)
- probability of domestic exposure (i.e. risk of pathogens from the imported product
reaching susceptible animal populations in the importing country).
The first three of these factors are discussed below.
FOOT AND MOUTH DISEASE CARRIER STATUS
AND VIRUS EXCRETION
The F M D carrier status of cattle represents a major problem for disease control in
situations where the disease is enzootic or where a vaccination policy is adopted. Studies
have shown that both vaccinated and non-vaccinated cattle are likely to become carriers
after exposure to F M D V infection. T h e virus was isolated from recovered,
non-immunised cattle for several months after clinical infection (32,18). Vaccinated
cattle exposed to the virus became carriers without showing clinical signs of the disease
(32, 3 0 , 1 0 , 1 8 , 31,19). Virus multiplication occurred primarily in the oropharyngeal
1137
mucosae (10). Observations conducted by Hedger (19) in a group of vaccinated cattle
exposed to severe n a t u r a l field challenge with FMDV, revealed that 8 5 % b e c a m e
carriers without showing clinical symptoms. Virus-negative animals in the group were
found to possess very high antibody titres, and it was suggested that vaccinated animals
may possess sufficient immunity to resist clinical disease, as well as being carriers of
infection. Partly-immunised cattle may exhibit clinical signs which could be overlooked
in field conditions, as the symptoms may not be as severe as those seen in susceptible
animals (27).
Several workers were unable to detect viraemia in immunised cattle exposed to
FMDV infection (31,19, 27). McVicar and Sutmoller (27) also reported the absence of
viraemia in partly-immunised cattle which exhibited clinical signs after challenge with
the virus. Doubts were expressed over the possible excretion of F M D V in the milk of
vaccinated cows, as L e e u w et al. (23) w e r e u n a b l e to isolate F M D V from the milk
of experimentally-infected, vaccinated cows. There is a high probability that the virus
may be absent from milk in a well-immunised herd in which no outbreak of the disease
has been reported for some years.
Some workers have also r e p o r t e d that spread of virus did not occur from carrier
animals to susceptible in-contact animals (32,10,19). However, Hedger (18) reported
evidence of transmission of the virus from a carrier animal to another animal within a
herd in the field. Studies by Sellers et al. (29) on airborne dispersal of F M D V revealed
that vaccinated and non-vaccinated cattle excreted virus in two phases - between 30 min
and 22 h, and 2 and 7 days after exposure. T h e authors suggested that the first phase
probably resulted from the mechanical shedding of the virus from the respiratory tract
and the coat of animals, or from the clothing of p e o p l e exposed in the experiment;
shedding in the second phase was thought to result from the multiplication of the virus in
the respiratory tract. Virus excretion was not detected after the first day in animals which
had been vaccinated more than once. This finding suggested that higher immunity levels
resulted in reduced or zero virus production. However, studies were not conducted on
the infectivity of the virus for susceptible animals. Donaldson and Kitching (17) found
that cattle vaccinated three weeks before challenge with F M D V did not transmit virus to
susceptible in-contact animals. In cattle vaccinated two weeks before challenge,
transmission of the virus to susceptible in-contact animals occurred but the infection was
subclinical. Transmission and s u b s e q u e n t clinical disease occurred in susceptible
in-contact animals which had b e e n exposed to cattle challenged four or seven days
after vaccination.
T h e above findings p e r t a i n i n g to the spread of F M D V from carrier animals to
susceptible in-contact animals support the conclusion by Anderson et al. (1) that in a
well-immunised herd where reintroduction of the virus is prevented through livestock
movement controls and other animal health measures, F M D V is unlikely to persist for a
lengthy period in the population and, therefore, that elimination of the virus from the
herd can be achieved.
FOOT AND MOUTH DISEASE VIRUS EXCRETION IN MILK
L i t e r a t u r e r e p o r t s confirm the i m p o r t a n t role of milk in the s p r e a d of F M D V .
Dawson (16) m a d e references to early l i t e r a t u r e in which l a b o r a t o r y e x p e r i m e n t s
supported r e p o r t s on the role of milk in the s p r e a d of F M D . D a w s o n p r e s e n t e d
1138
evidence on the involvement of milk and milk movement in the natural spread of the
disease during the 1967-1968 outbreak in the U n i t e d Kingdom. Studies by Burrows
et al. (12) and Blackwell et al. (8) confirmed that F M D V replicated in the mammary
gland.
T h e fact that F M D V is excreted in milk before the onset of clinical disease in
infected cattle is of major epidemiological significance in the control of the disease.
Burrows (11) had demonstrated virus excretion before the onset of clinical symptoms in
experimental studies. Hedger and Dawson (20) demonstrated high titres of infective
virus in milk, at least 33 h before the detection of clinical disease.
It is important to note that Leeuw et al. (23) were unable to demonstrate F M D V in
milk from vaccinated cows infected by the intranasal route. Milk samples obtained from
cows exposed to the virus between two and nine months after their last vaccination were
found to be negative for F M D V when inoculated onto cell cultures, fed to susceptible
pigs and calves or injected intramuscularly and/or intradermolingually into susceptible
steers. The absence of viraemia in immunised cattle exposed to F M D V infection and in
partly-immunised cattle which had exhibited clinical signs after challenge with the virus,
support the findings of Leeuw et al.
CATEGORISATION OF COUNTRIES
As scientific evidence supports the contention that the probability of F M D V being
present in milk from well-vaccinated herds would be low, the feasibility of importing
milk products from countries with well-protected herds was assessed. Accordingly,
countries were categorised according to disease status, disease control programmes and
the ability to implement such programmes effectively.
AQIS categorised potential exporting countries as follows:
Category 1: F M D vaccination is prohibited, and no outbreaks of the disease have
been reported in the past twelve months.
Category 2: Compulsory official control p r o g r a m m e s are i m p l e m e n t e d and no
outbreaks of F M D have been reported in the past two years. The veterinary authorities
in these countries have d e m o n s t r a t e d the ability to implement disease control
programmes effectively and could b e relied upon to diagnose and notify any disease
occurrence rapidly.
Category 3: F M D outbreaks continue to occur, or the disease status is unclear. These
countries have difficulty in the effective implementation of disease control programmes
and in the early detection, diagnosis and notification of the disease.
Draft quarantine conditions were then developed to facilitate the safe importation of
milk products from these countries, subject to a full public consultation.
FOOT AND MOUTH DISEASE VIRUS SURVIVAL IN MILK
AND MILK PRODUCTS
T h e investigation of F M D V inactivation in milk has b e e n conducted largely on
samples o b t a i n e d from susceptible cows inoculated by the i n t r a m a m m a r y and
1139
intravenous routes, although Blackwell et al. (7) reported similar results for susceptible
cows infected by oral exposure.
Several workers h a v e shown that c u r r e n t t e m p e r a t u r e / t i m e regimes used for
pasteurisation (either 60-65.5°C for at least 30 min, or high-temperature, short-time
[HTST] treatment at 71-72°C for at least 15 sec) are inadequate to inactivate F M D V
reliably in milk.
Earlier work, such as that by Sellers (28), used relatively insensitive cell-culture
techniques for residual virus detection, and added virus to milk instead of using milk
from infected animals (Table I). Hyde et al. (22) and Leeuw et al. (25) demonstrated that
virus in milk from infected cows survived longer than virus added to milk from healthy
cows, due to the protection afforded by cells, proteins and fat in the milk. These authors
also showed that susceptible cattle provided a more sensitive means for the detection of
residual viral activity than cell cultures.
TABLE I
Results of inactivation
Temperature
of foot and mouth disease virus
(28)
Time taken to inactivate FMDV to a
survival of 0.00001%
at pH 6.7
56°C
63°C
72°C
80°C
85°C
pH (at 4°C)
2
4
5.5
5.8
11
12
13
(FMDV)
6
1
17
<5
<5
at pH 7.6
min
min
sec
sec
sec
30
2
55
<5
<5
min
min
sec
sec
sec
Time taken for inactivation
1
2
30
18
2
2.5
2.5
min
min
min
h
h
min
min
Note: traces of virus were detected after the times recorded
Hyde et al. (22) infected susceptible cows by the intravenous and i n t r a m a m m a r y
routes. Infected milk samples, heated at 72°C and 80°C for 15-17 sec, were found to be
positive for F M D V by cell-culture techniques. Milk treated at the above temperatures
and times and then heated at 65°C for 64.5 min to reduce the volume by 5 0 % tested
negative for F M D V in cell cultures. However, susceptible steers inoculated with the
evaporated milk samples developed clinical disease (Table II).
1140
TABLE II
Results of inactivation
Sample
of foot and mouth disease virus (FMDV) in milk
(22)
Treatment
Milk obtained 24 h
post-inoculation
80°C
Milk pasteurised at
72°C and 80°C
for 15-17 sec
72°C for 15-17 sec
for
15-17
Virus survival
sec
65°C for 64.5 min
(evaporated to 50%
of original volume)
Detected in cell cultures
Detected in cell cultures
Not detected in cell cultures
Detected in cattle inoculation
tests
Similar results (Table III) were obtained by Blackwell and Hyde (6), who found that
F M D V survived pasteurisation at 72°C for 15 sec in skimmed milk, cream and pelleted
cellular components. F M D V survived in whole milk heated at 72°C for 5 min and 85°C
for 15 sec, skimmed milk heated at 72°C for 2 min and 85°C for 15 sec, and in cream
TABLE III
Results of heat treatment on components of milk from cows infected
with foot and mouth disease virus (FMDV)
(6)
Cattle inoculation
Sample Heat treatment
for FMDV
Whole milk
72°C for 5 min
85°C for 15 sec
Skimmed milk
72°C for 2 min
85°C for 15 sec
Cream
93°C for 15 sec
Pelleted cellular debris
72°C for 15 sec
Whole milk
(heated at 72°C for 5 min)
65°C for 1 h maximum
(evaporated to 50%
of original volume)
Skimmed milk
(heated at 72°C for 15 sec)
as above
Skimmed milk
(heated at 72°C for 30 sec)
as above
+ positive
-
negative
1141
heated at 93°C for 15 sec. F M D V survived in whole milk which was heated at 72°C for
5 min and then evaporated. The virus survived in skimmed milk after this was heated at
72°C for 15 sec, but not when it was heated at 72°C for 30 sec and then evaporated.
Most of the h e a t e d samples were found to b e negative for F M D V by cell-culture
assay but were positive in cattle inoculation tests. These findings reinforced the report
by Hyde et al. (22) that inoculation into susceptible cattle was a more sensitive method
for the detection of residual virus.
The results support the findings of Hyde et al. (22) that conventional pasteurisation is
insufficient to inactivate F M D V in milk and milk components.
Blackwell (4) showed that pasteurisation of cream at 93°C for 16 sec, followed by
processing to butter and storage for up to 45 days at 4°C at a p H which fell from 5.9 to 5.4,
did not completely inactivate the virus. F M D V was detected in cattle inoculation tests.
Leeuw et al. (25) also found that cell-culture assay was less sensitive t h a n cattle
inoculation in the detection of residual virus. Using cell-culture assay, F M D V was not
detected in skimmed milk heated at 72°C for 30 sec. However, the same sample heated
at 80°C for 30 sec was found to be positive for virus when inoculated into cattle.
Cunliffe and Blackwell (14) found that F M D V survived in casein p r e p a r e d from
skimmed milk pasteurised at 72°C for 15 sec. Casein had been maintained at p H 4.5-4.7
for 2-3 h during processing. F M D V was d e t e c t e d by the i n t r a d e r m o l i n g u a l and
intramuscular inoculation of cattle.
F M D V was not detected by either cell-culture assay or cattle inoculation in whey
constituents manufactured from virus-positive milk (5).
The effectiveness of ultra-high temperature ( U H T ) processes in virus inactivation
was examined by Cunliffe et al. (15), who found that heating milk at 138°C for 2-3 sec
did not inactivate the virus completely, while heating at 148°C for 2-3 sec was effective
(Table IV). Intradermolingual and intramuscular inoculation of susceptible steers was
conducted to detect residual virus in the treated milk samples.
Walker et al. (33) confirmed that heating infected milk at 148°C for 2.5 sec could
effectively inactivate F M D V and also d e m o n s t r a t e d that m o r e than 20 min w e r e
required to inactivate virus at 100°C. The data presented by Walker et al. were obtained
from a collaborative project c o n d u c t e d at the Plum Island A n i m a l Disease C e n t r e
(PIADC) in the United States of America and the Central Veterinary Institute (CVI) in
the N e t h e r l a n d s . D a t a on the e x p e r i m e n t s c o n d u c t e d at the CVI have also b e e n
reported separately by L e e u w and Van B e k k u m (24). Milk was o b t a i n e d from
susceptible cows infected by the intramammary and intravenous routes. T h e CVI used
whole and skimmed milk, whereas the P I A D C used only whole milk. However, no
differences w e r e d e t e c t e d in the t h e r m a l inactivation of F M D V in either whole or
skimmed milk. All assays for infectivity following heat t r e a t m e n t were conducted in
susceptible cattle which were inoculated by the intradermolingual and intramuscular
routes. The results are presented in Table V.
DISCUSSION
The literature review confirms that conventional pasteurisation of milk and milk
products is inadequate for the reliable inactivation of FMDV.
1142
TABLE IV
Results of ultra-high temperature treatments on milk from cows
infected with foot and mouth disease virus (FMDV)
(15)
Time
Cattle inoculation
for FMDV
102°C
2-3 sec
+
123°C
2-3 sec
+
130°C
2-3 sec
+
138°C
2-3 sec
+
148°C
2-3 sec
-
Temperature
+
positive
-
negative
TABLE V
Results of inactivation of foot and mouth disease virus in milk
heated at various temperatures and times
(33)
Time
80°C
30 sec
+
85°C
30 sec
+
90°C
3 min
+
100°C
9 min
+
100°C
27 min
-
110°C
30 sec
+
3 min
120°C
30 sec
-
130°C
10 sec
+
135°C
17 sec
-
138°C
2.5 sec
+
138°C
5 sec
-
148°C
3 sec
—
110°C
+
Results of virus assay
in susceptible cattle
Temperature
positive
- negative
Most of the work reported was conducted in milk obtained from susceptible cows
infected by intravenous and intramammary inoculation of the virus, although reports
were also made on milk from cows infected by contact exposure ( 7 ) . Reports on virus
excretion in milk ( 1 6 , 1 9 ) in the 1967-1968 outbreak in the United Kingdom pertained to
susceptible cattle.
Leeuw et al. (23) failed to detect F M D V in milk from vaccinated cows up to 19 days
after intranasal challenge, although virus was detected in oropharyngeal fluids during
1143
this period. F M D V was not detected when the milk samples were inoculated into cell
cultures, fed to pigs and calves, or injected intramuscularly and/or intradermolingually
into susceptible steers. T h e contention of Leeuw et al. (23) is that infected, vaccinated
cattle are unlikely to excrete virus in milk and that, even if excretion did occur, the
concentration of virus would not approach that recorded for susceptible cows infected
via the intramammary route. These authors further suggested that standard commercial
heat treatments, coupled with the activity of neutralising antibodies in pooled milk,
should be a d e q u a t e to inactivate the virus. R e p o r t s by some workers that viraemia
could not be detected in either well-immunised or partly-immunised cattle exposed to
FMDV, and the suggestion made by Anderson et al. (1) that total disease eradication in
a well-immunised herd is possible, would support this argument.
The above a r g u m e n t provides a sound basis for the d e v e l o p m e n t of q u a r a n t i n e
conditions to facilitate the safe importation of milk and milk products from Category 2
countries.
In the case of Category 3 countries, there is a high risk of F M D V contamination in
milk and milk products. As commercial pasteurisation procedures have b e e n shown
to be inadequate to inactivate the virus, only milk products which have been subject to
commercial sterilisation treatments in cans or retort pouches (or the equivalent) would
be considered to present an acceptable risk.
*
ÉVALUATION DES RISQUES LIÉS À L'IMPORTATION DE LAIT ET DE PRODUITS
LAITIERS (À L'EXCEPTION DES FROMAGES) EN PROVENANCE DE PAYS NON
INDEMNES DE FIÈVRE APHTEUSE. - N.H. Heng et D.W. Wilson.
Résumé : Les auteurs étudient la méthode utilisée par les Services australiens de
quarantaine et d'inspection (Australian Quarantine and Inspection Service :
AQIS) pour évaluer les risques liés à l'importation de lait et de produits laitiers
(à l'exclusion des fromages) en provenance de pays non indemnes de fièvre
aphteuse. Cette évaluation a été entreprise pour répondre aux demandes de pays
désireux d'exporter des produits laitiers en Australie.
L'AQIS a effectué une enquête publique en la matière, conformément à la
réglementation australienne sur la transparence et la responsabilité dans le
processus de prise de décision relatif à la quarantaine.
Les auteurs examinent les procédures appliquées à la recherche du virus de
la fièvre aphteuse dans le lait de vaches vaccinées et non vaccinées, ainsi qu'à
l'étude des paramètres de traitement thermique efficaces pour l'inactivation du
virus. Les données ainsi obtenues constituent une aide précieuse pour
l'évaluation des facteurs de risques liés à l'importation de lait et de produits
laitiers, ainsi que pour l'élaboration d'une réglementation de la quarantaine
applicable aux marchandises importées.
MOTS-CLÉS : Enquête publique - Evaluation des risques - Facteurs de
risques - Lait - Produits laitiers - Traitement thermique - Virus de la fièvre
aphteuse.
*
* *
1144
EVALUACIÓN DE RIESGOS POR IMPORTACIÓN DE LECHE Y DE PRODUCTOS
LECHEROS (EXCEPTO QUESOS) PROVENIENTES D E PAÍSES NO LIBRES DE
FIEBRE AFTOSA. - N.H. Heng y D.W. Wilson.
Resumen: Los autores estudian el método utilizado por el Servicio australiano
de cuarentena e inspección (Australian Quarantine and Inspection Service:
AQIS) para evaluar los riesgos derivados de la importación de leche y
productos lecheros (excepto quesos) provenientes de países no libres de fiebre
aftosa. Este método fue establecido en respuesta a los pedidos de países
deseosos de exportar sus productos lecheros a Australia.
El AQIS llevó a cabo una encuesta pública al respecto, conforme a la
reglamentación australiana que se refiere a la transparencia y la responsabilidad
en el proceso de tomar decisiones sobre cuarentenas.
Los autores examinan los procedimientos que se aplican en la investigación
del virus de la fiebre aftosa en la leche de vacas vacunadas y no vacunadas y los
que se aplican en el estudio de los parámetros de tratamiento térmico eficaces
para la inactivación del virus. Los datos así obtenidos son de gran ayuda para
la evaluación de los factores de riesgo en la importación de leche y de productos
lecheros, así como también para la elaboración de una reglamentación de la
cuarentena aplicable a las mercaderías importadas.
PALABRAS CLAVE: Encuesta pública - Evaluación de riesgos - Factores de
riesgo - Leche - Productos lecheros - Tratamiento térmico - Virus de la fiebre
aftosa.
*
* *
REFERENCES
1. ANDERSON E.C., D O U G H T Y W . J . & ANDERSON J . (1974). - The effect of repeated
vaccination in an enzootic foot-and-mouth disease area on the incidence of virus carrier
cattle. J. Hyg., Camb., 73, 229-235.
2. ANDREWES C , PEREIRA H . G . & WILDY P. (1978). - Bovine virus diarrhoea. In Viruses of
vertebrates, 4th Ed. Baillière Tindall, London, 102-104.
3. BEVERIDGE W.I.B. (1983). - Bacterial diseases of cattle, sheep and goats. Anim. Hlth
Aust, 4,155-164.
4. BLACKWELL J . H . (1978). - Persistence of foot-and-mouth disease virus in butter and
butter oil. J. Dairy Res., 45, 283-285.
5. BLACKWELL J . H . (1978). - Potential transmission of foot-and-mouth disease in whey
constituents.J. Food Protec, 41 (8), 631-633.
6. BLACKWELL J . H . & HYDE J.L. (1976). - Effect of heat on foot-and-mouth disease virus
(FMDV) in the components of milk from FMDV-infected cows. J. Hyg., Camb.,
77, 77-83.
7. BLACKWELL J . H . , M C K E R C H E R P.D., KOSIKOWSKI F.V, CARMICHAEL L.E. & GOREWIT
R.C. (1982). - Concentration of foot-and-mouth disease virus in milk of cows infected
under simulated field conditions. J. Dairy Sci., 65,1624-1631.
1145
8. BLACKWELL J . H . , M C K E R C H E R P.D., KOSIKOWSKI F.V., CARMICHAEL L.E. & G O R E W I T
R.C. (1983). - Histological and histochemical characterisation of mammary gland tissue
of cows infected with foot-and-mouth disease by contact exposure. Res. vet. Sci.,
35,106-113.
9. BRADSHAW J . G . , P E E L E R J.T., CORWIN J . J . , H U N T J.M., T I E R N E Y J.T., L A R K I N E . P .
& TWEDT R.M. (1985). - Thermal resistance of Listeria monocytogenes in milk. J. Food
Protec, 48 (9), 743-745.
10. BURROWS R. (1966). - Studies on the carrier state of cattle exposed to foot-and-mouth
disease virus. J. Hyg., Camb., 64, 81-90.
11. BURROWS R. (1968). - Excretion of foot-and-mouth disease virus prior to the
development of lesions. Vet. Rec., 82,387-388.
12. BURROWS R., M A N N J . A . , G R E I G A., CHAPMAN W.G. & G O O D R I D G E D . (1971). -
The growth and persistence of foot-and-mouth disease virus in the bovine mammary
gland. J. Hyg., Camb., 69,307-321.
13. CHUNG Y.S., P R I O R H.C., D U F F Y P.F., R O G E R S R . J . & M A C K E N Z I E A.R. (1986). -
The effect of pasteurisation on bovine leucosis virus-infected milk. Aust. vet. J., 63 (11),
379-380.
14. CUNLIFFE H.R. & BLACKWELL J.H. (1977). - Survival of foot-and-mouth disease virus in
casein and sodium caseinate produced from the milk of infected cows. J. Food Protec.,
40 (6), 389-392.
15. CUNLIFFE H.R., BLACKWELL J . H . , D O R S R. & W A L K E R J . S . (1979). - Inactivation of
milkborne foot-and-mouth disease virus at ultra-high temperatures. J. Food
42 (2), 135-137.
Protec.,
16. DAWSON P.S. (1970). - The involvement of milk in the spread of foot-and-mouth disease,
an epidemiological study. Vet. Rec., 87,543-548.
17. DONALDSON A.I. & KITCHING R.P (1989). - Transmission of foot-and-mouth disease by
vaccinated cattle following natural challenge. Res. vet. Sci., 46, 9-14.
18. HEDGER R.S. (1968). - The isolation and characterisation of foot-and-mouth disease
virus from clinically normal herds of cattle in Botswana. J. Hyg., Camb., 66,27-36.
19. HEDGER R.S. (1970). - Observations on the carrier state and related antibody titres
during an outbreak of foot-and-mouth disease. J. Hyg., Camb., 68,53-60.
20. HEDGER R.S. & DAWSON P.S. (1970). - Foot-and-mouth disease virus in milk: an
epidemiological study. Vet. Rec., 87,186-188 & 213.
21. HEEVER L.W. VAN DEN, KATZ K.W. & BRUGGE L A . TE (1982). - On the inactivation of
B. abortus in naturally contaminated milk by commercial pasteurisation procedures.
JlS. Afr. vet. Assoc., 53,233-234.
22. H Y D E J . L . , BLACKWELL J . H . & CALLIS J . J . (1975). - Effect of pasteurization and
evaporation on foot-and-mouth disease virus in whole milk from infected cows. Can.
J. comp. Med., 39 (3), 305-309.
23. LEEUW P.W. DE, V A N BEKKUM J . G . & TIESSINK J.W.A. (1978). - Excretion of
foot-and-mouth disease virus in oesophageal-pharyngeal fluid and milk of cattle after intranasal infection. J. Hyg., Camb., 81,415-425.
24. LEEUW P.W. DE & VAN BEKKUM J.G. (1979). - Some aspects of foot-and-mouth dises
virus in milk. In Report of the Session of the Research Group of the Standing Technical
Committee of the European Commission for the Control of Foot-and-Mouth Disease.
Lindholm, Denmark. Appendix C4,79-87.
1146
2 5 . L E E U W P.W. D E , TIESSINK J . W . A . & V A N BEKKUM J.G. ( 1 9 8 0 ) . - Aspects of heat
inactivation of foot-and-mouth disease virus in milk from intramammarily infected
susceptible cows. J. Hyg., Camb., 84,159-172.
2 6 . L O V E T T J., W E S L E Y I.V., MAATEN M . J . VAN DER, BRADSHAW J.G., FRANCIS D . W . ,
CRAWFORD R.G., DONNELLY C.W. & MESSER J.W. ( 1 9 9 0 ) . - High-temperature short-time
pasteurisation inactivates Listeria monocytogenes. J. Food Protec, 53 ( 9 ) , 7 3 4 - 7 3 8 .
2 7 . M C V I C A R J.W. & SUTMOLLER P. ( 1 9 7 6 ) . - Growth of foot-and-mouth disease virus in the
upper respiratory tract of non-immunized, vaccinated, and recovered cattle after
intranasal inoculation. J. Hyg.,
Camb.,16,467-481.
28. SELLERS R.F. ( 1 9 6 9 ) . - Inactivation of foot-and-mouth disease virus in milk. Br. vet. J.,
125,163-167.
2 9 . SELLERS R . F , HERNIMAN K.A.J. & G U M M I . D . ( 1 9 7 7 ) . - The airborne dispersal of
foot-and-mouth virus from vaccinated and recovered pigs, cattle and sheep after
exposure to infection. Res. vet. Sci., 23,70-75.
30. SUTMOLLER P. & GAGGERO A . C . ( 1 9 6 5 ) . - Foot-and-mouth disease carriers. Vet. Rec.,
77 ( 3 3 ) , 9 6 8 - 9 6 9 .
3 1 . SUTMOLLER P., MCVICAR J.W. & COTTRAL G.E. ( 1 9 6 8 ) . - The epizootiological importance
of foot-and-mouth disease carriers. I. Experimentally produced carriers in susceptible and
immune cattle. Arch. ges. Virusforch., 23,227-235.
32. VAN BEKKUM J.G., F R E N K E L H . S . , FREDERIKS H.H.J. & F R E N K E L S . ( 1 9 5 9 ) . -
Observations on the carrier state of cattle exposed to foot-and-mouth disease virus.
Tijdschr. Diergeneesk., 84,1159-1164.
3 3 . WALKER J.S., LEEUW P.W. DE, CALLIS J.J. & VAN BEKKUM J.G. ( 1 9 8 4 ) . - The thermal
death time curve for foot-and-mouth disease virus contained in primarily infected milk.
J. biol. Stand., 12,185-189.