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International Journal for Parasitology 30 (2000) 1369±1377
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The role of companion animals in the emergence of parasitic zoonoses
I.D. Robertson*, P.J. Irwin, A.J. Lymbery, R.C.A. Thompson
Division of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, Western Australia 6150, Australia
Received 31 May 2000; received in revised form 7 August 2000; accepted 1 September 2000
Abstract
Pets offer individuals and the community signi®cant bene®ts, however cognisance must be taken of the potential for transmission of
infectious agents from these animals to humans. The prevalence of many parasites, such as Giardia and Cryptosporidium, has increased over
the past few decades while others, such as Toxocara and Ancylostoma, have decreased. These changes could be real, associated with the
ready availability of ef®cacious anthelmintic products or could be arti®cial due to the type of surveys conducted, the animals surveyed and
the diagnostic tests used. Immunocompromised people, in particular, must be aware of the potential risk of acquiring parasitic infections from
their pets. However, with the adoption of good hygiene and a thorough knowledge of the transmission of these parasites, immunocompromised people should be able to continue to enjoy the signi®cant bene®ts of pet ownership. As many owners are not aware of the zoonotic
parasites that could be carried by their pets or their mode of transmission, it is concluded that veterinarians need to play a greater role in the
education of their clients. q 2000 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.
Keywords: Parasites; Zoonoses; Pets; Companion animals
1. Introduction
Pet animals, particularly dogs and cats, play an important
role in societies throughout the world. They are important
companions in many households, contributing to the physical, social and emotional development of children and the
well-being of their owners (in particular elderly people) [1±
7]. Furthermore, pet owners are reported to visit their doctor
less often, use fewer medications and have lower blood
pressure and cholesterol levels than non-pet owners [8].
Although pets offer signi®cant bene®ts to our society,
there are well-documented health hazards associated with
owning a pet. Animal bites and allergy to pets are the
commonest health hazards, however a diverse range of
infections, including parasitic, bacterial, fungal and viral
diseases are capable of being transmitted to humans from
domestic pets [9,10]. Schantz [11] reported that the potential
health risk to humans of enteric parasites harboured by pet
dogs and cats remains a signi®cant problem in most parts of
the world.
Details of the speci®c zoonotic parasites of pets have
been presented elsewhere in this issue, and this review
will concentrate on factors affecting the prevalence of para-
* Corresponding author. Fax: 161-893-107-495.
E-mail address: [email protected] (I.D. Robertson).
sites, the risk of infection and managing and reducing these
risks.
2. Changes in the distribution of parasites in dogs and
cats
Recent parasitological surveys of dogs and cats have indicated that the prevalence of intestinal helminths has
declined over the last 20 years in the developed world
[12±14]. This may be a real decrease associated with the
ready availability and adoption of safe and effective drugs to
eliminate these infections from pets. Alternatively, differences in sampling protocols including source and age of
animals; prior anthelmintic usage in sampled animals; diagnostic techniques; or environmental conditions between
surveys may account for the decrease in prevalence [15±17].
2.1. Sampling protocols
Bugg et al. [14] demonstrated the signi®cant effect of dog
source in their study where stool samples were collected
from dog refuges, pet shops, veterinary clinics, breeding
kennels and from exercise areas in Perth, Australia. The
prevalence of zoonotic helminths was low in dogs sampled
at veterinary clinics, exercise areas or breeding kennels (less
than 1.5%). In contrast, the prevalence of Toxocara canis
and Ancylostoma caninum in dogs from refuges and pups
0020-7519/00/$20.00 q 2000 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.
PII: S 0020-751 9(00)00134-X
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from pet shops was higher (up to 7%). Schantz et al. [18]
and Blagburn et al. [19] also reported that in the USA, dogs
from refuges and shelters had higher prevalences of T. canis
and Ancylostoma spp. than did dogs sampled at veterinary
clinics. Similarly for cats, higher prevalences of zoonotic
parasites have been reported in shelter cats (18.2%) than
client owned cats (10.1%) [20]. The lower prevalence in
well cared for pets is probably associated with the type of
owner surveyed, as Schantz [11] reported that clients of
veterinarians were more likely to treat their pets with anthelmintics than were other pet owners. Animals in refuges are
less likely to have received a high degree of care or veterinary attention and are unlikely to receive routine anthelmintic treatments.
2.2. Demographic factors
Young animals are at a greater risk of being infected with
parasites such as Toxocara. For example, Schantz et al. [18]
observed that the prevalence of T. canis in dogs aged from 7
weeks to 3 months in shelters in the USA was nearly 100%.
However, even after allowing for age differences in the
study of Bugg et al. [14], different levels of infection were
found between sources, indicating the importance of other
factors, such as frequency of anthelmintic use, in in¯uencing the prevalence of parasites.
Infections of T. canis and hookworms have been reported
to be more prevalent in intact male and female dogs than in
neutered animals [16]. This may be related to hormonal or
behavioural differences between intact and neutered
animals. However, Robertson et al. [21] observed that
mature entire animals (other than those used for breeding)
were taken to veterinarians less frequently and received
fewer prophylactic treatments, such as anthelmintics, than
did similar aged neutered animals.
2.3. Anthelmintic usage
While early studies in dogs from Australia found high
levels of infection with T. canis of up to 38% [22,23],
more recent surveys have reported a lower prevalence of
this parasite and evidence of the increasing importance of
zoonotic protozoa such as Giardia and Cryptosporidium
[24,25]. These earlier surveys involved only small numbers
of animals, predominantly sampled younger animals, were
restricted to high risk groups and were concerned with
speci®c parasites [26±28]. Bugg et al. [14] considered that
one of the major reasons for the reduction in the prevalence
of parasites such as T. canis and Dipylidium caninum was a
growing awareness by dog owners about these parasites and
methods for controlling them. They proposed that targeted,
strategic treatments for parasites such as T. canis and Diro®laria immitis, where owners could associate treatment with
a particular parasite and its consequences, were more likely
to be successful and therefore reduce the risk of transmission of these potentially zoonotic parasites. In contrast,
advocating regular anthelmintic treatment of dogs of all
ages, without an association with a particular parasite or
strategy (e.g. [29], was considered to be less effective and
more likely to hasten the appearance of drug resistance, as
has been reported in farmed livestock [30].
Bugg et al. [14] found that Giardia duodenalis was the
most common enteric parasite of domestic dogs in Perth,
Western Australia. Although the clinical signi®cance of this
parasite appears to be minimal in dogs, Hopkins et al. [31]
demonstrated that some strains were shared between
humans and dogs. The apparent increase in the prevalence
of protozoa, such as Giardia, may have arisen from the use
of more sensitive diagnostic techniques in recent studies.
Alternatively Bugg et al. [14] proposed that as enteric protozoa are unaffected by the anthelmintics currently in
common use, the intestinal protozoa may be colonising
the niche vacated by parasites such as T. canis and D. caninum.
2.4. Geographical location
In contrast to Australia, Blagburn et al. [19] found high
levels of helminths in a large study in the USA (A. caninum
19.2% and T. canis 14.5%) but low levels of protozoa.
Samples were, however, only collected from animal shelters
and also were collected from a wide geographical area. In
contrast, a survey of pups for sale at pet stores in Atlanta
showed that 34% were infected with Giardia [32] even
though none had diarrhoea or clinical illness. Similarly a
survey of dogs in California detected 35.9% of dogs shedding Giardia, with similar rates of infection and also the
absence of clinical signs in both shelter and privately owned
pups [33].
The high level of pet ownership by American households
and the reported high carrier rates of Toxocara and Ancylostoma by dogs could lead to widespread environmental
contamination with eggs and infective larvae. These represent a signi®cant source of infection for humans, in particular children [34,35] and highlight the importance of
giving preventive treatments particularly to young pups.
Unless anthelmintic treatments are administered to puppies
before the age of 4 weeks, female worms can become gravid
and produce eggs when the pups are as young as 3 weeks of
age. Furthermore, these eggs can remain infective in some
environments for months or even years [16].
Toxocaral larva migrans has been reported to be the most
common zoonotic parasitic infection acquired from pets in
the USA [36] and it has been estimated that this infection
causes hundreds of cases of unilateral blindness and less
permanent forms of illness in children each year in the
USA [34,35]. However these estimations were based on
data collected between 10 and 20 years ago. Given the
reduction in the prevalence of intestinal helminths reported
elsewhere [14] it is probable that a similar reduction in the
number of human cases in the USA has occurred.
Differences in the parasites detected between studies may
also be attributable to factors related to the environment
I.D. Robertson et al. / International Journal for Parasitology 30 (2000) 1369±1377
(e.g. soil type and elevation) and climate (rainfall, temperature and humidity) [15,16].
2.5. Diagnostic techniques
Use of different diagnostic tests may account for, in part,
differences in prevalences observed between studies.
Schantz [16] emphasised that stool examinations were not
reliable in young pups, as often worms were prepatent.
Schantz et al. [18] reported that although approximately
80% of dogs younger than 6 weeks of age had Toxocara
in their intestine, Toxocara eggs were detected in only 20%
of their stools.
Other reasons for false negative stool results include the
use of poor or inappropriate diagnostic techniques, provision of insuf®cient faeces or inappropriate handling of faecal
material. Chomel [37] highlighted the need for sensitive
diagnostic tests to ®rstly detect and identify parasites and
then to establish an effective control program against them.
Although faecal ¯otation tests have been used widely in
parasitological studies, the accuracy of the technique varies
depending upon the procedure employed. Faecal ¯otation is
not reliable for detecting eggs of many of the tapeworms.
Furthermore, eggs of most of the tapeworms are not regularly released from proglottids into the faecal stream leading
to false-negative results. Although necropsy is considered
the `Gold standard' diagnostic procedure, it has the disadvantage of being time consuming and is obviously not
appropriate for domestic pets.
With the development of new diagnostic techniques, such
as the polymerase chain reaction (PCR) for Cryptosporidium [38,39] and the enzyme-linked immunosorbent assay
(ELISA) for detecting coproantigens of Echinococcus [40],
the sensitivity of tests used to detect parasites, in particular
protozoa, has increased signi®cantly.
Most studies of parasites of pets have been limited by the
fact that only single stool samples have been collected and
examined. With the intermittent shedding of many parasites
[41], the prevalence of infection with zoonotic parasites in
pets may, in fact, be underestimated and the real risk to
owners could be greater than that currently assumed.
3. Risk factors
Speci®c groups of the population have been identi®ed as
having a higher risk of acquiring parasitic zoonoses than
others. These groups include small children, pregnant
women and the immunocompromised [42]. These groups
are at a higher risk partly because of behavioural characteristics and partly because of immunological reasons.
Pet ownership is an important risk factor for the occurrence of many zoonoses. For example, Toxocariasis has
been reported to occur more frequently in households with
a pup (rather than an adult dog). This risk is further elevated
if the household contains a child who has a habit of geophagia [11,43,44].
1371
The presence of multiple dogs in a household was identi®ed by Bugg et al. [14] as increasing the chance of dogs
being infected with Giardia. They proposed that given the
apparent ease with which infection could be transmitted
between dogs, members of households with multiple dogs
had a greater chance of acquiring infection with Giardia
from their pets. A similar risk would be expected for people
who have close associations with pets such as veterinarians,
veterinary nurses and breeders. Paradoxically it would seem
that while the risk of acquiring infections with Toxocara or
Ancylostoma by owners has decreased because of the
routine use of anthelmintics, this has led to a concurrent
rise in the risk of infection with Giardia. Bugg et al. [14]
demonstrated a positive correlation between the number of
doses of anthelmintics and the prevalence of Giardia and a
negative correlation between parasitism with helminths and
the number of doses of anthelmintics administered.
As trade in animal products and the movement of human
populations continues to increase, so does the risk that
zoonotic diseases will be introduced or reintroduced into
certain areas [45]. Effective control of many parasitic
zoonoses depends upon a detailed knowledge of life cycle
patterns, particularly the role humans play in perpetuating
transmission cycles [46]. Juckett [42] proposed that simple
procedures to break the faecal-oral route, such as washing
hands, not emptying a litter box if pregnant, regular worming of pets and supervising toddler-pet interactions could all
reduce the likelihood of infection with parasitic zoonoses.
Furthermore, Overgaauw [47] believed that education,
along with an appropriate anthelmintic regime and consequent reduction in environmental contamination, would
lead to a reduction in the incidence of most parasitic
zoonoses. Bugg et al. [14] reported that most owners
(56%) collected and removed their dog's faeces from their
yard at least four to ®ve times per week. Such regular disposal of faeces would help reduce the risk of viable eggs of
Toxocara and other helminths being maintained in the
household environment.
Advances in chemotherapy, the emergence of human
immunode®ciency virus and advances in organ transplants
have led to an increase in the number of immunocompromised individuals and consequently zoonoses have become
a more important threat. Hill et al. [20] proposed that pets
residing in the homes of immunocompromised people
should be screened for enteric zoonotic organisms.
However, Wong et al. [7] believed that, if appropriate
care was taken, the risk of transmission of zoonoses to
immunocompromised people was minimal.
The United States Public Health Service considered that
Cryptosporidium, Giardia and Toxoplasma were of signi®cant risk to immunocompromised people. However, these
risks are not necessarily associated with pet ownership as
Luft and Remington [48] determined that the risk of cerebral
toxoplasmosis was not increased by cat ownership as most
cases occurred because of reactivation of previously latent
infections acquired from eating undercooked meats [49].
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Although pet ownership may present a health risk, it is
equally important to take into account adverse physical and
emotional effects that may result if immunocompromised
individuals are advised to give up loved pets [7]. In a
study of HIV infected persons by Angulo et al. [50], pet
ownership was associated with a decreased prevalence of
depressive symptoms and Chomel [51] believed that
provided immunocompromised patients were diligent in
washing their hands after handing animals or cleaning
cages, the risks of acquiring zoonotic infections were minimal. Angulo et al. [50] also believed that immunocompromised individuals need to be more vigilant of their pet's
health than do other pet owners.
Factors that in¯uence the likelihood of human infection
with zoonotic parasites vary depending upon the genus and
even species of the parasite involved. Many of these risk
factors are changing as the habits of humans and their pets
alter, leading to the emergence or reemergence of parasitic
disease or new pathways for transmission. Of particular
interest are Echinococcus, Ancylostoma, Giardia, Cryptosporidium, Toxoplasma, Neospora, Leishmania and Pneumocystis and these will be discussed in further detail.
3.1. Echinococcus
The distribution and perpetuation of the hydatid tapeworm, Echinococcus has resulted from human activity. In
Australia, the sheep strain of Echinococcus granulosus was
introduced during European colonisation with the subsequent establishment of a sheep-dog cycle that has been
perpetuated by poor husbandry practices. This subsequently
has resulted in establishment of wild animal cycles that
serve as reservoirs of infection for both cattle and sheep
[52,53]. However, an emerging public health problem has
developed through the establishment of urban foci of transmission. These have been detected in several regions of
Australia and are a consequence of interaction between
wild animals and domestic dogs as a result of increased
hunting and recreational activities on the outskirts of
urban areas [54±59]. The carcasses of animals shot in
areas of bush close to urban areas or killed by motor vehicles, are potential sources of infection for dogs. Management factors, such as not feeding raw offal or allowing dogs
access to carcasses of dead animals, can contribute to a
reduction in the incidence of dogs with E. granulosus [14].
In the USA and Europe [17,60] foxes represent a signi®cant source of Echinococcus multilocularis (e.g. up to 67%
of foxes in areas adjacent to Zurich, Switzerland have been
shown to be infected with this parasite [60]). Rodents can
act as intermediate hosts for E. multilocularis [11] and
Hofer et al. [60] detected infection in up to 20% of sampled
rodents. These rodents form a signi®cant source of infection
to domestic cats and dogs that acquire patent infections after
eating these animals. These pets then represent a signi®cant
risk to their owners. Restricting the hunting of animals and
the regular use of appropriate anthelmintics has been recom-
mended as an appropriate and easy method of controlling
infection with E. multilocularis [61].
3.2. Ancylostoma
Contact with soil or sand contaminated by the faeces of
dogs and cats infected with hookworms can lead to the
development of cutaneous larva migrans in humans. This
is particularly so in areas of higher humidity or in people
who have to crawl beneath buildings, or in sunbathers
reclining on wet sand contaminated by hookworm larvae
[17].
Recently, however, it has been demonstrated that enteric
infections with A. caninum can lead to eosinophilic enteritis
[62]. Of particular concern with this condition is the fact that
single worms have been incriminated in its development
[63]. Croese et al. [64] concluded that climate directly in¯uenced the rate of human enteric infections with A. caninum
and the frequency of eosinophilic enteritis has been shown
to be higher in tropical climates [63,65]. Because the worm
could easily be overlooked in pathological specimens,
Walker et al. [63] proposed that this condition was likely
to be underdiagnosed and may be more widely distributed
than is currently appreciated. As with many other parasitic
zoonoses the faecal-oral route is important in the development of eosinophilic enteritis in humans, and appropriate
hygiene procedures such as hand washing and removal
and disposal of faeces will reduce the probability of infection.
3.3. Giardia
Giardiasis is a common illness of humans [17,66±69].
Epidemiological evidence would suggest that humans are
likely to be the main reservoir of human giardiasis and it is
likely that direct person to person transmission is more
important than zoonotic transmission [67,70]. However,
dogs and cats can carry strains of Giardia which are potentially infective to humans [31] and therefore the zoonotic
potential must be considered, especially for immunocompromised people. Although Giardia is common in dogs and
cats, it is rarely associated with overt symptoms or clinical
disease. However, if clinical giardiasis is reported, it is
usually associated with kennel or cattery situations [71],
which are important sources of new pets for households.
Therefore treatment of Giardia-infected dogs and cats
may be advocated, whether or not they are clinically ill,
because of the potential for zoonotic transmission [71±
74]. The recent development of vaccines against Giardia
and their apparent ability to reduce the duration of shedding
of cysts [75,76], may provide an effective method for reducing carrier rates in pets and subsequent environmental
contamination. This could be of particular importance to
immunocompromised individuals.
I.D. Robertson et al. / International Journal for Parasitology 30 (2000) 1369±1377
3.4. Cryptosporidium
Cryptosporidium is a common cause of human diarrhoea
[77]. Cryptosporidium has also been detected in dogs and
cats [24,25,78] and these animals may represent an important reservoir of infection for humans. Molecular studies
have indicated that C. parvum is not a single uniform
species but is instead composed of at least six genetically
distinct but morphologically identical genotypes, only two
of which appear to be capable of infecting immunocompetent humans [38,79±82]. However, the signi®cance of
immunocompetence is important since it is possible that
individuals whose immune system is de®cient, as in
AIDS, may be susceptible to other genotypes of the parasite,
including the genotype commonly found in dogs [83,84].
The clinical signi®cance of cryptosporidial infection in
dogs and cats is unclear but appears to be most severe in the
very young where the effects are exacerbated by stress,
overcrowding and immune suppression [84]. Most infections of dogs with C. parvum are asymptomatic, however
concurrent infection with distemper virus can lead to clinical illness. Although only a few dogs have been found to be
shedding the oocysts, high rates of seropositivity have been
documented, suggesting a history of previous exposure [85].
The infective oocyst may be transmitted directly by the
faecal/oral route, or through contamination of water
supplies.
Although there are several drugs that can be used to treat
infections with Giardia and numerous preparations effective
against intestinal helminths, there currently are no reliable
forms of chemotherapy for cryptosporidial infections [86].
Thus with Cryptosporidium early diagnosis is important and
clinical intervention will be required to treat the effects of
infection. Fortunately in persons with a normal immune
system, symptoms of cryptosporidiosis usually last only a
few days or weeks, however in the immunocompromised
infection can lead to serious, life threatening illness.
3.5. Toxoplasma
Although infection with T. gondii is common in humans,
most infections occur without the presence of clinical signs.
The cat is the only de®nitive host for this parasite and after
oocysts are passed in the faeces they must mature for
between 2 and 5 days before they are infective for humans
and other animals [50]. Furthermore oocysts are usually
only passed for a few weeks during a cats life and ,1%
of cats examined in the USA have been shown to be shedding oocysts [41]. The risk of acquiring toxoplasmosis from
freshly passed cat faeces while cleaning a litter box would
therefore appear to be extremely low. However, daily cleaning of cat litter trays, to ensure that if Toxoplasma oocysts
are shed they do not have time to sporulate and become
infective, and wearing gloves while cleaning the trays
have been recommended, particularly in high risk groups
[87,88]. Many other animals can also be infected with T.
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gondii and it is likely that most humans are infected by
eating raw or poorly cooked meat rather than from a cat
[50].
3.6. Neospora
At present there is no evidence to suggest that Neospora
caninum is a cause of disease in humans. However, some
non-human primates are susceptible to infection [89] and
the parasite can be a signi®cant cause of abortions in cattle
[90,91]. Based on the results of a serological survey, Tranas
et al. [92] concluded that humans could be exposed to N.
caninum, however the possible route of exposure or
outcome of infection were not known. Given the similarities
between T. gondii and N. caninum, there is the possibility of
human infection with N. caninum after ingesting raw or
under cooked meat containing tissue cysts or by exposure
to sporulated oocysts present in the faeces of dogs. As with
many other zoonoses, it is probable that immunocompromised individuals are at a greater risk of infection. However,
common sense would suggest that every-day hygienic practices should afford protection from what is currently
perceived to be a very low risk of disease.
3.7. Leishmania
The distribution of zoonotic forms of Leishmania is
expanding into areas of Europe not previously regarded as
endemic [45,93]. Leishmaniasis caused by L. infantum is
found throughout the Mediterranean region where dogs
are considered to be the main reservoir host [94±96].
Recently, new foci of transmission have been reported,
and studies from several areas have found the levels of
canine infections to be higher than expected [97±98].
More signi®cant, however, is the high level (up to 75%)
of serologically positive dogs that have subclinical, inapparent infections with L. infantum [98]. This highlights the
dangerous reservoir that dogs represent with regard to the
spread of visceral, and possibly cutaneous leishmaniasis
[95,99]. Furthermore, pet dogs which accompany their
owners on short visits to endemic foci of transmission
may act as reservoirs and serve to introduce the infection
to other geographical areas. Orndorff et al. [98] emphasised
the risk to public health of bringing naõÈve dogs into such
endemic areas.
3.8. Pneumocystis
Pneumocystis carinii is a common cause of life-threatening pneumonia in immunocompromised patients [100±102].
Several authors [100,103] have proposed that, although
some infections with Pneumocystis involve reactivation of
previously acquired infections (particularly from childhood), the environment is an important reservoir of infection and nosocomial transmission is possible. P. carinii has
been considered a zoonotic parasite with morphologically
similar forms occurring in a range of mammalian species
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I.D. Robertson et al. / International Journal for Parasitology 30 (2000) 1369±1377
including humans. However, recent molecular studies
suggest that the taxon `P. carinii' in fact consists of a
number of genetically distinct groups associated with single
host species [104,105]. Although infections with Pneumocystis are important in both immunocompromised patients
and pets [106], it would appear that pets play an insignificant role in the transmission and distribution of this organism.
4. Education
Since infections of pets and humans are often the result of
human activity, and a major aim in controlling zoonotic
infections is to break the cycle of transmission, education
must play a key role in their control. The role of education in
preventing infection with parasitic zoonoses has been highlighted by numerous authors [11,14,47,84,107,108] and
veterinarians have been identi®ed as a potential provider
of this education. The value of education in reducing the
incidence of hydatids by changing management practices
(such as not feeding the offal of home-killed sheep to
dogs) has been well-documented [42,109]. Kornblatt and
Schantz [110] and Harvey et al. [107] found that although
veterinarians were an effective source of information many
were providing incorrect, minimal or out of date information. Although Thompson and Morgan [84] believed that
veterinarians and other professionals including medical
practitioners could play a vital role in the education of the
public, they considered that these `educators' must be aware
of the changing trends in the transmission of zoonotic parasites. For example, they considered it was important to be
aware that: foodborne transmission of Toxoplasma was a
more important source of infection to humans than were
cats in many endemic areas [93]; canine scabies was probably not a zoonotic condition [111]; and dogs and cats could
harbour strains of Giardia which are potentially infective to
humans [31].
Bugg et al. [14] reported that in Perth, Western Australia,
many dog owners (62.5%) were aware that canine parasites
could be transmitted to humans. However, the awareness of
the zoonotic potential of some parasites was variable; eg
only 1.5% of pet owners were aware of the zoonotic potential of Cryptosporidium in contrast to 70% for Toxocara.
Although the risks of some parasites were appreciated, only
one third (34%) of interviewed pet owners could provide
correct information on the mode of transmission of these
parasites. Correct information was obtained from veterinarians, the health department or from schools, highlighting the
value of these information sources. In contrast, incorrect
information was being commonly acquired from the mass
media. Robertson [112] observed that although most
Australian pet owners (and non-pet owners) were aware of
the exotic disease rabies and how this disease was transmitted, few were aware of endemic parasitic zoonoses
such as toxocariasis or toxoplasmosis. Schantz et al.
[43,44] also reported that, in the USA, most pet owners
did not know that their pets might carry illnesses transmissible to people.
Strategically timed preventive anthelmintic treatments
for dogs and cats have been recommended as an important
tool in the control of parasites [16]. In the study of Bugg et
al. [14], although a high percentage (82.4%) of owners had
wormed their dogs in the twelve-month period preceding the
survey, many were giving fewer doses of anthelmintics than
recommended. De-worming is most effective for averting
morbidity and preventing environmental contamination
when aimed at pups and kittens and their dams because
they have the highest worm burdens, are most vulnerable
to the effects of these infections, and are the main sources of
infective stages [16].
Most households acquiring new animals obtain a young
pet rather than an older animal [1]. Frequently puppies or
kittens are acquired by a household when that household
also has a young family. Therefore the risk of infection is
heightened by having animals with a potentially higher
prevalence of parasitism in a household with members
who have a greater chance of becoming infected through
close contact with the pet. These owners need to be made
aware of the potential dangers from their pets and the simple
precautions that can be adopted to reduce these dangers. In
the study of Bugg et al. [14], only 23% of owners/managers
of pet shops advised customers of the potential zoonotic
risks from puppies and no owners/managers were aware
that Giardia was potentially zoonotic. Little advice was
given on the management or husbandry procedures that
could be adopted to minimise the risk of infection of zoonotic parasites, to both puppies and humans. In fact four petshops advised customers to worm themselves and/or their
children as preventive measures for parasitic infection of
their dogs. Stehr-Green et al. [32] observed that pups that
received regular anthelmintic treatments in pet shops were
infected less commonly with intestinal helminths than those
receiving none or only a single treatment. This again highlights the signi®cant bene®t of routine prophylactic anthelmintic treatment in minimising the risk to owners.
Although animals in shelters do not constitute a signi®cant risk to the general public, some of these animals are
subsequently rehomed and then represent a real risk to their
new owners. It is therefore essential that these animals
receive treatment to eliminate existing infections prior to
rehoming. Without this, environmental contamination with
infective stages will continue, facilitating infection of
susceptible animals, reinfection of existing pets and potential infection of humans.
Education has an important place in reducing the prevalence and incidence of infections with potentially zoonotic
parasites in pets and their owners. Practicing veterinarians,
knowledgeable of the potential dangers and how to minimise them, are ideally placed to provide pet owners with
sound advice about prevention and to recommend appropriately-timed preventive medications to reduce the zoonotic
I.D. Robertson et al. / International Journal for Parasitology 30 (2000) 1369±1377
risks [11,107,108]. Increased awareness that some diseases
may be associated with animals and more effective prevention and treatment programmes will help to reduce the
prevalence of zoonotic infections so that pets can continue
to be integral members of households throughout the world
[107,113].
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