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Giardia in
Dogs |
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GUARDING AGAINST
GIARDIA
By Maggie Fisher BVetMed MRCVS
Early in 1995, the
kennel population of more than 100 dogs at the Guide Dogs for the Blind
Association's Midlands Regional Center in Leamington Spa was hit by an
unusually severe outbreak of diarrhea. The cause turned out to be an infection
of the intestine by a commonly-occurring, single celled organism - or protozan
known as Giardia. A combined treatment and disinfection strategy was then
introduced that brought the infection under control. Maggie Fisher, a
veterinary surgeon with a special interest in parasitology, was called in to
help deal with the Giardia outbreak, and in the following paper she describes
the infection and how it can be treated and controlled.
The division of
Giardia into groups according to species is still somewhat confused; the
organisms that infect mammals look very similar but it remains unclear to what
extent they form one or a number of species. It is for this reason that, while
Giardia infection in some mammals, including dogs, is suspected of being
infectious to man (ie: a zoonosis), it has not been conclusively shown that the
species in, for example, dogs and man is the same.
The Giardia
trophozoite - which is the active stage of the organism - inhabits the small
intestine of the dog. It attaches to the cells of the intestine with its
adhesive disc and rapidly divides to produce a whole population of
trophozoites. As they detach they may be swept down the intestine. If
intestinal flow is fast then they may appear in the feces. However, if they
have time, they will develop into the inactive, more durable, cyst form of the
organism and these will be passed in the feces. The cyst is more able to
survive in the environment than the trophozoite, which is very fragile.
How do Giardia cause disease in dogs?
Like all infectious
agents, in order to cause disease Giardia depends on being able to overcome the
dog's defense against infection, either by its virulence or by the number of
the organisms becoming established. It has been observed that as few as 10
cysts can cause disease in humans. Different animals may respond to infection
in different ways, which may be due to different strains of the same Giardia
population, with varying levels of pathogenicity. Another explanation for
observed differences in the host response to infection is that protective
immunity with age and/or exposure. This may be temporarily lost if the animal
is stressed or immunosuppressed, for example with corticosteroid
treatment.
What is the source of infection for dogs?
The original
source of an outbreak may be cysts in contaminated water or the environment. In
addition, infected dogs which may be either carriers (ie: show no clinical
signs but continue to harbor infection and pass cysts into the environment) or
dogs that have diarrhea associated with infection may act as the source.
Surveys have shown that about 14% of the adult dog population and over 30% of
dogs under one year of age were infected. Once passed, the cysts can survive in
cold water for several months.
The cysts are infective as soon as they
are passed, unlike other parasites where a lag period is necessary before the
organism is infective. The most common route of infection is feco-oral. For
example, dogs may accidentally eat cysts as they lick around the environment or
lick other dogs' coats (particularly if the other dog has diarrhea). Another
major source of infection in human cases is drinking contaminated water. Once
eaten, the cyst breaks open in the animals' intestine and releases two new
trophozoites to initiate infection. If a dog is left in a dirty environment it
may act as its own source of further infections it eats cysts passed in its own
feces.
What are the clinical signs associated with infection?
The
trophozoites divide to produce a large population, then they begin to interfere
with the absorption of food, so feces from affected animals are typically light
colored, greasy and soft. These signs, together with the beginning of cyst
shedding, begin about one week post-infection. There may be additional signs of
large intestinal irritation, such as straining and mucus in the feces, even
though the Giardia do not colonize the large intestine. Usually the blood
picture of affected animals is normal, though occasionally there is a slight
increase in the number of eosinophils (one of several types of white blood
cells) and mild anemia. Without treatment, the condition may continue, either
chronically or intermittently, for weeks or months.
How can infection be
diagnosed?
Diagnosis is based on demonstration of the infection and the
elimination of other possible causes of diarrhea (eg: Salmonella or
Campylobacter), Giardia cysts may be observed directly in fecal samples or
indirectly using an elisa technique. Direct examination of feces, using zinc
sulphate centrifugal flotation. followed by staining the supernatant with
Lugol's iodine, has been found to be up to 70% effective at detecting infection
from a single fecal sample. The cyst output is very variable from day to day so
the detection rate may be improved by pooling fecal samples collected over
three days. Fecal examination is the cheapest method but is time consuming and
requires an experienced technician for reliable results.
The elisa
technique requires a kit and some method of reading a color change or
production of florescence. Studies examining the reliability of some
immunoflorescent kits have found them to be over 90% accurate, with relatively
few false negatives or false positives. However, the tests are costly and
probably only worthwhile where there are a large number of samples to be
processed and a technician who is familiar with carrying out elisas.
How can infection be treated?
Infection may be treated using one
of a number of drugs. Unfortunately there is no treatment licensed for the
control of giardias in dogs, though fenbendazole (Panacur, Hoechst Animal
Health) is licensed for treatment of worms in dogs. Whatever treatment is
chosen, it is very unlikely to eliminate 100% of the infection in all dogs.
Adaptations that may be made to try to improve the success rate of a treatment
regime include extending the duration and dose of the treatment.
Care
must obviously be taken with this approach to make sure that an adequate safety
margin is always maintained. Another approach is to retreat after an interval
of one week. Alternatively, repeat fecal samples may be collected one week
after the treatment and dogs which are still passing cysts can be identified
and treated. It should be recognized that, when treating a large number of
dogs, whichever of these treatment strategies is adopted, there maybe one or
two dogs that remain as carriers of infection that will act as a potential
sources of infection in future.
How can infection, once present , be
controlled?
Once infection is present in a kennels, control may be
approached in two ways:-
1. identification, isolation and treatment of
infected dogs.
2. mass treatment of all dogs.
Option 1 is only
practical where a few dogs in a discrete area have been identified as being
infected and where complete isolation is feasible, either within their own
block or in a specific isolation block. Such isolation includes segregation of
exercise areas and these animals should be fed and cleaned after all others on
the premises, preferably using separate cleaning and feeding equipment and
separate staff if possible. Treatment of all dogs should commence on the same
day when option 2 is adopted.
Thorough cleaning of all kennel area
where infected dogs have access is essential. Once organic debris has been
removed, thorough disinfection will help to further reduce the level of
environmental contamination and reduce the risk of dogs becoming re-infected
after the completion of treatment. Disinfectants containing quaternary ammonium
compounds have been found to kill Giardia cysts at the manufacturers'
recommended dilutions (dilutions of one disinfectant up to 1:704 were found to
be effective at both low and high environmental temperatures). Efficacy of
killing is increased by prolonged contact time, therefore disinfectant solution
should be left for 20 minutes to half an hour before being rinsed off kennel or
run surfaces. Since disinfection of grass runs is impossible, such area should
be regarded as contaminated for at least a month after infected dogs last had
access.
Introduction of new dogs into the infected area should be
avoided until the period of treatment and fecal sample checking has been
completed. It should not be overlooked that some of the infected dogs may
continue to excrete low numbers of cysts even after all treatments and
examinations have been completed. It is therefore important that rigorous
disinfection is maintained and a careful check is kept on the condition of all
treated and introduced animals.
How can infection be
prevented?
It is very difficult to prevent entry of an infection that is
known to be carried by a percentage of normal dogs into a kennels. However, an
initial period of isolation for all new entrants into kennels, for perhaps ten
days, would reduce the risk of an infected dog spreading a large number of
cysts around the main kennel area. All dogs could be observed and any infection
present, which in the case of Giardia might be exacerbated by the stream of
entry in kennels, could be identified and treated before entry into the main
kennels.
Dogs should be prevented from access to foul water that may
contain large numbers of cysts (eg: river-flooded paddocks).Small numbers of
cysts may occasionally be present in the potable water supply but the risk of
this being a major source of infection is small. |
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