GUARDING AGAINST GIARDIA
Maggie Fisher BVetMed, MRCVS
(Member Royal College Veterinary Surgeons)
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, (UK)
was hit by an unusually severe outbreak of diarrhoea. The cause turned out to be an
infection of the intestine by a commonly-occuring, 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 (Figure 1/pic not avail here) - 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 faeces. 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
faeces. The cyst (Figure 2/photo not avail here) 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 defence 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 harbour infection and pass cysts into the environment) or dogs that have
diarrhoea 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 faeco-oral. For example, dogs may accidentaly eat cysts as they lick around
the environment or lick other dogs' coats (particularly if the other dog has diarrhoea).
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 infection as it eats cysts passed in its own faeces.
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 faeces from affected animals are typically light coloured,
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 faeces, even though the Giardia do not colonise 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 anaemia. 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 diarrhoea (eg: Salmonella or Campylobacter), Giardia cysts may be observed
directly in faecal samples or indirectly using an elisa technique. Direct examination of
faeces, using zinc sulphate centrifugal flotation. followed by staining the supernatant
with Lugol's iodine, has been found to be upto 70% effective at detecting infection from a
single faecal sample. The cyst output is very variable from day to day so the detection
rate may be improved by pooling faecal samples collected over three days. Faecal
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 colour change or production of flourescence. Studies examining the reliability
of some immunoflourescent kits have found them to be over 90% accurate, with relatively
few false negatives or false postives. However, the tests are costly and probably only
wothwhile 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 traeted using one of a number of drugs. Unfortunately there is no
treatment licenced for the control of giardias in dogs, though fenbendazole (Panacur,
Hoechst Animal Health) is licenced for treatment of worms in dogs. Treatments from the
literature are shown in table 1. Whatever treatment is chosen, itis 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 faecal samples may be collected one week after
the treatment and dogs which are still passing cysts can be identified and treated. It
should be recognised that, when treating a large number of dogs, whichever of these
treatment strategies is adopted, there may be one or two dogs that remain as carriers of
infection that will act as a potential source of infection in future.
Treatments for Giardias in dogs
Drug Name - Trade Name - Dose Rate - Duration of Treatment
Metronidazole Flagyl 25-30 mg/kg bid** 7 days
Furazolidone Neftin 4 mg/kg bid* 10 days
Tinadazole - 44 mg/kg once daily 7 days
Fenbendazole Panacur *** 50 mg/kg once daily 5 days
Albendazole Valbazen 25 mg/kg bid 2 days bid Twice daily
* Maximum daily dose 200 mg
** Contra-indicated in pregnancy
** Licenced for the treatment of worm infections in dogs
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 complet 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, therfore 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 faecal sample checking has been completed. It should not be overlooked that
some of those 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 case of Giardia might be exacerbated by the
street 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.
More Information http://www.priory.com/vet/giardia.htm
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