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 Centre in Leamington Spa 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) - 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) is more able to survive in the environment than the trophozoite, which is very fragile.

Figure 1 - Figure 2
Diagram of Giardia Tophozoite Diagram of Giardia Cyst

How do Giardia cause disease in dogs?

Like all infectious agents, in order to cause disease Giardia depaends 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 cycsts can cause disease in humans. Different anaimals may respond to infection in different ways, which may be due to different strains of the sam 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 theenvironment 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 infectionas 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 abou tone 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 alarge 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 sources of infection in future.

Treatments for Giardias in dogs

Drug NameTrade NameDose RateDuration of
Metronidazole Flagyl25-30 mg/kg bid**7 days
Furazolidone Neftin4 mg/kg bid*10 days
Tinadazole -44 mg/kg once daily7 days
Fenbendazole Panacur ***50 mg/kg once daily3 days
Albendazole Valbazen25 mg/kg bid2 days

bidTwice 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 thes 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 upto 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 atleast 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 samle checking has been completed. It should not be overlooked that some of thoe 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 stree 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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