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Giardia is an intestinal infection caused by a parasitic protozoan (single celled organism) called Giardia lamblia. These protozoans are found in the intestines of many animals, including dogs and humans. This microscopic parasite clings to the surface of the intestine, or floats free in the mucous lining the intestine. Veterinary research documents suggest that 5% to 10% of all dogs in North America have giardiasis at any given time.


Giardia occurs in two forms: a motile feeding stage that lives in the intestine, and a non-motile cyst stage that passes in the feces. The giardia trophozoite, which is the active stage of the organism, inhabits the small intestine of the dog. The trophozoite stage is tear-drop shaped, binucleated, and has four pairs of flagella. 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, encystment occurs as the parasite travels to the large intestine. The cyst is fairly resistant and can survive for several months outside of a host’s body as long as sufficient moisture is provided. The cyst is oblong in shape with four nuclei that are sometimes distinctly visible. Mature cysts are usually found in the feces of infected animals. Other animals become infected by ingesting the cysts that passed from the body in feces. These ingested cysts then break open inside the small intestine to release the motile feeding stage (trophozoite). Giardia increases their numbers by each organism dividing in half which is called binary fission.

Giardia infections are transmitted via ingestion of trophozoites or cysts in contaminated water or food. If a giardia cyst is ingested, the cyst wall is broken down during the digestive process and the trophozoite stage begins to colonize the upper small intestine. Transmission also occurs by direct contact, especially with asymptomatic carriers. The ingestion of as few as one or more giardia cysts may cause the disease, as contrasted to most bacterial illnesses where hundreds to thousands of organisms must be consumed to produce illness.


Giardia causes its unpleasant effects on the body not by invading the tissues, but simply by being in the way. It multiplies to the point where it sort of paves the lining of the intestine and blocks normal digestion and causes malabsorption. This causes only partially digested food to get lower in the digestive tract than it should, causing diarrhea. Clinical signs range from none in asymptomatic carriers, to mild recurring diarrhea consisting of soft, light-colored stools, to acute explosive diarrhea in severe cases. Other signs associated with giardiasis are weight loss, listlessness, fatigue, mucus in the stool, and anorexia. 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 even small amounts of blood in the feces. Without treatment, the condition may continue, either chronically or intermittently, for weeks or months.

Diagnosing giardia is not easy. A fecal sample is not straightforward. Even when a flare is at it’s worst, the cysts will not be shedding in every single stool. Therefore, a negative report does not rule out giardia. The most thorough way to assess is to collect a sample from every single stool produced for 48 to 72 hours and have a vet examine it using the giardia test kit.

3D Giardia

Infection may be treated using one of a number of different drugs. The treatment of choice is often with Metronidazole. Metronidazole has two interesting properties–the action is largely confined to the gut and it also seems to stimulate the local immune system. Metronidazole kills off the giardia and reduces the numbers to the level the dog’s immune system can handle. Fenbendazole is an antiparasitic drug that kills some intestinal worms and can help control giardia. It may be used alone or with metronidazole. It is possible these treatments only remove the cysts from the feces but do not kill all the Giardia in the intestine. This means even though the fecal exams after treatment may be negative, the organism is still present in the intestine. This is especially true of the older treatments. Therefore, treated animals could still be a source of infection for others.

Remember that the cysts can live several weeks to months outside the host in wet, cold environments. As with other parasites of the digestive system, prevention of the spread of Giardia centers on testing and treating infected animals and using sanitary measures to reduce or kill the organisms in the environment. Solutions of bleach or quaternary ammonium compounds are effective against Giardia.

Remember, Giardia of dogs may infect people, so good, personal hygiene should be used by adults when cleaning kennels or picking up the yard, and by children who may play with pets or in potentially contaminated areas.


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