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October 30, 2008, By Jeff Grognet, D.V.M., B.S.c.(Agr.), ARTICLE, HEALTH

Once considered to be a rare infestation in dogs, advancements in testing tell us this intestinal parasite found in water is more prevalent than previously estimated. In fact, between five and 10 per cent of dogs carry the organism. In puppies, infestations can run as high as 50 per cent, while in kennel settings, an entire population can have giardia.

Giardia infects many mammals, even birds. Older reference books described a giardia species for every animal. For example, Giardia bovis infested cattle while Giardia canis infested dogs. Now researchers believe there are fewer species of giardia and each is capable of infesting more than one type of animal.

Once a host consumes a giardia cyst, it invades the intestine, hatches into a motile form, feeds, then multiplies. As the cyst progresses through its life cycle, it generates inflammation in the intestine. The motile form eventually passes into the large intestine where it changes into a non-motile cyst. This is shed in the host feces, serving as the source of infestation for the next animal.

Giardia cysts are hardy. They can remain viable for months, particularly in moist and cool environments. They thrive in clear, clean water. Stagnant, contaminated water harbours bacteria that compete with the cysts, decreasing their survival time.

The giardia organisms settle on the intestinal villi – microscopic projections in the lining of the gut. As they damage cells, they shorten the villi, reducing the surface area of the intestine. This alters digestion of food as well as nutrient and water absorption. The result is diarrhea – a mushy, cow-pie stool that is often malodourous.

Risk and diagnosis

The effect giardia has on the host depends on the dog’s age as well as its immune status. Puppies with poor immunity suffer from more-severe clinical symptoms. When roundworm or coccidial infestations combine with giardia, the poor pup gets severe fluid diarrhea that results in dehydration. In mature dogs, giardia has a minimal effect, often causing just soft stool. However, older dogs with failing immunity or those on cancer chemotherapy are also at risk.

Giardia should be suspected when a dog suffers from recurring diarrhea consisting of soft, light-coloured stools. It can also be a reason for acute explosive diarrhea. Weight loss as well as listlessness, anorexia and mucus in the stool may also occur with a
giardia infestation.

Clinical signs generated by giardia can mimic other diseases, so it’s important to make a specific diagnosis. Unfortunately, detecting giardia is a challenge. Veterinarians have traditionally relied on analyses of feces – a small stool sample is mixed with a concentrated salt solution and the cysts float to the top. The cysts are then viewed with a microscope.

Regrettably, fecal tests are not very accurate because grass pollens and yeasts can be mistaken for giardia cysts, creating a false positive. As well, because dogs shed cysts only intermittently, its easy to miss an infestation – a false negative. To get around this problem, veterinarians recommend doing at least three fecal examinations before declaring a dog free of giardia.

A newer solution to this dilemma is a test that detects giardia protein in a fecal sample. This is a good screening test because it can detect minute quantities of giardia, unveiling infestations that would normally be missed.


A confirmed case of giardia is treated with medications such as metronidazole and albendazole. Metronidazole is the most economical, but the least effective. Only two-thirds of patients clear the parasite. Albendazole is 90-per-cent effective following a two-day course of medication.

Even though treatment may resolve a dog’s clinical symptoms, he can still be a carrier, shedding cysts with his feces. Follow-up fecal examinations are needed to confirm whether the organism has been cleared from the intestine.

A giardia vaccine, containing chemically inactivated motile stages of the organism, is available for dogs. Studies show that vaccines mean less chance of diarrhea occurring, and it if does, it lasts for a shorter time. Vaccination also helps reduce the amount of
cyst shedding.

The vaccine has also been touted for dogs with existing giardia infections. Theoretically, it boosts the dog’s immune response so the organism can be fought off more effectively. Studies on the benefit of this vaccine under these circumstances offer mixed results.

We are probably missing many cases of giardia in dogs because, especially in adult animals, it causes only mild symptoms. It’s also challenging to diagnose. Thankfully, new testing can help us find cases that have previously run under the radar, but we have to keep it in mind and test for it.

A multi-published writer, Jeff Grognet, D.V.M., B.S.c.(Agr.), runs a veterinary practice in Qualicum Beach, B.C., along with his wife, Louise Janes, D.V.M.Originally published in the August 2007 issue of Dogs in Canada.