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Giardia (Giardia canis, Giardia cati)
by Holly Frisby, DVM,
Drs. Foster & Smith Veterinary Services Department

Giardia are protozoa (one-celled organisms) that live in the small intestine of dogs and cats. Giardia are found throughout the United States and in many other parts of the world. Infection with Giardia is called giardiasis.

There are many things we don't know about this parasite. Experts don't agree on how many species of Giardia there are and which ones affect which animals. Veterinarians do not even agree on how common Giardia infections are and when they should be treated. Generally it is believed that infection with Giardia is common but disease is rare. There is much about the life cycle we don't know either.

How do Giardia reproduce and how are they transmitted?

A dog or cat becomes infected by eating the cyst form of the parasite. In the small intestine, the cyst opens and releases an active form called a trophozoite. These have flagella, hairlike structures that whip back and forth allowing them to move around. They attach to the intestinal wall and reproduce by dividing in two. After an unknown number of divisions, at some stage, in an unknown location, this form develops a wall around itself (encysts) and is passed in the feces. The Giardia in the feces can contaminate the environment and water and infect other animals and people.

What are the signs of a Giardia infection?

Most infections with Giardia are asymptomatic. When disease rarely occurs, it is most common in younger animals, and the usual sign is diarrhea. The diarrhea may be acute, intermittent or chronic. Usually the infected animals will not lose their appetite, but they may lose weight. The feces are often abnormal, being pale, having a bad odor, and appearing greasy. In the intestine, Giardia prevents proper absorption of nutrients, damages the delicate intestinal lining, and interferes with digestion.

Can Giardia of dogs and cats infect people?

This is another unknown. There are many species of Giardia, and experts do not know if these species infect only specific hosts. Sources of some human infections have possibly been linked to beavers, other wild animals and domestic animals. Until we know otherwise, it would be wise to consider infected animals capable of transmitting Giardia to humans.

You may have heard about Giardia outbreaks occurring in humans due to drinking contaminated water. Contamination of urban water supplies with Giardia is usually attributed to (human) sewage effluents. In rural settings, beavers most often get the blame for contaminating lakes and streams. Giardia outbreaks have also occurred in day care centers fueled by the less than optimal hygienic practices of children.

How do we diagnose giardiasis?

Giardiasis is very difficult to diagnose because the protozoa are so small and are not passed with every stool. Tests on serial stool samples (one stool sample every day for three days) are often required to find the organism. Special diagnostic procedures, beyond a routine fecal examination, are necessary to identify Giardia. The procedures we use to identify roundworms and hookworms kill the active form of Giardia and concentrate the cyst form.

To see the active form, a small amount of stool may be mixed with water on a microscope slide and examined under high magnification. Because these forms have flagella, you can see them move around on the slide. The active forms are more commonly found in loose stools. If you ever have the opportunity to see the active form of Giardia under the microscope, take it! It’s an interesting looking creature. It is pear-shaped and its anatomy makes it look like a cartoon face, with eyes (which often look crossed), nose and mouth. Once you see it, you won't forget it.

Cysts are more commonly found in firm stools. Special solutions are used to separate the cysts from the rest of the stool. The portion of the solution that would contain the cysts is then examined microscopically.

Tests which detect antigens of Giardia in the feces are becoming more available. These tests are more difficult to run, are more expensive, and their accuracy may not be better than other methods. In the future, these tests will hopefully be fine-tuned so we can more accurately diagnose infection with Giardia.

We've done the tests, now what?

Now we come to how to interpret the test results. It can be a dilemma for your veterinarian. What you see (or We've see) is not always what you have. A negative test may mean the animal is not infected. It may also mean there were too few Giardia present in the small portion of stool that was examined. Negative test results are common in infected animals. If a negative test occurs, your veterinarian will often suggest repeating the fecal examination at least two more times on different samples taken on different days. Repeat tests are often necessary to finally find the organism.

What about a positive test? That shouldn't be hard to interpret, right? Wrong. Giardia can be found in many dogs and cats with and without diarrhea. If we find Giardia, is it the cause of the diarrhea or is it just coincidence we found it? The animal could actually have diarrhea caused by a bacterial infection, and we just happened to find the Giardia. Test results always need to be interpreted in light of the signs, symptoms and medical history.

If we find Giardia, how do we treat it?

Here we go again; treatment is controversial too. There’s a question about when to treat. If Giardia is found in a dog or cat without symptoms should we treat the animal? Since we shouldn't know if G. canis and G. cati can infect man, we often err on the side of caution and treat an asymptomatic infected animal to prevent possible transmission to people.

If we highly suspect infection with Giardia, but can't find the organism, should we treat anyway? This is often done. Because it is often difficult to detect Giardia in the feces of dogs and cats with diarrhea, if there are no other obvious causes of diarrhea (e.g., the dog didn't get into the garbage several nights ago) we often treat the animal for giardiasis.

There are several treatments for giardiasis; some of them have not been registered to treat giardiasis in dogs or cats. Metronidazole is one of these, but is the old standby. The nice thing about this drug is that it also kills some types of bacteria that could cause diarrhea. So if the diarrhea was caused by bacteria, and not Giardia, we still kill the cause of the diarrhea and eliminate the symptoms. Makes us look pretty sharp! Unfortunately, metronidazole has some drawbacks. It has been found to be only 60-70% effective in eliminating Giardia from infected dogs. In some cats and dogs it can cause vomiting, anorexia and some neurological signs. It also can be toxic to the liver in some animals. It is suspected of being a teratogen so it should not be used in pregnant animals. Finally, it has a very bitter taste and many animals resent taking it – especially cats.

Quinacrine hydrochloride has been used in the past, but is not very effective and can cause side effects such as lethargy, vomiting, anorexia and fever.

Furazolidine has been used effectively in treating giardiasis in cats. It can cause vomiting and diarrhea and should not be used in pregnant cats.

A newer drug, albendazole, has been shown to be 50 times more effective than metronidazole and 10-40 times more effective than quinacrine hydrochloride in killing Giardia in the laboratory. It has not been registered for use in dogs and cats. Some serious side effects of albendazole have been noted, including injury to the bone marrow. Since it may also cause birth defects, it should not be used in pregnant animals.

In a recent small study, fenbendazole, which has been approved for treatment of roundworm, hookworm and whipworm infections in dogs, has been shown to be effective in treating giardiasis in dogs. It is safe to use in puppies but has not been approved for use in cats.

Most recently a combination of praziquantel, pyrantel pamoate and febantel has been shown to decrease cyst excretion in infected dogs.

This table summarizes the information above.

Drug Species Dose Use in pregnant animals, puppies or kittens?


Dog, cat Dogs: 11.5 mg/lb twice daily for 5 days

Cats:5.5-11.5 mg/lb twice daily for 5 days


Quinacrine hydrochloride


Dog, cat 3 mg/lb twice daily for 5 days




Dog, cat 11.5 mg/lb twice daily for 2 days




Dog, not evaluated in cats Dogs: 22.5mg/lb once daily for 3 days

Safe in puppies 6 weeks or older



Cat 2mg/lb twice daily for 5-10 days


Praziquantel/ pyrantel pamoate/ febantel


Dog Use manufacturer’s suggested dosage


(puppies must be 3 weeks old and weigh more than 2 pounds)

But now we come to yet another unknown. 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. So treated animals could still be a source of infection for others.

How can I prevent my pet from becoming infected with Giardia?

The cysts can live several weeks to months outside the host in wet, cold environments. So lawns, parks, kennels and other areas that may be contaminated with animal feces can be a source of infection for your pet. You should keep your pet away from areas contaminated by the feces of other animals. This is not always easy.

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 Lysol, bleach, and quaternary ammonium compounds are effective against Giardia.

How do I control Giardia in my kennel or cattery?

Infection with Giardia can be a big problem in kennels and catteries. Veterinarians at Cornell University have developed a specific protocol. They recommended a four-pronged approach.

Treat Animals: Treat all nonpregnant animals with fenbendazole or albendazole for 5 days. On the last day of treatment move them to a holding facility while a clean area is established. When the animals are moved back to the clean area, treat them once again with a 5-day course of fenbendazole or albendazole.

Decontaminate the Environment: Establish a clean area. If possible, this can be the whole facility. Otherwise create a few clean runs or cages, separate from the others. Remove all fecal material from the areas since the organic matter in feces can greatly decrease the effectiveness of many disinfectants. Steam clean the area and then clean it with a quaternary ammonium disinfectant according to the manufacturer's directions. These solutions will generally kill the cysts within one minute. Then let the area dry for several days before reintroducing the animals.

Clean the Animals: Cysts can remain stuck to the haircoats of infected animals. So before moving the treated animals to the clean area, they should be shampooed and rinsed well. Especially concentrate on the perianal area. The Cornell researchers then recommend washing the animals with a quaternary ammonium compound, using the manufacturer’s recommended dilution. Be sure all shampoo has been rinsed from the animal, as it will neutralize the effect of the quaternary ammonium compounds. Leave the compound on the animal for 3 minutes, then completely rinse the animals. These compounds can be irritating. Do not leave them on for more than 3 minutes. Do not get these compounds on mucous membranes or in the eye. Always use an ophthalmic ointment to protect the eyes.

Prevent Reintroduction of Giardia : Giardia can be brought into the kennel or cattery either by introducing an infected animal or on your shoes or boots. Any new animal should be quarantined from the rest of the animals and be treated and cleaned as described above. You should either use disposable shoe covers or clean shoes/boots and use a footbath containing quaternary ammonium compounds to prevent people from reintroducing Giardia.

Remember, Giardia of dogs and cats 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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Reprinted with permission from  courtesy of Dr. Race Foster and Dr. Marty Smith, 2000 Drs. Foster and Smith, Inc. Free pet supply catalog: 1-800-323-4208

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