Giardia is a protozoan parasite found all over the world.  It can affect humans, most types of domesticated mammals, and birds. The parasite lives in the intestinal tract and causes damage to the intestines.  Giardia can affect either sex and any breed, although it is most commonly seen in young animals and animals confined together in groups, such as in kennels, shelters, and pet stores.  Although most cases of human Giardia point to other sources of infection (person-to-person contact, or by contaminated water), animals do harbor strains of Giardia that are infective to humans, and animal to human transmission is theoretically possible.

Diarrhea is the most common sign of infection.  Some animals may vomit in addition to the diarrhea.  Weight loss may occur secondary to the diarrhea.   In many instances, a cat may be infected with Giardia, but show no clinical signs at all.

There are several ways to diagnose Giardia infection.  The most common methods involve analysis of a fecal sample. A fecal sample can also be sent to a diagnostic laboratory for more sophisticated immunology-based diagnostic tests.

Several drugs have been used to treat Giardia infections.  Antiparasitic drugs are the mainstay of therapy, however, additional measures, such as adding extra fiber to the diet, can help hasten recovery.

Metrodinazole (Flagyl) has been used extensively to treat Giardia in dogs and cats, as well as in people.  This drug has reasonable efficacy, and has an added advantage of being effective against other protozoans and some bacteria that might also be contributing to the diarrhea.  Side effects involving the nervous system have been reported in some animals, although this is uncommon.  Fenbendazole (Panacur) is a dewormer that kills common worms such as hookworms, roundworms, and whipworms.  When given at the normal deworming dose, fenbendazole is also very effective in treating Giardia.  This drug is very safe and fairly inexpensive.  A recent study has shown that administering both metronidazole and fenbendazole concurrently is the most efficacious treatment.

Home Care
Cats with Giardia need to have their prescribed medication administered faithfully.  High fiber diets often provide additional help in controlling  the diarrhea.

Prevention involves several approaches:

Decontamination of the environment: in multiple pet households and in crowding situations (kennels, shelters, pet stores), proper sanitation is key to prevent cross contamination from one animal to another.  All fecal material needs to be removed from cages, runs, and yard.  Kennels need to be cleaned with proper disinfectants and let totally dry before allowing animals into them. All animals should be treated with appropriate medication before being introduced into a multi-animal environment   Shampooing: bathing animals before introducing them into an uncontaminated environment allows for removal of feces and infective cysts from the haircoat.

There is considerable controversy as to the potential for animals to spread Giardia to humans.  There is scant evidence linking human infections to dogs and cats.  Human cases of Giardia are usually caused by person-to-person contact, or by drinking contaminated water.  Until these controversies are resolved, it is best to err on the side of caution and treat all infected animals regardless of whether they are experiencing symptoms or not.  The best way to detect infection is to have us regularly check a fecal sample (at least once a year).