Giardia is one of the most common findings in fecal examinations from pets living in our Downingtown area. Not all pets are symptomatic, but the doctors at the Animal Clinic at Thorndale always discuss with pet owners the zoonotic potential of this parasite. They are a common zoonotic disease, meaning the infection can be passed from animals to humans (and back again as well.) Giardia are not actually worms; they are one-celled parasites classified as protozoa. They invade the gastrointestinal tract and can cause diarrhea.
Most dogs that are infected with Giardia do not have diarrhea or any other signs of illness. When the eggs (cysts) are found in the stool of a dog without diarrhea, they are being shed into the environment and can still be infective to other mammals. In puppies and debilitated adult dogs, Giardia may cause severe, watery diarrhea that may be fatal.
Infection
A dog becomes infected with Giardia when it swallows the cyst stage of the parasite. Once inside the dog’s intestine, the cyst goes through several stages of maturation. Eventually, the dog is able to pass infective cysts in the stool. These cysts lie in the environment and can infect other mammals. They may also be transmitted through drinking infected water that is contaminated with stool from wildlife.
Diagnosis
Giardiasis is commonly diagnosed by performing a microscopic examination of a stool sample. The cysts are quite small and usually require a special floatation medium for detection, which is one reason our hospital sends all our patients’ stool samples to a diagnostic laboratory for proper testing. Occasionally, the parasites may be seen on a direct smear of the feces. A blood test is also available for detection of the antigens (cell proteins) of Giardia in the blood. This test is probably more accurate than the stool exam, but it requires several days to get a result from the laboratory performing the test.
Treatment
The most common drugs used to kill Giardia are metronidazole, an antibiotic-type drug, or fenbendazole, an anti-parasitic medication. The medications are generally given for 7 days. Other drugs are also used if diarrhea and dehydration occur.
Transmission of Giardia to Humans
Giardia can also cause diarrhea in humans. Contaminated water sources are an important cause (“backpacker’s diarrhea” from drinking water from streams.) If a pet in the house has giardia, environmental disinfection is important. The use of chlorine bleach, one cup in a gallon (500 ml in 4 liters) of water, is effective if the surfaces and premises can be safely treated with it. The most important preventative is good hygiene practices, especially when children are involved with pets. Hand washing after handling any pets and certainly before mealtimes can prevent accidental infection. Cleaning up the area where pets defecate can help prevent reinfection and spread to other animals and humans.