How Giardia are Diagnosed

The disease caused by infection with Giardia is not difficult for veterinarians to diagnose. Examination of a dog’s feces can reveal the trophozoite and/or cyst forms of the organism. Giardia can also be identified through more advanced, specialized tests. Most veterinarians evaluating a dog with diarrhea, abdominal pain and other signs of gastrointestinal upset will take a blood sample and perform a complete blood count and a serum biochemistry panel. They will also probably take a urine sample for a urinalysis. The results of these tests typically will be inconclusive if the cause of the dog’s discomfort is giardiasis, without some other contributing condition. However, they are useful diagnostic tools to evaluate the dog’s overall health.

The initial database also will typically include an examination of the dog’s feces. There are several different techniques that can be used to identify parasites in feces. Fecal suspension involves mixing a small amount of fresh feces with saline, and looking at drops of the mixture on a glass side covered with a thin glass coverslip. The goal here is to try to identify the trophozoite form of the parasite. Fecal floatation is a bit more involved, but not too much so. It involves mixing a fresh stool sample with zinc sulphate or another special substance, spinning it down in a centrifuge so that the cyst forms to float to the surface of a test tube and then looking at the top part of the sample under a microscope. This is usually done several times over a series of days. Diagnosis can also be made by finding the parasite or its cysts by examining smears of fresh stool under a microscope. Fecal smears are simple to make from a rectal swab.

Because Giardia cysts are only shed intermittently and not constantly, a Giardia-free fecal examination does not necessarily rule out giardiasis. Two or three negative fecal tests, done at least several days apart, usually are conducted before a Giardia infection is ruled out. Of course, a positive fecal examination is usually diagnostic of the parasite, depending upon the skill of the person looking at the sample.

A test kit is available for in-clinic screening for Giardia. Advanced testing can be done by either direct immunofluorescent assay (IFA) or enzyme-linked immunosorbent assay (ELISA), both of which are done on fecal samples. Fecal polymerase chain reaction (PCR) testing is also available, but is not normally used for routine diagnosis. Veterinarians can also take a physical sample of the upper small intestinal lining via endoscopy. This involves using a wand-like instrument with a camera on the end to go into the small intestine, visualize its lining and snip and retract a small piece for submission to a diagnostic laboratory. However, this procedure is usually unnecessary to diagnose Giardia; it is a bit of an overkill in terms of time, invasiveness and expense.

Owners should consult with their veterinarian about diagnosing and treating their dogs for any infection with this or any other gastrointestinal parasites.

Special Notes

Any of these diagnostic methods can produce false positives (a positive test result in a dog that is free from infection) or false negatives (a negative test result in a dog that is infected with Giardia). False negatives are far more common in dogs with giardiasis than are false positive test results.