D405 - Giardia spp.

Giardia is a protozoan parasite found in the intestinal tract of human beings and most domestic animals throughout the world. It has two morphologic forms; the trophozoite form, which is motile and dwells in the intestinal lumen; the cyst form, which contains two incompletely separated but formed trophozoites, and is responsible for transmission and environmental survival. The life cycle of Giardia is direct. After ingestion, cysts (trophozoites are noninfective) excyst in the duodenum, following exposure to gastric acids and pancreatic enzymes, and the two released trophozoites separate, mature, and attach to the brush border of the villous epithelium. In dogs, the organism seems to prefer the duodenum and jejunum but has been isolated from the ileum to the duodenum. In cats, trophozoites have been found throughout the intestinal tract. Trophozoites may be passed in diarrheic stools, but cysts are more routinely shed. The cysts can survive for days or weeks in cool, moist conditions while trophozoites cannot survive long outside the host. The prepatent period of infection ranges from 5 to 12 days in dogs and from 5 to 16 days in cats. The onset of disease, when it occurs, may precede cyst shedding by 1 to 2 days. Putative differences in virulence of Giardia strains, as well as host genetics and immune status, usually determine the outcome of an infection. Most infections (where cysts are being passed in the feces) are nonsymptomatic. Acute diarrhea tends to occur in very young puppies and kittens shortly after infection. In older cats and dogs, diarrhea may be acute and short lived, intermittent, or chronic. Feces are often malodorous, pale, and steatorrheic. Affected animals may experience weight loss secondary to diarrhea, but rarely will they become inappetent. Studies have shown that birds are also susceptible to the infection. Infected birds shed cysts and noninfective trophozoites in feces. Birds with clinical signs including chronic to intermittent diarrhea with loose, malodorous, mucoid stools; lethargy; anorexia; dry skin; and feather plucking should be screened for the infection.

Often, the infection goes undetected, even when the presence of the parasite is high in the animal. Reasons for this occurrence may include failure to consider it in the differential diagnosis, failure to recognize the organisms, use of inappropriate methods for fecal analysis, and intermittent excretion of organisms in feces of infected individuals. Commercial ELISA kits to detect fecal antigens in humans are available, but these kits have shown reduced sensitivity and specificity for detection in dogs. Also, ELISA kits are somewhat difficult to perform and fairly expensive. A PCR-based assay is now available for the detection in a fecal sample. This assay allows for high specificity and sensitivity of detection. Since PCR detects the presence of the parasite’s genetic material, only a few organisms need to be present in a sample, making detection very accurate and definitive.