The incubation period is >10 days. Giardia infections are often clinically inapparent in animals. The clinical symptoms depend on the pre-damage of the gut and the immune status of the animal. Also, bad housing conditions and high-carbohydrate diet contribute to the infection. Thus, the giardia infection represents a factor disease. It mostly affects young animals and is manifested in acute, but more often in chronic, catarrhous, intermittent diarrhea. The feces are watery, often with increased mucus and fat (Stearrhoe) admixture. Particularly in young animals or animals from poor housing conditions, an infection can cause chronic intermittent diarrhea. These animals show malabsorption syndromes like a shaggy coat, chronic weight loss, occasional vomiting, and apathy. The animals excrete large quantities of infectious cysts. Giardia infections in immunocompetent animals induce an immunity that eliminates the pathogen population completely or in part. Adult dogs and cats can also be chronic carriers, without getting ill themselves.

Treatment is often difficult because asymptomatic chronic carriers remain undetected and there are some extent resistances against used antiparasitics. Especially group housing with poor hygiene treatment is problematic, since even after successful treatment with antiparasitics often reinfections can occur.

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