Giardia duodenalis is a protozoan parasite that commonly infects a variety of mammals, including cats, dogs, and humans. Giardia is A species complex consisting of at least eight genetically distinct but morphologically identical collections (named A to H), with cats most commonly infected with collection F. Giardia infection in cats can be subclinical, but the most common clinical symptom is diarrhea. Historically, Giardia infection was diagnosed by microscopic examination of faeces, but immunological tests that detect antigens in faeces are now widely used as bedside tests.