Giardiasis, a gastrointestinal disease characterized by acute or chronic diarrhea, is caused by protozoan parasites in the genus Giardia. Giardia duodenalis is the major species found in mammals, and the only species known to cause illness in humans. This organism is carried in the intestinal tract of many animals and people, with clinical signs developing in some individuals, but many others remaining asymptomatic. In addition to diarrhea, the presence of G. duodenalis can result in malabsorption; some studies have implicated this organism in decreased growth in some infected children and possibly decreased productivity in young livestock. Outbreaks are occasionally reported in people, as the result of mass exposure to contaminated water or food, or direct contact with infected individuals (e.g., in child care centers).
People are considered to be the most important reservoir hosts for human giardiasis. The predominant genetic types of G. duodenalis usually differ in humans and domesticated animals (livestock and pets), and zoonotic transmission is currently thought to be of minor significance in causing human illness. Nevertheless, there is evidence that certain isolates may sometimes be shared, and some genetic types of G. duodenalis (assemblages A and B) should be considered potentially zoonotic.
Iowa State University
Iowa State University Center for Food Security and Public Health, "Giardiasis" (2012). Center for Food Security and Public Health Technical Factsheets. 66.