Definition
Giardia is a genus of anaerobic, flagellated protozoan parasites that colonize the small intestine of vertebrates. Members of this genus cause giardiasis, a diarrheal disease in humans and other animals.
Explanation
The genus Giardia includes several species, with Giardia intestinalis (also known as G. lamblia or G. duodenalis) being the primary species infecting humans. Giardia species have a two-stage life cycle consisting of a motile trophozoite and a resilient cyst. The pear-shaped trophozoite possesses two nuclei and four pairs of flagella, and attaches to the epithelial cells of the duodenum and jejunum using a ventral adhesive disc. Cysts are environmentally resistant and are shed in the feces of infected hosts. Transmission occurs through ingestion of cysts in contaminated water, food or by direct contact. After excystation in the small intestine, trophozoites multiply by binary fission, causing malabsorption and diarrhea. Giardia lacks typical mitochondria and relies on anaerobic metabolism. Host immunity, particularly secretory IgA, plays a role in controlling infection, but reinfection is common in endemic areas. Different assemblages of G. intestinalis infect humans, livestock and wildlife, and cross-species transmission can occur. Diagnosis of giardiasis involves microscopic detection of cysts or trophozoites in stool or the use of antigen-based assays.
Notable examples and facts
Giardia infections are a leading cause of waterborne outbreaks worldwide. The so-called “beaver fever” outbreaks in North America were linked to Giardia cysts in streams contaminated by beaver feces. Travelers visiting regions with poor sanitation are at risk, and children in daycare centers often experience giardiasis. Dogs, cats and livestock can carry distinct Giardia assemblages, though some strains are zoonotic. In laboratory settings, trophozoites are cultured in axenic media such as TYI-S-33 for research into metabolism and antigenic variation. Prevention focuses on treating drinking water with filtration and boiling, as Giardia cysts are resistant to standard chlorination.
Giardia represents a group of intestinal parasites with a simple life cycle but wide host range. Improving water quality and sanitation remains essential for reducing giardiasis prevalence.
Related Terms: Protozoa, Flagellate, Trophozoite, Cyst, Giardiasis