Giardia

Giardia lamblia, also known as Giardia intestinalis or Giardia duodenalis, is a flagellated protozoan parasite that colonizes the small intestine of humans and many mammals. It exists in two forms: cysts, which are environmentally resistant and excreted in feces, and trophozoites, the active stage in the host. Ingestion of cysts through contaminated water, food, or person-to-person contact leads to excystation in the small intestine, release of trophozoites that attach to enterocytes, and formation of cysts that are excreted in stool, continuing transmission.

Clinical presentation ranges from asymptomatic infection to diarrhea, abdominal cramps, bloating, nausea, and greasy stools with weight loss. Chronic infection can occur, particularly in immunocompromised individuals, and fat malabsorption may accompany disease.

Diagnosis relies on stool testing. Microscopy with concentration has limited sensitivity; modern approaches include antigen detection and nucleic acid amplification tests, which are more sensitive and specific. Because cysts may be shed intermittently, multiple stool samples are often collected.

Treatment typically uses nitroimidazole drugs such as metronidazole or tinidazole, with nitazoxanide or paromomycin as alternatives in some settings. Response is usually rapid, but retreatment or evaluation for other causes may be necessary in persistent cases.

Prevention emphasizes safe drinking water, proper sanitation, good hand hygiene, and avoiding swallowing water from natural sources. Cysts resist chlorine disinfection, so filtration and/or UV treatment are recommended in at-risk settings. Giardia is a common cause of diarrheal illness worldwide, with outbreaks linked to daycare centers, travelers, hikers, and contaminated recreational water.