Giardia lamblia (Giardia intestinalis): overview, biology, and public health

Biology and life cycle

Giardia exists in two main forms: the active trophozoite and the environmentally resistant cyst. Trophozoites are pear-shaped, have two nuclei, and multiple flagella; they attach to the intestinal lining using a specialized adhesive disc. Cysts are shed in feces, survive outside the host, and are the infectious stage that spreads disease. Transmission occurs by ingesting cysts from contaminated water, food, or direct contact (fecal–oral route).

Clinical features and diagnosis

Symptoms range from none (asymptomatic) to acute or chronic gastrointestinal illness. Common signs include watery or greasy stools, abdominal cramps, bloating, nausea, and weight loss due to impaired nutrient absorption. Diagnosis is typically by stool microscopy, antigen detection tests, or molecular methods that identify parasite DNA. Repeated sampling may be needed because cyst shedding can be intermittent.

Treatment, prevention, and control

Treatment uses specific anti-protozoal medications prescribed by clinicians; supportive care addresses dehydration and nutrition. Prevention emphasizes safe drinking water (boiling, filtration, and disinfection), good hand hygiene, proper sanitation, and food safety. Recreational water and backcountry water sources are common settings for outbreaks, so travelers and outdoor enthusiasts are advised to treat or filter untreated water.

History, ecology, and notable facts

Giardia was one of the first microbes observed in human stool and later named after early describers. It has multiple genetic groups (assemblages) with varying host ranges; some assemblages commonly infect humans while others are mainly animal-associated. Because of its hardy cyst form and low infectious dose, Giardia is an important cause of waterborne disease globally and a frequent target of public health measures for drinking-water safety.

Further information

• Transmission routes and prevention: more on human infection
• Pets and zoonotic risk: cats, dogs
• Wildlife reservoirs and ecology: beavers, deer, birds