Giardiasis (GEE-are-DYE-uh-sis) is a diarrheal illness caused by a one-celled, microscopic parasite, Giardia intestinalis (also known as Giardia lamblia). Once an animal or person has been infected with Giardia intestinalis, the parasite lives in the intestine and is passed in the stool. Because the parasite is protected by an outer shell, it can survive outside the body and in the environment for long periods of time.

Giardia lamblia exists in two forms, an active form called a trophozoite, and an inactive form called a cyst. The active trophozoite attaches to the lining of the small intestine with a “sucker” and is responsible for causing the signs and symptoms of giardiasis. The trophozoite cannot live long outside of the body, therefore it cannot spread the infection to others. The inactive cyst, on the other hand, can exist for prolonged periods outside the body. When it is ingested, stomach acid activates the cyst, and the cyst develops into the disease-causing trophozoite.

Symptoms include loss of appetite, lethargy, fever, explosive diarrhea, loose or watery stool, stomach cramps, upset stomach, projectile vomiting (uncommon), bloating, flatulence, and burping (often sulphurous). Symptoms typically begin 1–2 weeks after infection and may wane and reappear cyclically. Symptoms are caused largely by the thick coating of Giardia organisms coating the inside of the small intestine and blocking nutrient absorption. Most people are asymptomatic; only about a third of infected people exhibit symptoms. Untreated, symptoms may last for six weeks or longer.

Giardia is one of the chief causes of diarrhea in the United States, and is transmitted through contaminated water. It can survive the normal amounts of chlorine used to purify community water supplies, and can live for more than 2 months in cold water. As few as 10 of the microscopic parasites in a glass of water can cause a severe case of giardiasis in a human being who drinks it.

Anyone can get giardiasis but it tends to occur more often in people in institutional settings, people in daycare centers, foreign travelers and individuals who consume improperly treated surface water. Men who have sex with men may also be at increased risk of contracting giardiasis.

Giardia outbreaks can occur in communities in both developed and developing countries where water supplies become contaminated with raw sewage. It can be contracted by drinking water from lakes or streams, where water-dwelling animals such as beavers and muskrats, or domestic animals such as sheep, have caused contamination. It is also spread by direct person-to-person contact, which has caused outbreaks in institutions such as daycare centers.

Giardiasis is easy to catch if you drink untreated spring water or stream water. Many animals carry giardia in their feces and may introduce this parasite into rivers, streams and springs in rural areas. Infected stream water may look clean and safe when it really isn't. City water may also be infected if sewer lines flood or leak. If you travel overseas, you may get giardiasis by drinking water (even tap water) that hasn't been boiled or treated.

Infection may be caused by poor hygiene and most commonly occurs in daycare centers, nursing homes, and in sexually active homosexual males. Up to 50% of children infected with Giardia in daycare centers, and up to 20% of infected sexually active homosexual males, pass cysts in their stool. Family members, daycare workers, and others in contact with infected stool may then they become infected.

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