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What is Giardia Lamblia?

By Emma Lloyd
Updated May 17, 2024
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Giardia lamblia is a parasitic species of protozoa that was first recognized as harmful in the 1970s. Globally, infection by this organism is responsible for a large proportion of cases of diarrheal illness. It has a prevalence of up to 42 percent in developing countries, and in the United Kingdom is the most common parasite of the gut. In the United States, G. lamblia is the most commonly identified intestinal pathogen in water and is present in up to 80 percent of the country’s water supplies obtained from ponds, lakes and streams. This organism is also known as Giardia duodenalis and Giardia intestinalis.

The lifecycle of the Giardia lamblia protozoan has two stages. In the first stage, the organism lives in the human small intestine and is referred to as a trophozoite. After living and feeding in the small intestine, the trophozoite passes through to the bowel, where cyst formation is triggered by the neutral pH there. Giardia lamblia cysts are then excreted during bowel movements.

The cysts can remain dormant for a considerable period of time, until they are exposed to a pH of between 1.3 and 2.7, such as what exists in the human stomach. Infection of a new host occurs via contact with cysts in contaminated food or water, or via direct contact with an infected person. It is believed that as few as 10 cysts might be enough to cause infection.

An estimated 15 percent of cases of infection caused by G. lamblia are asymptomatic, but cyst excretion still occurs. Giardia lamblia symptoms are present in about 50 percent of cases. Symptoms might include diarrhea, abdominal cramping, bloating and flatulence. People with chronic infection can become malnourished, and a weight loss of up to 15 pounds is not uncommon. Children in developing countries are particularly at risk of chronic infection and malnutrition, which may lead to delayed growth.

Standard Giardia lamblia treatment includes a course of antibiotics capable of killing the parasites. These include metronidazole, albendazole and furazolidone. People infected with this parasite should ensure they receive an adequate intake of fluids and nourishment.

There are no special dietary requirements for people with an active infection, but most people find that bland foods such as rice, potatoes and crackers are well-tolerated. After an infection, many people become temporarily lactose intolerant and might need to eliminate dairy products for up to several months. Because cysts can be transmitted through person-to-person contact, observing good hygiene is crucial to prevent the infection from spreading.

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