Giardiasis is an infection of the small intestine caused by the single-celled protozoan parasite Giardia lamblia. The main symptoms are abdominal cramping and diarrhea.

• People may have abdominal cramping, gas, belching, diarrhea, and nausea and feel vaguely tired.
• People acquire the infection by drinking contaminated water or by having contact with contaminated stool.
• Doctors diagnose the infection by testing or examining a stool sample.
• Hikers should boil fresh water before drinking it.
• Infected people are treated with an antiparasitic drug, such as tinidazole.

Giardiasis occurs worldwide and is the most common parasitic infection of the intestine in the United States. Giardia protozoa are a common contaminant of fresh water, including many lakes and streams—even ones that appear clean. Poorly filtered municipal water supply systems contribute to some outbreaks. Most people acquire the infection from drinking contaminated water, but direct person-to-person transmission of cysts passed in the stool also occurs—typically between children or sex partners. Giardiasis is more common among children in day care centers, people who practice oral-anal sex, and people who have traveled to developing countries. Backpackers and hikers who drink untreated water from streams and lakes are also at risk. Wild animals can harbor the parasite.

Symptoms and Diagnosis

Some infected people have no symptoms. In other people, symptoms appear about 1 to 2 weeks after infection. Symptoms typically include abdominal cramps, gas (flatulence), belching, and watery, foul-smelling diarrhea. Nausea may come and go. People may feel tired and vaguely uncomfortable and lose their appetite. If untreated, the diarrhea may persist for weeks. A few people develop diarrhea that persists longer. These people may not absorb enough nutrients from food, resulting in significant weight loss. Occasionally, chronic giardiasis prevents children from growing as expected (a condition called failure to thrive).

The symptoms often suggest the diagnosis. The easiest way to make the diagnosis is by testing the stool for proteins (antigens) released by Giardia lamblia. Microscopic examination of stool samples or secretions taken from the small intestine may also detect the parasite. However, because people who have been infected for a long time tend to excrete the parasites at unpredictable intervals, repeated microscopic examinations of stool are often needed.

Prevention and Treatment

Boiling water kills the parasite and is the safest way for hikers to ensure that surface water is safe to drink. Wells, reservoirs, and swimming pools can sometimes be disinfected using iodine or chlorine. This method is less reliable because it varies depending on how cloudy or muddy the water is (turbidity), what its temperature is, and how often it is disinfected. The amount of chlorine routinely used in drinking water may be insufficient to kill the cysts. Some handheld filtration devices can remove cysts from water, but whether a particular filter system is effective is not always known.

Did You Know... Giardiasis is the most common parasitic intestinal infection in the United States.

Infected people who have symptoms can be treated with tinidazole, metronidazole, or nitazoxanide, taken by mouth. Treating infected people who do not have symptoms might help reduce spread of the infection but is impractical and expensive. Tinidazole, taken in a single dose, has fewer side effects than metronidazole, which requires several doses. Drinking alcohol within a few days of taking tinidazole or metronidazole may cause nausea, vomiting, flushing, and headaches. Nitazoxanide is available in liquid form, which is useful for children, and as tablets. It has few side effects. Pregnant women should not take metronidazole or tinidazole. The safety of nitazoxanide during pregnancy has not been assessed. Consequently, the treatment of pregnant women is delayed if possible until after pregnancy. If symptoms are severe and treatment cannot be delayed, paromomycin can be used.

Last full review/revision March 2007 by Richard D. Pearson, MD