(See also Overview of Parasitic Infections.)
Giardiasis is an infection of the small intestine caused by the single-celled protozoan parasite Giardia. The main symptoms are abdominal cramping and diarrhea.
People may have abdominal cramping, gas, belching, diarrhea, and nausea and feel tired.
People acquire the infection by drinking water or eating food that is contaminated with stool containing Giardia or by coming into contact with stool from an infected person.
Doctors diagnose the infection by testing or examining a stool sample.
Boiling water kills Giardiaprotozoa and is the safest way for hikers to ensure that water from streams and lakes is safe to drink.
Infected people are treated with an antiparasitic drug, such as tinidazole, metronidazole, or nitazoxanide.
Giardiasis is a protozoan infection that occurs worldwide and is the most common parasitic infection of the intestine in the United States. Giardia protozoa can form an outer shell (called a cyst). It enables them to survive outside the body for long periods of time (for example, in lakes and streams) and makes them less likely to be killed by chlorine (for example, in swimming pools). These cysts are passed in stool and can cause infection.
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 people can acquire the infection if they eat contaminated food or have contact with stool from an infected person, which can occur between children or sex partners.
Giardiasis is more common among
Giardia can live in wild animals.
Some infected people have no symptoms, but these people can pass Giardia cysts in their stool and can thus infect others. Symptoms, when they occur, appear about 1 to 2 weeks after infection.
Symptoms of giardiasis 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 (called malabsorption), resulting in significant weight loss.
Occasionally, chronic giardiasis prevents children from growing as expected (a condition called failure to thrive).
The symptoms often suggest giardiasis.
The easiest way to make the diagnosis of giardiasis is by testing the stool for proteins (antigens) released by Giardia lamblia or for its genetic material.
Microscopic examination of stool samples 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.
If these tests do not identify what is causing the intestinal symptoms, doctors may use a flexible viewing tube (endoscope) to examine the upper part of digestive tract, including the first part of the small intestine (duodenum). Doctors may use this procedure to obtain a sample of the contents of the small intestine for examination.
Giardiasis prevention requires
Boiling water kills the parasite and is the safest way for hikers to ensure that water from streams and lakes is safe to drink.
Water from streams and lakes can sometimes be disinfected using iodine. This method is less reliable because its effectiveness varies depending on how cloudy or muddy the water is (turbidity), what the water's temperature is, and how the water 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.
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 the spread of the infection but is impractical and expensive.
Tinidazole, taken in a single dose, has fewer side effects than metronidazole, which is taken three times a day for 5 to 7 days. 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 is taken twice a day for 3 days. 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.