What is giardiasis?
- Giardiasis (gee-ar-die-a-sis with a soft "G") is an infection of the small intestine that is caused by the parasite, Giardia duodenalis, also known as Giardia lamblia and Giardia intestinalis.
- It is the most common cause of parasitic gastrointestinal disease; it is estimated that 20,000 cases of giardiasis occur each year in the U.S., and there is a 20% to 40% prevalence in the world's population.
- 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. It takes ingestion of only ten cysts to cause infection. Trophozoites are important not only because they cause the symptoms of giardiasis, but also because they produce the cysts that exit the body in the feces and spread the infection to others.
- Cysts of Giardia are present in the feces of infected persons. Thus, the infection is spread from person to person by contamination of food with feces, or by direct fecal-oral contamination. Cysts also survive in water, for example in fresh water lakes and streams. As a result, giardiasis is the most common cause of water-borne, parasitic illness in the U.S.
- Domestic mammals (for example, dogs, cats, calves) and wild mammals (for example, beavers) can become infected with Giardia; however, it is not clear how often domestic or wild mammals transmit giardiasis to humans.
- Giardiasis also has occurred as outbreaks from recreational water sources such as swimming pools, water parks, and hot tubs, most likely because of an infected user rather than a source of water that was contaminated.
Who is at risk for giardiasis?
Giardiasis occurs where there is inadequate sanitation or inadequate treatment of drinking water. Giardiasis is one of the causes of "travelers diarrhea" that occurs during travel to less-developed countries, for example the Soviet Union, Mexico, Southeast Asia, and western South America. Giardiasis is a common cause of outbreaks of diarrhea in day-care centers because of the high probability of fecal-oral contamination from children; the children, their families, and day care center workers, all are at risk for infection. In fact, children are three times more likely to develop giardiasis than adults. Hikers exploring back-country areas who drink from contaminated fresh water lakes also are at risk for developing giardiasis. Individuals who practice anal/oral sex also may become infected.
What signs and symptoms does giardiasis cause?
The most common manifestations of giardiasis are diarrhea and abdominal pain, particularly cramping; however, diarrhea is not invariable and occurs in 60% to 90% of patients. Other common manifestations include:
Fever is unusual. The severity of the symptoms may vary greatly from mild or no symptoms to severe symptoms. Stools may be foul smelling when the Giardia interferes with the absorption of fat from the intestine (malabsorption). The illness or the malabsorption may cause loss of weight.
Symptoms and signs of giardiasis do not begin for at least seven days following infection, but can occur as long as three or more weeks later. In most patients the illness is self-limiting and lasts 2-4 weeks. In many patients who are not treated, however, infection can last for several months to years with continuing symptoms. Children with chronic infection may fail to thrive. Some patients recover from their giardiasis, with or without treatment, but symptoms continue, perhaps because of a condition referred to as postinfectious irritable bowel syndrome. The cause of the continuing symptoms is not clear but may be due to bacterial overgrowth of the small intestine.
How is giardiasis diagnosed?
The best single test for diagnosing giardiasis is antigen testing of the stool. For antigen testing, a small sample of stool is tested for the presence of Giardial proteins. The antigen test will identify more than 90% of people infected with Giardia. Giardia also can be diagnosed by examination of stool under the microscope for cysts or trophozoites; however, it takes three samples of stool to diagnose 90% of cases. Despite requiring three samples of stool, microscopic examination of stool identifies other parasites in addition to Giardia that can cause diarrheal illness. Therefore, microscopical examination of stool has value beyond diagnosing giardiasis, as it can identify and diagnose other parasites as the cause of a patient's illness.
Other tests that can be used for diagnosing giardiasis are collection and examination of fluid from the duodenum or biopsy of the small intestine, but these require tests that involve expense and discomfort. The string test is a more comfortable method for obtaining a sample of duodenal fluid. For the string test, a gelatin capsule that contains a loosely-woven string is swallowed. One end of the string protrudes from the capsule and is taped to the patient's outer cheek. Over several hours, the gelatin capsule dissolves in the stomach, and the string uncoils, with the last 12 inches or so passing into the duodenum. In the duodenum the string absorbs a small amount of duodenal fluid. The string then is untapped from the cheek and is removed. The collected duodenal fluid is expressed from the string and is examined under the microscope. Although more comfortable than some of the other tests, it is not clear how sensitive the string test is in diagnosing giardiasis.
How is giardiasis treated?
The most common treatment for giardiasis is metronidazole (Flagyl) for 5-10 days. It has an efficacy rate of 75% to 100%, but it often causes gastrointestinal side effects such as nausea and a metallic taste as well as dizziness and headache. Despite its effectiveness, metronidazole is not approved by the FDA in the U.S. for treatment of giardiasis.
Tinidazole has replaced furazolidone as the FDA-approved drug for treatment of the condition, and is highly effective at treating giardiasis (>90%). It also can be given as a single dose and is well tolerated. Furazolidone and Quinacrine are effective for treating giardiasis but are no longer available in the U.S. Albendazole and mebendazole are effective alternative agents, but these are not approved for used in the U.S. Paromomyin is less effective than other treatments.
Occasionally, treatment fails to eradicate Giardia. In such cases, the drug may be changed or a longer duration or higher dose may be used. Combination therapy also may be effective (e.g., quinacrine and metronidazole).
How can giardiasis be prevented?
- Drink only water that has been treated in established treatment facilities.
- If there is any doubt about the adequacy of the treatment of water, boil the water or filter it through a filter with a pore size of <1 micrometer, which will exclude trophozoites and cysts.
- Do not drink from fresh water streams or lakes without boiling or filtering the water.
- Do not use ice or drink beverages made from tap water that may be contaminated.
- Do not brush teeth with tap water that may be contaminated.
- Do not eat uncooked or unpeeled fruits or vegetables grown in conditions in which contamination with Giardia might occur.
- Wash hands before eating food, after changing diapers, using the bathroom, or touching pets.
- Avoid oral/anal sex.
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Medically reviewed by Robert Cox, MD; American Board of Internal Medicine with subspecialty in Infectious Disease