A disease caused by the protozoan parasite Giardia lamblia, characterized by chronic diarrhea that usually lasts one or more weeks. The presence of the protozoan parasite Giardia lamblia (also known as Giardia intestinalis and Giardia duodenalis) in the human small intestine is known as giardiasis. It is one of the most common types of waterborne disease. This condition also affects many mammals, birds, and reptiles, allowing for possible zoonotic transmission. The diarrhea caused by giardiasis may be accompanied by one or more of the following symptoms: abdominal cramps, bloating, flatulence, fatigue, or weight loss. The stools are malodorous and have a pale greasy appearance. Infection without symptoms is also common. As with most other protozoa inhabiting the intestinal tract, the life cycle of Giardia involves two stages: trophozoite and cyst (see illustration). Trophozoites stay in the upper small-intestinal tract, where they actively feed and reproduce. When the trophozoites pass down the bowel, they change into the inactive cyst stage by rounding up and developing a thick exterior wall, which protects the parasite after it is passed in the feces. Humans become infected either directly by hand-to-mouth transfer of cysts from feces of an infected individual (or animal) or indirectly by drinking feces-contaminated water. After the cyst is swallowed, the trophozoite is liberated through the action of digestive enzymes and stomach acids, and becomes established in the small intestine. See also: Diarrhea; Diplomonadida; Gastrointestinal tract disorders; Infant diarrhea; Protozoa; Water-borne disease; Zoonoses
Giardiasis occurs worldwide. Surveys conducted in the United States have demonstrated Giardia infection rates ranging from 1% to 20%, depending on the geographic location and the age of the persons studied. In community epidemics caused by contaminated drinking water, as many as 50–70% of the residents have become infected. Outbreaks also occur among backpackers and campers who drink untreated water from streams. Both human and animal (beaver) fecal contamination of stream water has been implicated as the source of Giardia cysts in waterborne outbreaks. Giardia species in dogs and possibly other animals are also considered infectious for humans.
Epidemics resulting from person-to-person transmission occur in day-care centers for preschool-age children. Infants and toddlers in day-care centers are more commonly infected than older children who have been toilet-trained. Epidemics also occur in institutions for individuals with intellectual disabilities. See also: Epidemic
Why some people become ill when infected with G. lamblia and others do not has not been fully explained. Host immunity undoubtedly plays a role, but the exact immune mechanisms involved have not been identified. Other factors, such as the number of Giardia cysts swallowed (dose), varying virulence between Giardia strains, and origin of the parasite (human or animal), have been postulated, but not proved, as having an influence on the clinical course of infection. See also: Epidemiology
The diagnosis of Giardia infection is most commonly made by identifying the causative agent, G. lamblia, in the feces. It is also possible to identify the parasite in digestive juices or biopsy material taken from the small intestine. In individuals with watery diarrhea, trophozoites are most commonly found in stools, but a few cysts may also be present. After the acute stage has passed, stools are more often semiformed or formed, and contain the more hardy cyst form of the parasite. Because Giardia cysts are passed in the feces on an intermittent basis, a minimum of three stool specimens (one every other day) should be obtained and examined to minimize the chance of missing an infection. The parasites may be stained in iodine or by more permanent staining methods for purposes of differentiating them from other bowel-inhabiting protozoa.
Several drugs are available for the treatment of giardiasis, including metronidazole, quinacrine (mepacrine), tinidazole, nitazoxanide, and furazolidone. Metronidazole, which is a type of nitroimidazole antibiotic, is generally well tolerated by both adults and children. Although quinacrine is effective in young children, the drug frequently causes vomiting in this age group; therefore, it is typically limited to adults and older children. See also: Antibiotic
Epidemic giardiasis most commonly results from ingestion of contaminated drinking water. The long-term solution to municipal waterborne outbreaks requires improvement in, and widespread use of, water filtration equipment in the water treatment process. Many cities in the United States rely solely on chlorination to disinfect drinking water; however, the amount of chlorine used does not kill Giardia cysts. Backpackers and campers should not drink water directly from streams or lakes, no matter how clean the water looks. If stream water must be used for drinking, it should be boiled for 1 min to kill Giardia as well as other infectious organisms that might be present. Chemical disinfectants such as laundry bleach or tincture of iodine may also be used to disinfect water of uncertain purity. These products work well against most bacterial and viral organisms, but they are not considered as reliable as heat in killing Giardia. If water is cloudy, it should be strained through a clean cloth into a container to remove any sediment or floating matter. Then, the water should be treated with chemicals. See also: Water pollution; Water treatment
Person-to-person transmission of Giardia can be prevented by practicing good personal hygiene and maintaining a sanitary environment. Washing of hands should be stressed, especially after using the toilet or handling soiled diapers of infants. Quick and thorough cleanup of fecal accidents at home or in institutions also reduces the risk of spreading Giardia to other individuals. See also: Medical parasitology; Public health