A disease caused by the protozoan parasite Giardia lamblia, characterized by chronic diarrhea that usually lasts 1 or more weeks. The diarrhea may be accompanied by one or more of the following: 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.
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.
People become infected either directly by hand-to-mouth transfer of cysts fromfeces of an infected individual 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.
Giardiasis occurs worldwide. Surveysconducted in the United States have demonstrated Giardia infection rates ranging from1 to 20%, depending on the geographic location and age of persons studied. In community epidemics caused by contaminated drinking water, as many as 50 to 70% of the residents have become infected. Outbreaks also occur among backpackers and campers who drink untreated stream water. 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 and institutions for the mentally retarded. Infants and toddlers in day-care centers are more commonly infected than older children who have been toilet-trained. 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. A number of 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.
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.
Three drugs are available in the United States for the treatment of giardiasis: quinacrine, metronidazole, and furazolidone. Quinacrine is considered the drug of choice for adults and older children. Although quinacrine is effective in young children, the drug frequently causes vomiting in this age group. Metronidazole gives cure rates similar to quinacrine, and is generally well tolerated by both adults and children. Furazolidone is also an effective drug; it is the only anti-Giardia preparation that is supplied in pediatric suspension.