What causes giardiasis?

Updated: Oct 01, 2018
  • Author: Hisham Nazer, MBBCh, FRCP, DTM&H; Chief Editor: Burt Cagir, MD, FACS  more...
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Giardiasis is caused by the flagellate protozoan Giardia intestinalis (formerly known as G lamblia) . Infection is transmitted through ingestion of infectious G lamblia cysts. [35] The organism is known to have multiple strains with varying abilities to cause disease, and several different strains may be found in one host during infection. The infective dose is low in humans; 10-25 cysts are capable of causing clinical disease in 8 of 25 subjects. Ingestion of more than 25 cysts results in a 100% infection rate.

Person-to-person transmission, often associated with poor hygiene and sanitation, is a primary means of infection. Diaper changing and inadequate hand washing are risk factors for transmission from infected children. Children attending day care centers, as well as day-care workers, have a higher risk of infection secondary to fecal-oral transmission.

Water-borne transmission is responsible for a significant number of epidemics in the United States, generally following ingestion of unfiltered surface water. Giardia cysts retain viability in cold water for as long as 2-3 months. Giardia was implicated in 90 waterborne outbreaks in the United States from 1964-1984, affecting 23,500 persons.

Venereal transmission occurs through fecal-oral contamination. Food-borne epidemics have been reported, most commonly secondary to contamination by infected food-handlers. [36, 37] Pets frequently harbor Giardia in their GI tracts, but they are not thought to be a significant cause of outbreaks in humans.

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