Transmission of Giardia

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Giardia cysts are spread via the fecal-oral route. Cysts may be ingested with contaminated food or water, or acquired from unwashed hands.

Infection can be spread from person to person indirectly by water, by fecal-oral contamination with cysts or occasionally by food.1,2 Giardia cysts are able to survive in fresh water, and it is the most common cause of epidemic waterborne diarrheal disease.3 From 1991-1994, giardiasis was one of two leading etiologies in the 129 waterborne diarrhea outbreaks reported.4 In 1991, the Environmental Protection Agency (EPA) reported Giardia cysts in 81% of raw water samples from 66 surface water treatment plants and from 17% of filtered water samples in 14 states and one Canadian province.5

Giardia cysts are quite resistant to chlorination, making water filtration the key to effective water treatment. Since Giardia cysts can sometimes be isolated from filtered drinking water, however, boiling water is the best way to kill chlorine-resistant Giardia cysts.6

In addition to outbreaks associated with ingestion of unfiltered or poorly chlorinated water, Giardia can also be a cause of recreational water outbreaks of gastroenteritis—swimming pools, water parks, hot tubs and spas. Cysts in recreational water generally come from other users versus a primary contaminated water source. In Washington state, an outbreak of giardiasis in an infant and toddler swimming class reported that 61% of 70 children had giardiasis following exposure to an infected child in the class.7

G. lamblia is one of the first pathogenic parasites to infect infants in both industrialized and developing countries, making it one of the most common causes of day-care center outbreaks of diarrhea. U.S. day-care centers have reported the prevalence of Giardia to range from 17%-90%. Infection often spreads among the family members of infected children as well.8

Giardia Species that Infect Animals13
Species Host Morphology
G. duodenalis Mammals (beavers, coyotes, cattle, cats, dogs) Pear-shaped, trophozoite, claw shaped median body
G. muris Rodents Round trophozoite, small median body

Food borne transmission of giardiasis has also been documented, although not very well characterized. There have been several reports where food has been identified as the source in some cases. Outbreaks attributed to infected foods have usually been linked to an infected food handler.9 Male homosexual contact has also been implicated as another transmission factor with oral-anal sexual practices serving as a route of transmission.10

Although humans are the main reservoir of the parasite, a variety of animals carry Giardia species, which can be passed on to humans. Giardia duodenalis cysts from beavers may contaminate community water supplies.11 Although the possibility of Giardia transmission from dogs to humans (zoonosis) has historically been a subject of debate, recent studies have provided strong evidence that this can and does happen.12

  1. Craun GF. Waterborne giardiasis in the United States 1965-84. Lancet. 1986 Aug 30;2(8505):513-4.
  2. Petersen LR, Cartter ML, and Hadler JL. A food-borne outbreak of Giardia lamblia. J Infect. Dis. 1988;157:846-848.
  3. Mank TG, Diagnostic advantages and therapeutic options for giardiasis. Expert Opin Investig Drugs. 2001 Aug;10(8):1513-9.
  4. Furness BW, Beach MJ, Roberts JM. Giardiasis surveillance--United States, 1992-1997. MMWR CDC Surveill Summ. 2000 Aug 11;49(7):1-13.
  5. LeChevallier MW, Norton WD, Lee RG. Giardia and Cryptosporidium spp. in filtered drinking water supplies. Appl Environ Microbiol. 1991 Sep;57(9):2617-21.
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  7. Harter L, Frost F, Grunenfelder G, Perkins-Jones K, Libby J. Giardiasis in an infant and toddler swim class. Am J Public Health. 1984 Feb;74(2):155-6.
  8. Wolfe MS. Giardiasis. Clin Microbiol Rev. 1992 Jan;5(1):93-100.
  9. Adam RD. The biology of Giardia spp. Microbiol Rev. 1991 Dec;55(4):706-32.
  10. Peters CS, Sable R, Janda WM, Chittom AL, Kocka FE. Prevalence of enteric parasites in homosexual patients attending an outpatient clinic. J Clin Microbiol. 1986 Oct;24(4):684-5.
  11. Davies RB, Hibler CP. Animal reservoirs and cross-species transmission of Giardia, p. 104-126. In W. Jakubowski and JC Hoff (ed.), Waterborne transmission of giardiasis. Environmental Protection Agency, Cincinnati.
  12. Inpankaew T, Traub R, Thompson RC, Sukthana Y. Canine parasitic zoonoses in Bangkok temples. Southeast Asian J Trop Med Public Health. 2007 Mar;38(2):247-55.
  13. Smith JW, Wolfe MS. Giardiasis. Annu Rev Med. 1980;31:373-83.