CDC - Giardia - Epidemiology & Risk Factors

Giardia

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Epidemiology & Risk Factors

Giardia trophozoites. Credit: Waterborne Disease Prevention Branch, CDC

Wet mount of a Giardia cyst. Credit: Waterborne Disease Prevention Branch, CDC

Giardiasis is a diarrheal illness caused by the parasite Giardia intestinalis (also known as Giardia lamblia or Giardia duodenalis). A parasite is an organism that feeds off of another to survive.

Giardiasis is a global disease. It infects nearly 2% of adults and 6% to 8% of children in developed countries worldwide. Nearly 33% of people in developing countries have had giardiasis. In the United States, Giardia infection is the most common intestinal parasitic disease affecting humans^[1].

People become infected with Giardia by swallowing Giardia cysts (hard shells containing Giardia) found in contaminated food or water. Cysts are instantly infectious once they leave the host through feces (poop)^[2]. An infected person might shed 1-10 billion cysts daily in their feces (poop) and this might last for several months^[2,6,7]. However, swallowing as few as 10 cysts might cause someone to become ill^[2,6]. Giardia may be passed person-to-person or even animal-to-person^[2,3]. Also, oral-anal contact during sex has been known to cause infection^[4,5]. Symptoms of giardiasis normally begin 1 to 3 weeks after a person has been infected^[6].

Giardia infection rates have been known to go up in late summer^[8-10]. Between 2006-2008 in the United States, known cases of giardiasis were twice as high between June-October as they were between January-March^[2].

Anyone may become infected with Giardia. However, those at greatest risk are^[2,11-16]:

Travelers to countries where giardiasis is common
People in child care settings
Those who are in close contact with someone who has the disease
People who swallow contaminated drinking water
Backpackers or campers who drink untreated water from lakes or rivers
People who have contact with animals who have the disease
Men who have sex with men

Reference

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Hoque ME, Hope VT, Scragg R, Kjellstrom T, Lay-Yee R. Nappy handling and risk of giardiasis. Lancet. Mar 31 2001;357(9261):1017-1018.
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Welch TP. Risk of giardiasis from consumption of wilderness water in North America: a systematic review of epidemiologic data. Int J Infect Dis. 2000;4(2):100-103.
Stuart JM, Orr HJ, Warburton FG, et al. Risk factors for sporadic giardiasis: a case-control study in southwestern England. Emerg Infect Dis. Feb 2003;9(2):229-233.