Sources of Infection & Risk Factors

Multiple Giardia trophozoites

Giardia trophozoites. 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 from person-to-person or even from 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:

a Giardia cyst

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

  • Travelers to countries where giardiasis is common
  • People in childcare 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

The risk of humans acquiring Giardia infection from dogs or cats is small 17-19,13. The exact type of Giardia that infects humans is usually not the same type that infects dogs and cats 17,18,13.

Learn more about Giardia and pets.

Giardia intestinalis (aka: G. duodenalis, G. lamblia) can be subdivided based on molecular analysis into what are known as different genetic assemblages (A,B,C,D,E,F, and G). Some of these assemblages can be classified even further into subtypes like for example A-I, A-II, A-III, A-IV. Each assemblage is capable of infecting certain species, and some assemblages are more commonly seen than others 18,20,21.

Molecular Characterization
Assemblages Some Species Commonly Infected
A-I Humans and animals (cats, dogs, livestock, deer, muskrats, beavers, voles, guinea pigs, ferrets)
A-II Humans (more common than A-I)
A-III and A-IV Exclusively animals
B Humans and animals (livestock, chinchillas, beavers, marmosets, rodents)
C and D Dogs, coyotes
E Alpacas, cattle, goats, pigs, sheep
F Cats
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