Pathogen and Environment

Causal Agent

Giardia duodenalis is a protozoan flagellate (Diplomonadida). This protozoan was initially named Cercomonas intestinalis by Lambl in 1859. It was renamed Giardia lamblia by Stiles in 1915 in honor of Professor A. Giard of Paris and Dr. F. Lambl of Prague. However, many consider the name Giardia duodenalis to be the correct taxonomic name for this protozoan.

Giardia is protected by an outer shell called a cyst that allows it to survive outside the body for long periods of time and makes it somewhat tolerant to chlorine disinfection.

Life Cycle

  • Giardia cysts can contaminate food, water, and surfaces, and they can cause giardiasis when swallowed in this infective stage of their life cycle. Infection occurs when a person swallows Giardia cysts from contaminated water, food, hands, surfaces, or objects.
  • When Giardia cysts are swallowed, they pass through the mouth, esophagus, and stomach into the small intestine where each cyst releases two trophozoites through a process called excystation. The Giardia trophozoites then feed off and absorb nutrients from the infected person.
  • Giardia trophozoites multiply by splitting in two in a process called longitudinal binary fission, remaining in the small intestine where they can be free or attached to the inside lining of the small intestine.
  • The Giardia trophozoites then move toward the colon and transform back into cyst form through a process called encystation. The Giardia cyst is the stage found most commonly in stool.
  • Both Giardia cysts and trophozoites can be found in the stool of someone who has giardiasis and may be observed microscopically to diagnose giardiasis. Giardia cysts are immediately infectious when passed in the stool or shortly afterward, and the cysts can survive several months in cold water or soil.
Life cycle of Giardia

Life cycle image and information courtesy of DPDx.