Outbreaks of giardiasis linked to drinking water and food handlers [2] have been documented. Large, prolonged outbreaks linked to recreational water, particularly chlorinated sources, are less common.
Giardia exists in two forms: the trophozoite is responsible for clinical illness, and the cyst for transmission of infection. Transmission of G. lamblia is by the faecal–oral route, and can be person-to-person, animal-to-person, foodborne or waterborne. The infection is acquired by ingesting cysts, which are excreted in the feces. As few as 10-25 cysts are sufficient to establish an infection in some humans. Some livestock may be infected by as few as 1–10 cysts.
As trophozoites pass through the small intestine they encyst where the trophozoites then multiply. Many of the dividing trophozoites are carried toward the colon, and encyst along the way, in response to bile salts and other stimuli. Cysts can appear in the feces from 3 days to 3 weeks after infection, depending on the host species.
Excretion usually begins around the same time the first symptoms (if any) appear. Cysts voided in the feces are the infective stage and are immediately infective if ingested. The cysts may be transmitted directly from one individual to another under circumstances that are conducive to fecal-oral transfer such as in daycare centers or in environments where hygiene levels are compromised
Giardiasis: Cycle and mode of transmission
