Encystation is a continuous process during infection. After 15 hours, cyst wall proteins are visible. Within 24 hours after the appearance of ESVs, the trophozoite is covered with these cyst wall proteins, the form of the cyst has emerged, and new antigenic differences are present. From 1964-84, G. lamblia caused at least 90 waterborne outbreaks of diarrhea, affecting more than 23,000 people. It is the most common gut parasite in the U.K., and infection rates are especially high in Eastern Europe.
Death is rare and usually occurs in malnourished children.
G. lamblia has been implicated as the chief cause of growth retardation in infected children, even after control for other diarrhea-causing agents. There are no racial differences in infection. Infection occurs equally in both sexes. Many of the epidemics documented over the last 2 decades have originated in day-care centers. The pathogenesis of diarrhea in giardiasis is unknown.