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. Proposed mechanisms include occlusion of the mucosa by large numbers of the organisms, competition with the host for nutrients, epithelial damage, immune mediated absorptive changes, altered mucus secretion, and alterations in motility. The parasite is known to invade the colonic epithelium, but the significance of invasion is not known. The most important result of invasion by the parasite is subsequent activation of host immune processes.