Giardiasis: Pathogenesis of Chronic Diarrhea and Impact on Child Growth and Development

In a letter to the Royal Society (London) dated November 4, 1681, Antony van Leeuwenhoek gave what seems to be the first account of the human intestinal protozoan pathogen Giardia lamblia (1). Historically this is of great interest since he used his own stools as a source of the parasite, and his own invention, the microscope, to visualize it. Despite the three centuries that have now intervened, little is known of the mechanism by which Giardia colonizes the upper small intestine and causes diarrheal disease. Indeed, it is only during the past few decades that the organism has been elevated from its place as a commensal to that of an important pathogen. Giardia lamblia is now considered the most common human enteric protozoan parasite and is found throughout temperate and tropical regions of the world (2,3). Prevalence of Giardia cyst excretion varies from 5 to 10% in the United States and Europe to much higher rates in the tropics (2). Increased prevalence is also found in residential institutions (4), day-care facilities (5), and in sexually active male homosexuals (6,7), when person-to-person spread by fecal-oral contact is the mode of transmission. In most countries Giardia is endemic, but water-borne epidemics have been reported with increasing frequency in Japan (8) and the United States (9). In all instances cyst-contaminated water was thought to be the cause of the epidemic, and in some of these cases clear evidence of fecal contamination of domestic water supplies with recovery of cysts from apparently treated water has been shown to be the basis of the outbreak (10). An interesting addition to our knowledge of the epidemiology of Giardia has been the observation that both wild and domestic animals, particularly the beaver and the dog (11), may harbor the parasite and therefore act as potential reservoirs of human infection.