Immune response-mediated pathology in human intestinal parasitic infection

Parasites have colonized the human gastrointestinal tract since the beginning of time, often coexisting harmoniously with their host. Asymptomatic carriage is the most common form of intestinal parasitic infection worldwide. Even for now well established pathogens such as Giardia intestinalis intense debate continued throughout the first half of the 20th century as to whether this organism was a true pathogen, presumably because it was so commonly identified in subjects without symptoms (1). In this setting parasites are usually present in relatively small numbers, producing little if any perturbation of intestinal structure or function and probably controlled by a combination of innate and acquired host immune responses. There is even some evidence to suggest that some forms of intestinal parasitism may be advantageous to the human host. Chronic infection with some intestinal helminths, for example, may reduce immune-mediated pathology to other bacterial infections, such as Helicobacter pylori, by converting the host immune response to the latter to a Th2 rather than a Th1 response and thereby reducing tissue damage (2). There are, however, continuing concerns about the enormous human burden of intestinal parasites worldwide and the deleterious effects this has on health particularly for children and adolescents with growth potential.