Host parasite interactions and pathophysiology in Giardia infections.

Giardia is a protozoan parasite of the small intestine, and a leading cause of diarrhoeal disease worldwide in a variety of animals, including humans. The host-parasite interaction and pathophysiological processes of giardiasis remain incompletely understood. Current research suggests that Giardia-induced diarrhoeal disease is mediated by small intestinal malabsorption and maldigestion, chloride hypersecretion and increased rates of small intestinal transit. Small intestinal malabsorption and maldigestion results from the CD8+ lymphocyte-induced diffuse shortening of brush border microvilli. Activation of CD8+ lymphocytes occurs secondary to small intestinal barrier dysfunction, which results from heightened rates of enterocyte apoptosis and disruption of epithelial tight junctions. Both host and parasite factors contribute to the pathogenesis of giardiasis and ongoing research in this field may elucidate genotype/assemblage-specific pathogenic mechanisms. Giardia infections can result in chronic gastrointestinal disorders such as post-infectious Irritable Bowel Syndrome and symptoms may manifest at extra-intestinal sites, even though the parasite does not disseminate beyond the gastrointestinal tract. The infection can cause failure to thrive in children. Furthermore, there is now evidence suggesting that Giardia symptoms may vary between industrialised and developing areas of the world, for reasons that remain obscure. More research is needed to improve our understanding of this parasitic infection which was recently included in the World Health Organisation "Neglected Disease Initiative".