Giardia is a genus of protozoa discovered in 1681. Six morphologically distinct species are recognized. It mainly attaches in the upper GI tract of a wide variety of vertebrates (including zebrafish), often with beaver and muskrat as reservoirs/carriers but exhibiting minimal—if any—disease in some animals. Giardia is usually non-pathogenic in the human population, even in children if exposed early in life. Although Giardia can be pathogenic, some strains colonize the gut with no malady. This parasite is not invasive and only serious infections depress the small intestine. Giardia are pear-shaped, have an adhesive disc for attaching to enterocyte cells in the small intestine villus, and move with eight designed flagella. In the post-Fallen world, Giardia infection occasionally has resulted in digestive dysfunction. However, Giardia may function in non-parasitic, possibly mutualistic, ways. For example, it may have been designed to aid digestion having a role as a “primer.” The presence of Giardia muris causes a fundamental change in the microbiome in mice and Giardia may have other influences on the microbiome such as enhancing digestion in certain animals and possibly shifting ratios of bacteria from anaerobic to aerobic. Giardia may play a role in host metabolism and provide nutritional enhancement via its association with enteric bacteria, like E. coli. The function of Giardia may parallel with non-parasitic tasks found in Trypanosoma lewisi, and also termite systems that contain protozoa and bacteria for plant digestion. Giardiahas two “faces” even in today's world: a harmless commensal in wildlife and a pathogenic parasite in humans.
Gillen, Alan L. and Sherwin, Frank, "The Design of Giardia and the Genesis of Giardiasis" (2017). Faculty Publications and Presentations. 126.