15 “New” Intestinal

Abstract

This chapter discusses the biology and epidemiology of new intestinal parasitic protozoa. Many new intestinal parasitic protozoa have been identified as potential and actual human pathogens. Although most of these organisms were recognized over a century ago either in nature or in multiple animal and bird species, their role in causing human disease is new or was not appreciated for a long time. Most of these new parasitic protozoa are intestinal pathogens, such as Giardia and the spore-forming Cryptosporidium, microsporidia, Isospora , and Cyclospora . Other organisms that have been appreciated include Babesia infecting human erythrocytes and Naegleria causing a form of meningoencephalitis. The characteristic morphological features of the genus, Giardia, are well established. The life cycle of Giardia infection in humans or other animal hosts is simple. Infection is established through the ingestion of parasite cysts or in rare occasions the trophozoite stage. Excystation occurs in the stomach and may be aided by several factors, including gastric acidity, pancreatic secretions, and carbon dioxide concentration.