Giardia, a genus of primitive eukaryotes, is a flagellated enteric protozoan of the class Zoomastigophorea. Giardia lamblia, also known as Giardia intestinalis or Giardia duodenalis, is the species known to infect humans. Its name comes from Vilem Lambl, who first reported the organism in 1859.
After ingestion of G lamblia cysts, 5-15% of patients will have asymptomatic cyst passage, and 25-50% of patients will have diarrhea. From 35% to 70% of these patients will have no evidence of infection. The three manifestations of infection include asymptomatic cyst passage, self-limited diarrhea, and chronic diarrhea with associated malabsorption and weight loss. Factors related to each of these manifestations are unknown but are believed to be related to specific host factors, parasite load, and virulence variation among G lamblia isolates.
Within the last decade, the AIDS epidemic has heightened awareness of several gastrointestinal spore-forming protozoan pathogens. The genera Cryptosporidium, Isospora, and Cyclospora are members of the subclass Coccidia and phylum Apicomplexa; the microsporidia are a group of organisms belonging to the phylum Microspora.
Microsporidia were first discovered in 1857, but it was not until 1973 that a human case of microsporidiosis was confirmed from a case described in 1959. Awareness of the diversity of microsporidial infections has heightened, especially in light of the AIDS epidemic. Central nervous system, respiratory, corneal, muscular, and gastrointestinal microsporidial infections have all been identified. Microsporidiosis has been found worldwide.
Isospora infection is endemic in several tropical and subtropical climates in areas of South America, Africa, and southwest Asia. In the United States, Isospora belli infection occurs primarily in patients with AIDS but is still quite rare in this population, accounting for = 0.2% of AIDS-defining illnesses. Isospora infection is more common in patients with AIDS from developing countries in which the prevalence of spore passage is 15% compared with 5% in industrialized nations.
Cyclospora is a coccidian that had been described as a “large cryptosporidium” or “cyanobacterium-like body” before being confirmed as a member of the phylum Apicomplexa in 1993. The life cycle in humans has not been fully detailed.
Enteric cryptosporidiosis is the most common clinical presentation in patient populations. In addition, immunocompromised patients may present with cholecystitis or respiratory infections attributed to C parvum (Box 1).
There are numerous distinct species of ameba within the genus Entamoeba, and the majority of these do not cause disease in humans. E histolytica is a pathogenic species that is capable of causing disease, such as colitis or liver abscess, in humans. E dispar is prevalent and is indistinguishable from E histolytica by conventional laboratory methods. E dispar exists in humans in only an asymptomatic carrier state and does not cause colitis.
Amebic liver abscess is the most common extraintestinal manifestation of amebiasis. Patients may note right-upper-quadrant pain that is either dull or pleuritic in nature.
Of patients infected with E histolytica, > 90% are asymptomatic carriers who are colonized with the organism and pass cysts in the stool. This carrier state often resolves without treatment, although relapses and reinfection are common.