Tags: Complex

Diphyllobothrium Latum Infection

D latum is found worldwide, and infection is acquired by ingestion of contaminated raw or improperly cooked freshwater fish. Because of enthusiasm for raw or undercooked fish, Siberia, Europe, Canada, Alaska, and Japan are endemic regions for D latum infection. Once the D latum cyst has been ingested, the worm matures within the human intestine and begins to produce eggs after 5 weeks.


Nematodes (roundworms) are nonsegmented, tapered, bilaterally symmetrical, cylindrical organisms that have complete digestive tracts and reproduce sexually. Although > 500,000 species of nematodes have been described, only a small number are commonly encountered as human parasites. Most nematodes have complex life cycles, sometimes involving several larval forms and intermediate hosts or free-living stages. The pathogenic nematodes may be categorized as primarily intestinal or extraintestinal tissue parasites (Box 1).

Intestinal Nematode Infections

Worldwide, more than 1 billion people are infested with Ascaris lumbricoides, the causative agent of ascariasis or roundworm. More than 4 million people are estimated to be infected in the United States. Infection occurs predominately in the southeastern states and more commonly in younger children, and it is associated with lower socioeconomic status. The organism is acquired through ingestion of embryonic forms of the worm, which are found in fecally contaminated soil.


The genera Leishmania and Trypanosoma are members of the family Trypanosomatidae. These protozoans cause diseases with widely varied clinical presentations as well as geographic distributions, including leishmaniasis, American trypanosomiasis (Chagas’ disease), and African trypanosomiasis (sleeping sickness).

Cryptosporidium, Cyclospora, & Isospora Species & Microsporidia

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.


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.

Toxoplasma Gondii

Toxoplasma gondii infection, or toxoplasmosis, is a zoonosis (the definitive hosts are members of the cat family). The two most common routes of infection in humans are by oral ingestion of the parasite and by transplacental (congenital) transmission to the fetus. Ingestion of undercooked or raw meat that contains cysts or of water or food contaminated with oocysts results in acute infection.