infectious diseases
Approach to Parasitic Infections
Introduction
Human parasites are organisms that live on or in a person and derive nutrients from that person (its host). There are 3 types of parasites: Single-cell organisms (protozoa, microsporidia) Multicellular helminths (worms) Ectoparasites such as scabies and lice Parasitic infections due to protozoa and helminths are responsible for substantial morbidity and mortality worldwide. They are prevalent in Central and South America, Africa, and Asia. They are much less common in Australia, Canada, Europe, Japan, New Zealand, and the United States. By far, the greatest impact is on residents of resource-limited tropical areas with poor sanitation, but parasitic infections are encountered in resource-rich countries with adequate sanitation systems among immigrants and travelers returning from endemic regions and, on occasion, even among residents who have not traveled, particularly those with HIV infection or other conditions that cause immunodeficiency. Some parasites have adapted to living in the lumen of the intestine or vagina where conditions are anaerobic; others reside in blood or tissues in aerobic conditions. Many intestinal parasitic infections are spread through fecal contamination of food or water. They are most frequent in areas where sanitation and hygiene are poor. Some parasites, such as hookworms, can enter the skin during contact with contaminated dirt or, in the case of schistosomes, with freshwater. Others, such as malaria, are transmitted by arthropod vectors. Rarely, parasites are transmitted via blood transfusions or shared needles or congenitally from mother to fetus. Some parasites are endemic in the United States and other resource-rich countries. Examples are the pinwormEnterobius vermicularis,Trichomonas vaginalis,Toxoplasma gondii, and enteric parasites such asGiardia intestinalis(also known asG. duodenalisorG. lamblia) andCryptosporidiumspecies. The characteristics of protozoan and helminthic infections vary in important ways. 