There are two classes of infectious agents called parasites. Protozoa are single cell animals which characteristically divide and multiply within the host, are usually direct fecal-oral in transmission, and do not cause an eosinophilia. Helminths (worms) are multi-cellular animals and with rare exceptions do not multiply within the host and are often associated with some degree of eosinophilia that correlates with the degree of mucosal invasiveness. Worms have a limited life span within the host and without reinfection eventually die on their own.
Most worms require either a prolonged incubation period outside the host before being infectious or need a specific vector for transmission. A notable exception to this rule is Enterobius vermicularis (pinworm), the eggs of which are infectious shortly after being passed, so auto-infection occurs readily. Direct person-to-person transmission is uncommon.
The likelihood of acquiring an intestinal parasite depends on several factors - the presence of the specific infectious agent, an appropriate "vector" or mode of transmission, and a host who is susceptible to the infectious agent. The world-wide distribution of parasites is determined by geographic factors, socio-economics, age, and crowding with poor food preparation and a break in the standard of water and personal sanitation being the major factors.
Risk factors include: Age (children), Low socioeconomic status, Poor sanitation (personal, food, water), International travel, Crowding (day care centers, mental institutions), Intercurrent medical conditions, pregnancy, gastric hypoacidity, and immunosuppression (AIDS).
A single stool specimen provides an accurate diagnosis in 90% of patients. Additional specimens will need to be examined for greater diagnostic accuracy. Special techniques for the detection of Cryptosporidium, Isospora belli, Cyclospora, and microsporidia often require that the laboratory be informed of the "risk" profile of the patient before these tests will be done. Pinworm paddles provide a greater diagnostic yield when Enterobius vermicularis is being considered. Multiple tests (5) may be needed to exclude the diagnosis of pinworms. A parasite culture is possible for a few organisms - Giardia lamblia, Entamoeba histolytica, Strongyloides stercoralis, but are rarely indicated and are usually available only in referral laboratories.
String tests and upper bowel intubations are rarely needed to diagnose the upper intestinal parasites. Rarely, a biopsy will demonstrate the presence of an invasive helminth on tissue section. Worms can be extremely difficult to diagnose in this manner, usually needing the expertise of a tissue parasite pathologist. The other parasites may be visualized on the mucosa or in the mucous layer.
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