Diagnosing Medical Parasites: A Public Health Officers Guide to Assisting Laboratory and Medical Officers.

摘要

Parasitology is the study of parasites and as such does not include bacterial, fungal or viral parasites. Human parasites are separated into intestinal and blood-borne parasites. For a parasite to be defined as intestinal it must have an intestinal life-cycle stage, though it may have life- cycle stages in the heart, circulation, lung, tissue, other animals or the environment. Parasites found in the intestines can be categorized into two groups: Protozoa and Helminths. Protozoa are single celled organisms. There are four classes of Protozoa commonly found in concentrated fecal samples. These are differentiated by the method of motility. Protozoa include Entamoeba, Giardia, Trichomonas, Cryptosporidium, Isospora, Pneumocystis and Balantidium. There are two diagnostic life-cycle stages commonly seen in parasites - the cyst and the adult trophozoite stage. The trophozoite stage is analyzed directly on a slide without concentration. Cysts require concentration. The key diagnostic factor is that Protozoan cysts are typically 5-30 m ( m = microns or micrometers) in diameter, and as such are smaller than most Helminth eggs. Due to the size they are particularly difficult to see under the microscope if the sample clarity is bad. The medically important Helminths are nematodes (roundworms), cestodes (tapeworms) and trematodes (flukes). Genera include: Fasciola, Schistosoma, Ascaris, Hookworm, Trichuris, Taenia and Enterobius. The normal stage for examination is the egg stage, although larvae may develop in some organisms (Strongyloides); the diameter of the eggs range from 30 m - 150 m. The other major grouping of parasites is known as blood-borne parasites which are transmitted through an arthropod vector. By far the most important arthropod for transmitting parasitic infections is the mosquito. Mosquitoes are known to carry malaria and filarial nematodes. Different types of biting flies transmit African trypanosomiasis, leishmaniasis and several kinds of filariasis.