Classification of human parasites, vectors, and similar organisms.

When common names are used to describe parasites and parasitic infections, these names may refer to different parasites in different parts of the world. To eliminate these problems in interpretation of names, a binomial system of nomenclature is used in which the scientific name consists of the genus and species. Classification systems have been developed to indicate the relationship among the various species of parasites, and these schemes have been based primarily on life cycles and morphology of the organism. Closely related species are placed in the same genus, related genera in the same family, related families in the same order, related orders in the same class, and related classes in the same phylum, one of the major categories in the animal kingdom. Organisms in each category will have characteristics in common. Parasites of humans are classified in six major divisions. These include the Protozoa (amebae, flagellates, ciliates, sporozoans, coccidia, microsporidia), the Nematoda or roundworms, the Platyhelminthes or flatworms (cestodes, trematodes), the Pentastomids or tongue worms (may be grouped with the arthropods), the Acanthocephala or thorny-headed worms, and the Arthropoda (e.g., insects, spiders, mites, ticks). Although these categories appear to be clearly defined, there may be confusion in attempting to classify parasites, often due to the lack of known specimens or the presence of strains or races of the same species with slightly different characteristics. Reproductive mechanisms have been used as a basis for determining definitions of species, but there are many exceptions within parasite groups. Another difficulty in recognition of species is the ability and tendency of the organisms to alter their morphological forms according to age, host, or nutrition, which may result in different names for the same organism. In many cases, there may be alternation of parasitic and freeliving phases in the life cycle. These organisms may be very different and difficult to recognize as belonging to the same species. However, newer molecular methods of grouping organisms have often confirmed taxonomic conclusions reached hundreds of years before by experienced taxonomists. As studies continue in parasitic genetics, immunology, and biochemistry, the species designation will be defined more clearly by use of highly sophisticated molecular techniques. No attempt has been made to include every possible organism, but only those considered to be clinically relevant in the context of human parasitology. Not every organism listed below is a human pathogen. Also, although new classification names are proposed frequently and there may be some disagreement among scientists, this list is designed to provide nomenclature that is currently in use.