PARASITOLOGY. The object of parasitology is the study of those organisms (parasites) which during the whole or a part of their life live upon other organisms (hosts). The parasite does not destroy its host immediately or rapidly even when both are of the same size. On the contrary, a perfect parasite while de riving its food and often protection from the body of the host, lives economically in doing as little direct damage to the host as possible. In this respect it differs from a predatory or carnivorous organism which recklessly destroys and utilizes its victim. The association between the parasite and the host is beneficial and essential only for the parasite, while it is not beneficial, almost al ways harmful, and often fatal for the host. It is, however, impos sible to draw a sharp line separating parasitism from other modes of life such as saprophytism, carnivorism, commensalism, mutual ism and symbiosis.
Parasites, according to their mode of association with the host, can be divided into various groups such as: (I) permanent para sites which are parasitic throughout their life (tapeworm) : (2) temporary parasites which live upon their host only during certain periods of their development (maggot of a warble fly) : and (3) intermittent parasites which come only temporarily in contact with their host. According to the localization, the parasites may be separated into ecto- and endoparasites. The latter may invade the open cavities or the tissues of different organs; they may live inside the closed cavities of deeper organs ; or inside the proto plasm, or even nuclei, of the cells.
No sharp line can be drawn between the different categories of parasitic adaptation. The following examples will illustrate a typical transition between the free-living mode of life, intermittent and stationary ectoparasitism and endoparasitism. Among the mosquitoes only the females are blood-sucking, while the males are harmless and live upon plant juice. (I) There are, however, some wood mosquitoes which can complete their life history with out taking a single meal of blood. (2) The malarial mosquito
(Anopheles) can live without taking blood but the female will not lay eggs unless it has at least one blood meal. (3) The yellow fever mosquito (Stegomyia) lives and breeds near houses, and the female sucks blood more often. (4) The bed bug (Cimex) lives in the house and very near its food supply with which it can get in contact at more or less regular intervals. (5) The flea remains upon the host much longer than is required for the meal. (6) The body or head louse remains always upon the host, over the body of which it moves freely. (7) The crab louse is more sta tionary in habit, it moves very little, and is more adapted to the host. (8) In the case of the chigger flea (Sarcopsylla), common in tropical countries, the female penetrates deep into the skin forming a small tumour. (9) Finally, there are larvae of insects which form deep galleries under the skin or invade open cavities of the host producing more or less dangerous cases of myiasis.
Distribution of Parasitism.—The parasitic mode of life is very widely distributed among bacteria, fungi, higher plants and animals. The parasitic bacteria (see BACTERIOLOGY) are known to invade and produce various diseases in man, animals and plants. In man for instance, tuberculosis, leprosy, diphtheria, pneumonia, enteric fever, tetanus, cholera, plague and other diseases are caused by parasitic bacteria. Relapsing fever and syphilis are caused by parasitic spirochaetes, which are organisms allied to bacteria.
The entire class of fungi is composed of organisms which de rive their nourishment from other organisms, living or dead, and they contain an enormous number of forms parasitic on plants and on animals. They cause the smut and rusts of cereals ; mildew of grape-vine; and blight of potatoes. In animals and man they cause ringworm, actinomycosis and many other diseases.