MORBID CONDITIONS DUE TO ANIMAL PARASITES Introductory.—Animal parasites are invertebrate animals which have abandoned their free-range life and have taken up their abode in or upon the living bodies of other animals or plants. Freedom has been exchanged for a more constant and usually richer supply of food and a greater measure of protection.
The term "parasite" connotes habitat rather than zoological grouping (see PARASITISM ; PARASITOLOGY). Several of the main subdivisions of the animal kingdom contain both free-living and parasitic forms but with the specialised parasitic method of living certain physiological and morphological adaptations have often resulted which have rendered a free existence no longer possible. All gradations exist from the raiders which are parasitic only momentarily, for purposes of feeding, to obligatory parasites which cannot now survive if separated from their "host." Many forms which as adults are obligatory parasites still have a free living larval phase in an invertebrate during the free larval devel opment. The animal in which the adult stage of a parasite is at tained is termed the "definitive host." That in which an immature or "larval" stage develops is called the "intermediate host." Some parasites have no free stage or intermediate host. Some have a free stage without intermediate host and others have two succes sive intermediate hosts through which they must pass before reaching adult life in the definitive host. A knowledge of these developmental phases is obviously of great importance in preven tive medicine, for the control of a disease in a community may be attained more readily by destroying the essential intermediate hosts than by stamping out the disease in the members of the community. Climatic conditions limit the spread of parasites by acting upon the free-living stages, e.g., hookworm development is controlled by temperature and only occurs in warm climates. The effect may be indirect by action upon the intermediate host.
The vast majority of internal parasites inhabit the alimentary canal or ducts leading thereto. This indicates probably the origi nal as well as the present route of invasion of most parasites. Oral infection is from food, seldom from water. But in a few striking cases infection enters not by the mouth but by penetrat ing the skin, as in Bilharziasis (q.v.). In some instances penetration follows injection by some insect which has previously sucked up an earlier stage of larval development from another animal.
Animal parasites belong either to the unicellular Protozoa or to the multicellular (metazoan) invertebrates. Protozoan para sites can multiply in their definitive hosts and produce enormous numbers but in the metazoan parasites successive generations do not follow the attainment of sexual maturity in the definitive host. Here the progeny (eggs or embryos) must leave the host body and undergo some part of their larval development outside, or in an intermediate host, before the infective stage is reached when the definitive host can again be parasitized. As the majority of the Metazoa live in the gut, the eggs appear most frequently in the bowel discharges. Microscopical examination of the stools has become therefore an important routine in the detection of infec tion with animal parasites. Intestinal infections with protozoal forms can be diagnosed similarly from the passage of free or encysted stages of these organisms.
Animal parasites affect their hosts by absorbing nourishment, by feeding upon the living tissues or cells, by mechanical injury or obstruction, and by the poisonous effect of their excretions and secretions. Different species affect their hosts in varying degree. In some cases the effect is so slight that the parasite appears to be almost a commensal. Parasitism differs from commensalism in that the host does not benefit, in turn, from the presence of the parasite. But it should be noted that it is not in the interests of a parasite that great harm should result to the host from its presence. A certain amount of tolerance, i.e., of acquired im munity, develops in the host.
Within the definitive host the normal habitat of each parasite is usually strictly limited to some special organ or tissue. The malaria parasites for instance invade the red blood corpuscles. The trypanosomes live in the fluid of the blood. Some species of fluke live in the gut, others in the bile ducts (Plate fig. o) and others again in the lung (Plate fig. II). Adult tape worms almost without exception are inhabitants of the gut while in their lar val stages they infest the solid tissues of the body. Round worms are mainly intestinal in habitat but some live in the tissues, producing tumours, others in the lymphatic and haemal systems, while one of the largest invades the kidney. (Plate fig. 7.)