PARASITIC PROTOZOA The Protozoa (q.v.) are classified in four main groups based on the method of locomotion adopted by the organism. The first group, Sarcodina, move by means of pseudopodia, or extrusions of the body-wall, towards and into which the remainder of the body flows, e.g., Amoeba. The second group, the Flagellata, which possess a more definite body-shape, move by means of one or more long, very slender lash-like organs, which, by their rapid move ment, drag the body through the surrounding medium, e.g., Trypanosomes. Thirdly the Ciliata possess a fur-like coating of short, very fine out-growths, the cilia, which, by their rhythmical wave-like motion, propel the body. The fourth group, the Sporo zoa, form a heterogeneous group of organisms, all of which are parasitic. The adult stages possess no means of locomotion and they reproduce by spore-formation. In many of the Sporozoa the developmental forms have some power of movement, which may be effected by pseudopodia or by flagella, indication that the loss of movement is due to the parasitic mode of life.
This form of classification is beset with many difficulties, as, for example, that some of the Protozoa adopt different means of locomotion at different periods of their existence. Owing to the diversity present in the group and in the primary divisions or classes, numerous subdivisions have been formed within each class.
mission is probably contaminative.
Of the intestinal Flagellata only one species appears to be pathogenic. Giardia intestinalis, frequently associated with diar rhoea, is an eight-flagellated organism, which is also provided with a large sucker by which it clings to the intestinal epithelium. It passes out of the body in the form of cysts and is transmitted by the contamination of food with faeces.
The only ciliate pathogenic to man is Balantidium soli, which, like Entamoeba histolytica, penetrates the walls of the intestine causing ulceration and giving rise to dysenteric conditions. This form also produces cysts which are passed out of the intestine.
The most important sporozoan parasites of the alimentary canal of vertebrates are the Coccidia. These form a widely distributed group of parasites which attack almost all mammals and many of the lower vertebrates and invertebrates. Most species cause denudation of the intestinal epithelium, while a few invade the epithelium of the bile-ducts causing func tional disturbance of the liver. Only one species, Isospora hominis, is parasitic in man, and this has a very limited dis tribution and is not of great pathogenicity. Eimeria zurnii is the cause of epidemics of red dysentery in cattle. E. faurei and E. arloingi infect sheep and goats respectively. Dogs and cats harbour other species of Eimeria and the allied genus Isospora. Two species of Eimeria infect the rabbit. Poultry and game birds are also attacked.
The parasites are transmitted by resistant stages, the oocysts. These when mature contain sporozoites which hatch out on enter ing the intestine and make their way into the epithelial cells or travel up the bile-duct. Within the cells they multiply rapidly by an asexual process, producing merozoites. At the same time sexual forms are developed, the females after fertilization being passed out of the body in the form of resistant oocysts. Outside the body development proceeds, resulting in the formation of sporo zoites within the oocysts. When this development is completed the oocysts are infective if ingested by a new host.