Protozoa Inhabiting the Blood and Tissues.—These are both numerous and important. Only a few can be dealt with.
Malaria.—Plasmodium, the causative organism of malaria (q.v.), is represented by three species parasitic in man. Several other varieties are known to at tack other mammals. The para sites live in the red blood cor puscles, where they grow, form a pigment, and finally divide up into a number of small bodies which are liberated by the break ing down of the containing cor puscle. These small bodies, the merozoites, then penetrate other blood-corpuscles and again grow and divide, thus repeating the asexual cycle indefinitely. Since each cycle takes a definite time for its completion, the pigment formed by the parasite is liber ated into the blood at regular in tervals, and by its toxic action fever is set up. Some of the me rozoites develop into sexual forms which circulate in the bloodstream without further devel opment until they are ingested by a female anopheline mosquito. Fusion of the sexual elements takes place in the stomach of the mosquito and the resultant organism passes into the body-cavity where it forms a cyst. Within this cyst numerous small, spore-like bodies (sporozoites) develop. These make their way into the sali vary glands of the mosquito, and, at the next feed are injected into the wound creating a fresh infection. The three species of Plas modium causing human malaria are distinguishable by microscopic examination in all their stages. Further, while P. vivax and P. falciparum complete the asexual cycle in 48 hours, causing tertian fever, i.e., fever recurring every third day, P. malariae develops more slowly and produces quartan fever, i.e., fever recurring every fourth day.
Trypanosomes and Sleeping Sickness.—Trypanosomes, the causative organisms of sleeping sickness (q.v.) in man and similar diseases in cattle and game in Africa, are flagellate parasites which inhabit chiefly the liquid portion of the blood. They consist of an elongated, flattened, binucleate body, one edge of which forms an undulating membrane, the outer margin of which is supported by a flagellum which either becomes free at the anterior end or terminates with the membrane.
The life-cycle is passed partly in the mammalian host and partly in the gut of a blood-sucking insect, the transmittor. In African sleeping sickness the trypanosome, T. gambiense or T. brucei, is injected into the blood-stream of the definitive host, where it multiplies with great rapidity, by the bite of a tsetse fly. Some of the trypanosomes eventually make their way into the cere brospinal fluid, where they produce the symptoms of the disease. The ingested trypanosomes pass into the mid-gut of the fly where multiplication takes place and the resulting flagellates, the in fective forms, pass forward into the salivary glands from which they are ejected at the next feed to recommence the cycle in the definitive host. Other species of African trypanosomes differ slightly in their life cycle and mode of transmission.
Chagas's disease, prevalent among children in South America, is transmitted by certain bugs, the faeces of which are infective when rubbed under the skin. The trypanosomes multiply in the
tissues of the body where non-flagellate stages occur.
Leishmania Donovani and Leishmania Tropica.—Kala-azar, a disease widely distributed in India, China, Russia, Africa and the Mediterranean littoral, is caused by a flagellate parasite, Leishmania donovani, which has recently been shown to be trans mitted by a species of sand-fly. The disease attacks chiefly chil dren and young adults and is caused by the parasites invading the cells constituting the walls of the capillary blood-vessels in the bone-marrow and glands of the body. The parasite as seen in the definitive host is a minute, rounded, non-flagellate body, con taining two nuclei which differ from one another in form, one being rod-shaped, the other rounded. These bodies multiply by fission within the host-cells which become packed with parasites. The organisms then escape and enter other cells in which re production is continued. Many find their way into the white blood corpuscles and circulate in the peripheral blood, with which they are taken up by bloodsucking insects. In the sand-fly they be come flagellated. The body elon gates and a single long flagellum. arising from the neighbourhood of the rod-shaped nucleus, ex tends anteriorly, free of the body. Within the intestine and phar ynx these flagellates reproduce by longitudinal fission and give rise to forms infective to man.
L. tropica, while being mor phologically identical with L. don ovani, produces an infection that is limited to the skin and the mucosa of the nose, throat and mouth, where it produces ulcerat ing sores. This parasite occurs in the New as well as the Old World. It also is probably trans mitted by a species of sand-fly.
The Spirochaetes.—The spirochaetes are microscopic organisms with slender, thread-like, spirally twisted bodies. Movement is effected by wave-like motions of the flexible body, often accom panied by rotation. Their inclusion among the Protozoa is of doubtful validity, for, although they exhibit some protozoal char acters, they seem more closely allied to the Bacteria.
Spirochaetes occur in the intestines and blood of man and most domestic animals. In the latter they are usually non-pathogenic, but in man, when they invade the blood-stream and internal organs, they give rise to various diseases. Treponema recurrentis, the parasite of relapsing fever (q.v.), prevalent in Africa, Asia and America, is transmitted by lice and ticks to the human blood stream. Treponema pallidum, the causative organism of syphilis (see VENEREAL DISEASES), is a tissue parasite, as also are the spirochaetes associated with yaws and acute infective jaundice. The latter is thought to be trans mitted by water contaminated by the urine of naturally infected rats. Rat-bite fever, a disease following the bite of rats, or other small mammals, is due to an organism, Spirillum minus, which occurs in the mouths of these animals.