A New Look at Infectious Diseases JoxoplasB osEs

Toxoplasma gondii, the causal agent of toxoplasmosis, was first found in a North African rodent called gondi.1 It is an unusual parasite in that it is found in most parts of the world and it has little host specificity. It can infect any warm-blooded animal, including birds, and frequently does so. The prevalence of infection in man, as judged by antibody tests, varies from as low as 7% in Navajo Indians2 to 94% in Guatemala.3 In England the prevalence among adults varies from area to area but on average is about 30%.4 Similar variations in prevalence are found in domestic and semi-domestic animals. Infection becomes established in any animal when toxoplasma trophozoites penetrate and multiply in the cytoplasm of a nucleated host cell, most often a reticuloendothelial cell. After repeated multiplication of the parasites, the host cell becomes distended and finallyruptures, liberating 16 or more toxoplasms each capable of invading another host cell. Repetition of this process causes microscopic foci of damage. Spread may be local, by the blood stream, by the lymphatics, or across body cavities. Small foci are commonly found in the brain, lungs, heart, liver, and skeletal muscle. Multiplication and spread continue until the host develops immunity or, very rarely, dies. Immunity is partly due to antibodies and partly to sensitized reticuloendothelial cells. Reactive changes in lymph nodes and spleen are thought to be concerned with the development of this immunity. As immunity develops extracellular parasites are killed, intracellular multiplication is slowed down, and another stage in the life cycle of the parasite develops: the tissue cysts. While these are small, up to 1000i in diameter, each may contain thousands of toxoplasma zoites which are similar to, but smaller than, trophozoites. The cyst wall screens the zoites from the tissues and the host's defence mechanisms, as well as the tissues from the action of the zoites. Multiplication occurs within a cyst but at a much reduced rate. Cysts persist for