The Interrelation of Toxoplasmosis in Swine, Cattle, Dogs, and Man

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THE ORGANISM Toxoplasma gondii is an obligate intracellular parasite. Until i.iow, no form has been found which is capable of livin-ig for extended periods outside the cells of its niumerous hosts. It is capable of invading and multiplying in a wide variety of cell types, such as neurons, microglia, endothelium, reticulum, parenchyma cells of the liver, epithelial cells of the lung and glanids, and cardiac and skeletal nuscle. The parasite exists in two forms. The proliferative form., seen during the acute stage of the infection, undergoes rapid intracellular mtultiplication, and the numerous loosely grouped toxoplasmas thus produced are liberated by rupture and invade new cells. This form of the parasite is motile, showing twisting movements of its attenuated end and gliding movements unaccompanied by any changes in shape or surface visible by ordinary light microscopy. It measures about 3 by 6 microns, has a centrally loca.ted nucleus, and glycogen granules of small size. The pseudocyst form probably appears late in the subacute stage of the infection and is the only form which persists in chronic infections. Pseudocysts are generally larger in size than the cells parasitized by proliferating forms. The parasites within them, which are closely packed and more lanceolate, have a subterminal nucleus and larger glycogen gralnules (1). The latter can be considered characteristic of a resting organism. The wall of the pseudocyst is considered by soine worlkers a's the remains of the host cell wall to whlich are added some products of the parasite. Others regard it as primarily of parasitic origin. Whatever its origin, the wall of the pseudocyst is argyrophilic and elastic, and at least somewhat resistant to mechanical damage. Also, the psudocyst appears to be more resistant to environmental changes than are proliferative forms (2). According to a number of workers, proliferative forms of Toxop7cw?ma die rapidly outside the host and in the carcass of dead animals. These forms are destroyed on drying, on changes in osmotic pressure, and on exposure to low heat. Pseudocysts are also unable to withstand drying and are killed by low heat. However, they may survive in dead tissues for up to 2 weeks or longer at refrigerator temperatures, possibly with less attrition than proliferative forms (3). The poinit of greatest difference is in survival during digestion. It is revealed indirectly by feeding experiments. Tissues of mice dying of acute toxoplasmosis fed to other mice produce relatively few infections (4). However, when tissues from chroniDr. Jacobs is head of the Section on Protozoal Diseases, Laboratory of Tropical Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Public Health Service. Based on a paper presented before the Conference of Public Health Veterinarians, November 16, 1956, in Atlantic City, N. I.