Toxoplasma in the Norway Rat

Abstract

It is the consensus that human cases of toxoplasmosis originate from some animal reservoir or reservoirs. It is well known that Toxoplasma gondii Nicolle and Manceaux occurs in many diverse species of animals, and strains which have been isolated from birds and mammals appear immunologically and pathologically similar. A logical goal of epidemiological study wduld be to determine which species of animals could, by virtue of their habits and associations with man, serve as reservoirs from which the infection reaches man. The Norway rat (Rattus norvegicus) is always closely associated with man and serves as a source of human infection in endemic typhus fever, plague, and other diseases. Toxoplasma was first found in this rodent by Sangiorgi (1914), in an albino laboratory rat. Perrin et al. (1943) in Savannah, Georgia, conducted the only study on record in which the incidence of toxoplasmosis in a population of wild Norway rats was determined. They found infection in 8.7 per cent of the rats of that city examined during the period from March, 1941 through May, 1942. The purpose of this report is to extend the knowledge of Toxoplasmla prevalence in Rattus norvegicus to another geographical locality. METHODS Wild Norway rats were caught alive in various sections of the City of Memphis, Tennessee, in modified rabbit traps (Richter and Emlen, 1945). The rats were transported to the laboratory and killed with chloroform, usually the morning of the day they were captured. Weight, sex, and other characteristics were recorded following which dissection was carried out. An attempt was made to isolate Toxoplasma from the wild rats by inoculating the brains of the rats into either mice or guinea pigs. In the case of mouse inoculation, the brain of each individual rat was ground in about 10 cc. of physiological saline with a mortar and pestle. Three mice were then inoculated intraperitoneally with 0.5 cc. of the tissue suspension. All operations were performed as quickly as possible after the killing of the rat. After inoculation, the group of three mice was observed for four weeks. Any animals dying during this period were autopsied and tissue smears made of the peritoneal fluid, liver, spleen, lung, and brain. Autopsy smears were stained with Giemsa stain and 100 microscopic fields from each smear were examined for Toxoplasma. Tissues from any mice dying under circumstances indicating toxoplasmosis were put into clean mice in order to obtain the strain for comparative study. At the end of four weeks, if all three mice survived, two were sacrificed and smears made at autopsy. If one or more had died before this time, the remaining