Prevalence of Naturally Occurring Toxoplasma gondii Infections in Cats from U. S. Military Installations in Japan

Abstract

To assess the possible role of the domestic cat as a source of human infection with Toxoplasma gondii, cats belonging to U. S. Forces families were obtained from installations in the Kanto Plain (Tokyo) area. A total of 90 cats of all ages were examined for serum antibodies by the indirect fluorescent antibody test (IFAT), for tissue cysts by mouse inoculation, and for fecal oocysts by microscopic examination and mouse inoculation. Forty (44%) had antibodies to toxoplasma, and the organism was isolated from tissues of 11%. An isolation was made from feces of 1 adult cat. This indicates that domestic cats could be a significant source of soil contamination with infectious oocysts in U. S. military housing areas. For many years the mechanism of transmission of human-acquired toxoplasmosis was unknown. Other than the implication of raw or undercooked meat in a few situations (Desmonts et al., 1965; Kean et al., 1969; Magaldi et al., 1969) no sources of natural human infections have been identified. The recent discovery of a resistant oocyst form of Toxoplasma gondii, which is shed in the feces of the cat (Frenkel et al., 1970; Hutchison et al., 1970; Sheffield and Melton, 1970), has offered a simple and apparently logical explanation for the means of transmission of this widespread infection. However likely a source of human infection the oocyst might appear to be, the true role of this stage in the epidemiology of human infection has yet to be determined. The intestinal cycle producing the infectious oocyst is known to occur only in the domestic cat and in two species of wild felines (Frenkel, 1970). Experimental feedings of tissue cysts to ferrets and dogs (Witte and Piekarski, 1970), mice, rats, hamsters, guinea pigs, dogs, domestic rabbits, opossums, Japanese quail, chickens, and pigeons (Frenkel et al., 1970) have all failed to yield oocysts. Although these studies do not rule out the possibility of other species serving as reservoir Received for publication 17 March 1972. * Present address: Department of Biology, Northern Michigan University, Marquette, Michigan 49855. tPresent address: U. S. Army Medical Research Unit, Panama, Box 2011, Balboa Heights, Canal Zone. ho ts, they do leave the domestic cat as the only current likely suspect. The house cat has been shown to be frequently infected in many parts of the world (Dubey, 1968; Work, 1969). In Japan, only small numbers have been examined, but high infection rates, over 60%, were reported (Katsube et al., 1967). The domestic cat is frequently maintained as a household pet by U. S. military families, but the high cost of shipping usually precludes taking them back to the U. S. This, combined with the close official control over pets within U. S. Forces installati ns, provided an unusual opportunity to gain additional information regarding the prevalence of infection in cats in Japan, and specifically those of U. S. families, to assess their role as a potential reservoir of Toxoplasma gondii. MATERIALS AND METHODS Cats were obtained from donors and from pounds at triservice military installations throughout the Kanto Plain area of Japan. Immediately after acquisition, 1 to 3 stools were collected whenever possible from each cat and processed by sucrose (sp gr 1.18) flotation. Microscopic examination was made, and following incubation at room temperature for 6 to 10 days in 2% potassium dichromate, 0.5 ml of washed fecal suspension was injected ip into each of 5 mice. The cats were bled by heart puncture, killed by sodium phenobarbital injection, and examined as follows: (1) In the largest group consisting of 72 cats, approximately 3 g of brain tissue was homogenized in a tissue grinder in 10 ml of physiologic saline with 0.05 ml of penicillin-streptomycin solution, and 0.5 ml was injected ip into each of 6 mice. (2) In a second group of 9 cats, in addition to