Effects of Different Conditions on Duration of Infectivity of Toxoplasma gondii Oocysts

Abstract

Washed oocysts and cat feces containing oocysts were exposed to various environmental conditions in the laboratory and outdoors. Infectivity was tested every 14 days by injecting samples through tubes into stomachs of mice. The brains of the mice were examined (in wet films) 30 to 60 days later for T. gondii cysts. In the laboratory oocysts retained infectivity from 30 days (in uncovered dishes at 37 C) to 410 days or longer in covered and uncovered dishes at 4 C. Outdoors, infectivity was retained from 46 days (uncovered, exposed to direct sunlight, mean air temperature 20 C) to 410 days or longer (covered in shade, mean air temperature 19.5 C). Since cats normally bury their feces, the results suggest that T. gondii oocysts may remain infective for a year in warm climates, and longer in cool climates or in air-conditioned buildings. On the basis of 1969 discoveries, a number of authors in 1970 described the oocyst of Toxoplasma gondii which is passed for a brief period in the feces of experimentally infected cats. The literature, including recent discoveries, is reviewed by Dubey et al. (1970) and Hutchison et al. (1971). The oocyst of T. gondii seems morphologically similar but not identical to that of Isospora bigemina var. cati (Railliet and Lucet, 1891), called Isospora cati by Dubey et al. (1970). Toxoplasma is not considered a synonym of Isospora as Overdulve (1970) proposed because its life cycle is different. The coccidian oocyst is adapted to resist adverse conditions of the environment outside the host. As one phase of a complex study, we attempted to determine how long Toxoplasma gondii oocysts would remain infective under various environmental conditions, indoors and outdoors. MATERIALS AND METHODS Six 4-week-old kittens, born in the laboratory and negative for Toxoplasma by Sabin-Feldman dye test and fecal examination, were experimentally infected with the Beverley strain of T. gondii. This strain had been maintained in our laboratory through repeated passages from mouse to mouse Received for publication 17 March 1972. * From part of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Animal Parasitology, Texas AM monthly averages of daily maxima and minima were 5.5 to 35.5, monthly means were 12 to 29 C, and the mean for the year was 19.5 C. In each of the tests with 3 types of preparations under 5 sets of conditions, covered petri dishes we e paired with petri dishes having no cover. In each covered petri dish, the preparation was further protected by a pad of 10 layers of gauze whic was kept moist by rewetting every 3 to 4 days, or as often as needed to prevent complete drying. Covering and lack of covering in each of the 15 tests raised the total number of treatments