Quantitative Determinations of Critical Mortality Periods in Untreated and Treated Infections with the B Strain of Trypanosoma cruzi in Mice

Abstract

The mortality patterns of untreated experimental infections with the B strain of T. cruzi in female, Manor-Swiss mice are described, following intraperitoneal or subcutaneous inoculation with varying numbers of parasites. A positive correlation occurred between initial parasite densities and degrees of virulence following both routes of inoculation, but greater relative degrees of virulence and more uniform mortality patterns resulted when the subcutaneous route was employed. Examples are given of the use of quantitative survival-time data for measuring chemotherapeutic activity in highly standardized experimental infections with the B strain of T. cruzi in mice. The B strain of Trypanosoma cruzi, originally isolated from a Brazilian patient in 1942, has been used extensively in this country for experimental studies on Chagas' disease (von Brand et al., 1949; Hauschka, 1949; Hauschka et al., 1950; Goble, 1951, 1952; Norman et al., 1959). Published reports on its degree of virulence in laboratory animals are divergent, although the techniques of inoculation and the number and frequency of in vivo or in vitro passages have varied considerably. Von Brand et al. (1949) stated that this strain killed all rats during the 3rd week of infection. Survival times in male mice inoculated subcutaneously with 500,000 parasites from mouse donors ranged from 11 to 32 days (in later experiments, 9 to 28 days) according to Hauschka (1949). Median survival times of 17 days in male mice and 21 days in female mice were observed by Goble (1951) after intraperitoneal inoculation with 30 million cultural forms. In a later paper Goble (1952) stated that the median survival time in Webster female mice inoculated with 30 million cultural forms was about 28 days, with 85% mortality within 60 days. Norman et al. (1959) obtained only transient infections in mice inoculated with cultural forms, stating that virulence was lost during a 5-year period in the laboratory. Since March 1948, the B strain of T. cruzi, obtained through the courtesy of the National Institutes of Health, has been used in our laboratories for studies on experimental chemotherapy. More than 500 consecutive animal passages have been carried out to date, usually in mice but occasionally in weanling rats, and little relative change in virulence has been noted during this period. The course of infections has been altered markedly in different experiments by changes in the size and route of the inocula, but predictable mortality patterns occurred in different groups of mice when comparable experimental conditions were maintained. This paper presents representative data on the characteristics of mouse infections with the B strain of T. cruzi under the experimental conditions employed in our laboratories during the past 2 years, with particular reference to (1) changes in degree of virulence following different initial parasite densities or different routes of inoculation and (2) responses to chemotherapy. Emphasis has been placed on the use of quantitative survival-time data, rather than measurements of peripheral parasitemia, for determinations of degree of virulence and alterations in the course of infections following the introduction of chemotherapeutic agents. MATERIALS AND METHODS Female white mice, Manor-Swiss strain, weighing from 18 to 22 g, were used as experimental hosts throughout the studies described herein. Received for publication 7 July 1962. * From the Experimental Therapeutics Research Section, Lederle Laboratories Division, American Cyanamid Company, Pearl River, New York.