Antigenic Relationship of Toxoplasma gondii and Besnoitia jellisoni

Abstract

Morphologically Besnoitia and Toxoplasma resemble each other. Immunologically they are different but appear to have certain antigens in common. Rabbits were infected with Toxoplasma gondii and Besnoitia jellisoni both of which had been passed through chick embryo. Dye tests and hemagglutination tests were done using both Toxoplasma and Besnoitia as antigens. The organisms for the dye test and for preparation of HA antigens were derived from peritoneal exudates of intraperitoneally infected mice. The dye test was specific for both organisms with no titer demonstrated in the heterologous system. On the other hand, cross-reactions were demonstrated between Toxoplasma and Besnoitia in the hemagglutination test. The homologous titer was always higher than the heterologous titer. Complement-fixation test using only the Toxoplasma antigen showed a cross-reaction with Besnoitia antisera from these rabbits. Agar-gel diffusion on micro slides revealed precipitin lines in common with both systems. Rabbits infected with Besnoitia showed no immunity when challenged with a virulent strain of Toxoplasma and succumbed as rapidly as controls not previously infected. On the other hand, rabbits surviving infection with an avirulent Toxoplasma strain were immune to challenge with virulent Toxoplasma. Frenkel (1960) reviewed a number of organisms resembling Toxoplasma morphologically. The trophozoites of Besnoitia jellisoni are so similar to Toxoplasma gondii that it would be difficult to distinguish the two organisms with certainty in a mixed population. Goldman et al. (1957), using a silver protein stain, found similar internal structures and reproductive stages in Besnoitia and Toxoplasma trophozoites. Besnoitia cysts are larger than those of Toxoplasma and also have a thicker wall lined by large nuclei (Frenkel, 1955). From previous work it appears that Besnoitia and Toxoplasma are immunologically distinct. Frenkel (1953) using the dye test, found no cross-reactions between the two organisms. Goldman et al. (1957) reported that fluorescein-labeled Toxoplasma antibody did not stain Besnoitia under conditions in which Toxoplasma stained brightly. Although no protozoan parasites have been conclusively incriminated in producing Toxoplasma dye test antibodies, no studies including hemagglutination (HA) antibodies have been reported. Received for publication 28 July 1964. MATERIALS AND METHODS New Zealand white rabbits weighing approximately 2 kg were inoculated intradermally (id) with 10,000 relatively avirulent Toxoplasma harvested from the chorioallantoic membrane of chick embryos infected via the yolk sac 7 days earlier. Rabbits Nos. 1069 and 1070 were inoculated with the Beverley strain and rabbit No. 1071 was inoculated with strain 113-CE. Both of these strains had been maintained by weekly passage in chick embryo for over a year. One rabbit (No. 1066) had been inoculated id with 5,000 Toxoplasma trophozoites of the virulent RH strain, and received Spiramycin 125 mg/kg subcutaneously for 2 weeks. Chemotherapy was necessary to prevent death which would otherwise have occurred in 7 to 10 days. Besnoitia jellisoni was received from Dr. J. K. Frenkel (University of Kansas Medical Center). Prior to passage through chick embryo for the present study it had been maintained in mice by intraperitoneal (ip) passage twice a week for 3 months. Two rabbits weighing 2 kg each were given an initial id inoculation of 10,000 Besnoitia trophozoites from chick embryo followed by a second inoculation of 10,000 organisms id 2 months later. All rabbits were bled for serological studies 2 to 3 months after infection. These rabbits plus two normal rabbits were then challenged id with 1,000 Toxoplasma of the virulent RH strain. They