Virulence of different strains of Toxoplasma gondii and host response in mice

Abstract

THE protozoan Toxoplasma gondii has a world-wide distribution. It is an obligate intracellular parasite and appears in many species of mammals and birds. Animals that survive the acute stages of Toxoplasma infection probably remain infected for life¹. The RH strain of Toxoplasma gondii, originally isolated by Sabin² and widely used in the routine Dye test³ is extremely pathogenic to mice, LD₁₀₀ by intraperitoneal infection being below 10 tachyzoites⁴. Moreover, it is difficult to obtain survival after RH challenge of mice immunised with a strain of lower virulence₅. Such survival without the use of chemotherapy has, however, been reported_1,6,7. The RH strain was not totally eradicated⁷ from immunised mice even 7 weeks after challenge. How do immunised mice suppress the RH strain—by total elimination or making it possible for the RH strain to remain in the body as does the immunising strain? Furthermore, in the latter case, would the inherent virulence of the RH strain become changed? We here report that Toxoplasma possessing the full virulence of the original RH strain, could be isolated from mouse brains 14–18 months after RH challenge of mice which had first been immunised with the Beverley strain. This applied, however, to a proportion of the mice; in the brains of the other mice only the immunising strain (Beverley) was found.