Comparative study of tissue culture and mouse inoculation methods for demonstration of Toxoplasma gondii.

Two methods for the isolation of Toxoplasma gondii were analyzed and compared. Bradyzoites or tachyzoites of three strains of T. gondii were injected into mice and introduced in parallel onto MRC5 fibroblasts cultured on cover slips. In the cultures, the parasites were more readily identified by an indirect immunofluorescence assay than by examination of unstained or Giemsa-stained cultures. With the RH strain, the tachyzoites replicated actively, and large foci of parasites were observed in 24 h. The bradyzoites or tachyzoites of the other strains could also be cultivated, but grew rather slowly; 2 days after inoculation, early stages of multiplication could be observed: from day +4, Toxoplasma clusters or foci were easily identified at a x100 magnification. The course of infection in mice was greatly dependent on the virulence of the strain and on the parasitic stage inoculated. In the chronically infected mice, evidence of Toxoplasma infection was only detected 45 days after inoculation through the demonstration of cysts in the brain or the presence of specific antibodies in the serum. The mean ratio of infected mice and positive cultures was compared in relation to the inoculum size. The tissue culture method was found to be at least as sensitive as mouse inoculation. Since Toxoplasma organisms may be isolated within a few days in tissue culture, it is proposed that this method should be used when early isolation of the parasite is crucial for the diagnosis of toxoplasmosis.