Studies on cryopreservation of Toxoplasma gondii and its antigenicity in mice

Two strains (RH and GC, the latter of which is a Taiwan isolate of porcine origin) of Toxoplasma gondii were kept at -20 degrees C, -60 degrees C, and in liquid nitrogen (-196 degrees C) to follow the time course change in viability and virulence of the parasites by direct count and animal inoculation methods. Changes in antibody titers in some of the mice inoculated with the thawed organisms were assayed by the indirect immunofluorescent antibody test. Viability and virulence of T. gondii were best preserved by storage in liquid nitrogen. Tachyzoites kept in liquid nitrogen for eight years still can lead to the death of the injected mice in 2-3 weeks. Virulence of the tachyzoites could be maintained for eight weeks at most at -20 degrees C and -60 degrees C. Dimethylsulfoxide (DMSO) seemed to be a better cryoprotectant for T. gondii than glycerol, but the DMSO-preserved organisms resulted in fewer tachyzoite-containing peritoneal exudates in inoculated mice than the glycerol-preserved organisms. The local isolate (GC strain) tachyzoites tolerated cryopreservation less well than the RH strain parasites. Low antibody titers (at most 1:64) were produced in mice that survived more than 16 days after inoculation with thawed tachyzoites.