Toxoplasma gondii virulence: changing pattern under different maintenance conditions.

Toxoplasma gondii parasites were isolated in vitro and in vivo from a hare that had died of disseminated toxoplasmosis. Intraperitoneal inoculation of either homogenate of hare organs or parasites isolated on culture produced chronic infection in mice. However, a progressive increase of virulence following subsequent mouse-to-mouse transfections was observed. These findings suggest that host-parasite interaction plays a major role in determining the degree of pathogenicity, and that in vitro and in vivo isolation, if not followed by subsequent passages, may fail to reveal major differences in virulence between isolates.