Non-lethal infection parameters in mice separate sheep Type II Toxoplasma gondii isolates by virulence.

The zoonotic protozoan parasite Toxoplasma gondii can infect all warm-blooded animals, but virulence of isolates has previously been characterised mainly by the ability to kill mice after experimental infections. In the present study, 15 Type II strains of T. gondii, isolated from five adult sheep, six sheep abortions, two pigs, one cat and one fox were examined for their virulence to young mice by less dramatic parameters. Clinical disease of inoculated mice, directly evidenced by reduced weight gain, was correlated to increase in serum level of haptoglobin and level of specific antibodies. Although Type II T. gondii strains are non-virulent to mice by lethality studies, significant differences in mouse virulence were observed between the strains of T. gondii isolated either from adult sheep or from sheep abortions. It was not possible to characterise strains isolated from sheep abortions as being more or less virulent than strains isolated from adult slaughter sheep.