Ovine Toxoplasmosis: A New Look at its Pathogenesis

Conditions causing reproductive failure are a significant concern in the livestock industry, and amongst these, ovine toxoplasmosis is one of the most important. Despite decades of research, there remain gaps in knowledge about this disease, especially regarding the pathogenesis of abortion in sheep. As for other diseases causing ovine abortion, such as chlamydial abortion or border disease, the consequences of infection with Toxoplasma gondii depend largely on the stage of gestation, but the mechanisms involved are not well understood. Immunological modulation occurring during gestation has been proposed as the main mechanism accounting for this clinical variation. However, the extent and effect of such modulation has not yet been identified clearly in sheep and the involvement of other unknown factors has been proposed. Recent experimental studies have defined an unacknowledged clinical presentation of ovine toxoplasmosis, where abortions occur during the acute phase of infection, resulting in as high as 100% fetal loss in susceptible sheep. The pathogenesis of this clinical form differs from that of classically described ovine toxoplasmosis, and its pathological features resemble those of the perinatal syndrome known as cerebral palsy in man. A range of variables, including individual susceptibility, isolate virulence and infective dose, have been proposed as key factors in the development of one or the other of these clinical forms and warrants further investigation in this important disease.