TOXOPLASMOSIS - COMPARATIVE SPECIES SUSCEPTIBILITY AND HOST IMMUNE-RESPONSE

The protozoan parasite Toxoplasma gondii is capable of infecting all w arm blooded animals; however, the consequences of infection are very v ariable between different species of animal. Marsupials and New World monkeys, which have evolved largely separately from the cat, the defin itive host of the parasite, are among the most vunerable species where infection with T. gondii can prove fatal. In more resistant species s uch as humans and sheep, infection is generally unapparent, provoking only mild symptoms; thereafter the host remains infected for life. How ever, when the immune system is compromised, such as in the immunologi cally immature fetus, infection with the parasite can have very seriou s consequences. Much of the work examining host immune responses has b een done using experimentally infected mice. While there are many adva ntages in using this experimental model, care should be taken in extra polating results from mice to other species. Mice are extremely vulner able to the consequences of infection with T. gondii., and their use t o further our understanding of congenital toxoplasmosis may not be ide al, as fetal infection can occur in successive pregnancies. This is no t the case in rats or sheep: they are more resistant to the disease an d therefore may provide a more relevant model for human congenital tox oplasmosis. Studies of host immune responses have emphasised the impor tance of the cytokine interferon gamma (IFN gamma) in resistance to T. gondii. The efficiency of induction of this cytokine may be critical for determining the outcome of the host-parasite relationship. (C) 199 7 Elsevier Science Ltd.