ABSTRACT
J TOXOPLASMA gondii is one of the most common of all parasites. Its distribu-tion is global, ranging from Alaska to Australia, and it is estimated to infect about one third of the human population (Jackson and Hutchinson, 1989). The widespread distribution of the parasite may in part be due to its dual mechanisms of transmission, as infection may be either foodborne, due to ingestion of tissue cysts in uncooked meat, or "environmental" due to ingestion of oocysts excreted by cats. The spectrum of disease caused by the parasite is broad, but toxoplasmosis is mainly known as a cause of congenital disease and abortion both in humans and in livestock (Remington and Desmonts, 1990; Dubey and Beattie, 1988) and as a potentially lethal infection of AIDS patients (Luft and Remington, 1992). The cellular and molecular organization of Toxoplasma and host immune response to the parasite are well understood but parasite population biology has only recently been investigated (Sibley and Howe, 1996). The advent of molecular markers to analyze strain variation allows a reappraisal of several unresolved issues. Firstly, it is important to evaluate the extent to which parasite genotype influences the pathogenesis of infection and, related to this, whether the molecular mechanism of virulence can be determined. Secondly, the markers enable closer investigation of parasite epidemiology and in particular, allow us to assess the relative importance of the two transmission routes.
