Introduction

The rational design of vaccines should be considered within the context of the immunity that naturally develops against the targeted infection. It follows that each specific infection must be viewed in the context of the transmission cycle and epidemiology. The biological attributes of the microbe should be separated as much as possible from those of its hosts, and should be understood in terms of the evolution of the organism.

T. gondii has apparently evolved from a one-host intestinal coccidia of primitive cats. It has become a highly successful and stable two-host parasite. Its sexual cycle, which leads to oocyst production, occurs in the intestine of carnivorous cats, while extra-intestinal tissue cysts exist in the cats' prey. Prey animals that feed on the ground encounter the infectious oocyst stage from cat faeces. Evolution from the one-host primitive T. gondii to the two-host modern T. gondii was made possible by the evolution of the tissue cyst stage. Extension of infection into intermediate hosts with tissue cysts has enabled the transmission back to cats. Indeed, the T. gondii lifecycle has become so dependent upon tissue cysts that the transmission of infection to cats with tissue cysts is more efficient than the direct transmission from cat to cat via oocysts (Dubey & Frenkel 1976).

These facts are important for an understanding of immunity to T. gondii and the design of vaccines. The tissue cyst is an essential part of the evolved natural cycle.