Another factor which appears crucial for the induction of protective immunity, along with the identification of protective antigens, is the way in which these antigens are delivered to and/or presented at the host interface. A variety of microbial vectors are being used to target antigens to specific sites in the host; e.g. Salmonella spp. are being employed to target Eimeria antigens to the gut epithelium (64). The inclusion of genes encoding molecules with adjuvant- or immuno-modulating activity is being intensively studied to improve the effectiveness of recombinant vaccines (15, 38). In addition, more effort is being devoted to understanding how parasites evade the host immune response, with these effector molecules themselves becoming vaccine targets (32).