Authors

D Blake¹;  ¹ Royal Veterinary College, UK

Discussion

Eimeria can cause the disease coccidiosis in livestock and poultry, most notably in chickens where global costs incurred by these parasites exceed £10 billion every year. Current anticoccidial control is underpinned by routine chemoprophylaxis but resistance is widespread, with strong public and legislative pressure to reduce use. Alternatives include a range of live wild-type or attenuated vaccines that can be highly effective but cost more than drugs, have limited production capacity, and require expert management. Demand for novel, cost effective recombinant anticoccidial vaccines is high, but industry requirements are stringent. Host performance, measured as body weight gain (BWG) or food conversion ratio (FCR), must be protected, with parasite replication and infection-associated pathology minimised. For Eimeria, extensive efforts including candidate gene and genetic mapping strategies have identified a panel of immunoprotective antigens as priority vaccine candidates. Targets including Apical Membrane Antigen 1 (AMA1), Immune Mapped Protein 1 (IMP1), and Microneme Protein 3 (MIC3) show promise when administered as recombinant proteins or DNA vaccines, but more cost-effective and scalable approaches are required by the poultry industry. Options such as vectored vaccines that can be incorporated into poultry diets or inoculated at scale in the hatchery are needed. Examples include use of Saccharomyces cerevisiae to express and delivery recombinant antigens, supplementing the widespread use of this yeast as a feed additive in poultry diets with vaccine delivery. Killed S. cerevisiae spores, even those that have been genetically modified to express vaccine antigens, are Generally Regarded As Safe (GRAS) from a regulatory perspective, minimising challenges posed from use of Genetically Modified Organisms (GMOs). Alternatives include genetic modification of Eimeria vaccine strains to express antigens representative of different species to create a streamlined, multivalent anticoccidial vaccine. This approach has also been evaluated to express and deliver anti-Campylobacter vaccine candidates, offering broader appeal to industry. Persistent challenges include fine tuning vaccine formulations for optimal efficacy and economic margin when compared to industry staples such as ionophore prophylaxis. It is likely that legislative changes to reduce or remove chemoprophylaxis will be significant for the future of recombinant vectored anticoccidial vaccines.