Immunity and immunization against Clostridium perfringens-induced necrotic enteritis in broiler chickens
The objective of this thesis work was to identify proteins important in immunity to necrotic enteritis (NE) in broiler chickens and to develop a vaccine using candidate protein antigen(s). The first study identified seven secreted immunogenic proteins uniquely expressed by a virulent strain of 'C. perfringens' that reacted strongly to serum antibodies from immune birds. These were alpha-toxin, theta-toxin, fructose 1,6-biphosphate-aldolase (FBA), glyceraldehydes-3-phosphate dehydrogenase (GPD), pyruvate:ferredoxin oxidoreductase (PFOR), elongation factor-G (EF-G) and a Hypothetical Protein (HP). In addition, intestinal antibodies from immune birds reacted strongly with alpha-toxin and HP. The second study determined the role of these proteins in protection against NE following intramuscular immunization. All proteins significantly protected broiler chickens against relatively mild NE challenge but, in addition, alpha-toxin, HP and PFOR significantly protected birds against more severe challenge. Immunized birds developed high antigen-specific antibodies in serum and intestines. This study concluded that certain secreted 'C. perfringens ' proteins including alpha-toxin are involved in immunity to NE. The third study investigated the efficacy of HP, PFOR and FBA when delivered mucosally to chickens through an attenuated 'Salmonella 'vaccine vector in immunizing against NE. Two proteins, HP and FBA, showed significant protection against a severe challenge and birds developed serum and mucosal antibody responses to both 'C. perfringens' and 'Salmonella 'antigens. This study showed that it is possible to immunize broiler chickens against NE using attenuated 'Salmonella 'expressing ' C. perfringens' antigens, but that further work is required to optimize antigen expression and delivery by the vaccine strain. In conclusion, these studies showed that certain secreted 'C. perfringens' proteins including alpha-toxin are important in NE immunity and that it is possible to immunize broiler chickens orally using attenuated ' Salmonella 'as vaccine vectors to deliver certain 'C. perfringens ' protective antigens mucosally in protecting effectively against NE.