Providing an Economic Benefit to Using a Vaccine to Enhance Food Safety and to Reduce Antibiotic Use in Agriculture

Roy Curtiss, III, Ph.D. Washington University Contact Dr. Curtiss, III

Globally, Salmonella enterica serotypes cause a substantial number of human infections with significant morbidity and mortality, especially in the very young, the elderly and the immunocompromised. Numerous studies have implicated prevalent Salmonella transmission to humans through the food chain from farm animals via contamination of carcass meat and animal products such as eggs, cheese and milk. Licensed live attenuated Salmonella vaccines are highly effective in reducing and, in many instances, eliminating Salmonella contamination of eggs and broiler meat. Nevertheless, the poultry industry has employed improved management practices and is thus routinely able to significantly reduce Salmonella contamination of broiler carcasses below the 20% level of contamination permitted by federal food safety standards. Thus vaccines and vaccination, at a cost of about a penny a bird, are not often used. As a consequence, there has been no substantial reduction in Salmonella transmission from poultry meat to humans.

For over 50 years, antibiotics have been added to poultry feed to improve feed conversion efficiency and growth performance. While contributing a significant benefit in improving productivity in getting animals to market sooner, with consumption of less food and with production of less manure, there has been an ever-increasing frequency of antibiotic resistance in a diversity of commensal, opportunistic pathogenic and pathogenic microorganisms. These antibiotic-resistant bacteria are often transmitted through the food chain and directly or indirectly, by transfer of antibiotic resistance genes, compromise effective therapeutic treatment of bacteria infecting humans. The colonization of poultry by Clostridium perfringens has been associated with poor growth. Recent results have demonstrated that C. perfringens cell density in intestinal contents of chickens is influenced by diet components, is increased as a consequence of co-infection with Eimeria species causing coccidiosis, and is significantly reduced by adding growth-permitting antibiotics to the diets. The toxins produced by C. perfringens cause necrotic enteritis in severe cases and have the ability, at low doses, to cause subclinical necrotic enteritis with thickening of the intestinal mucosa and decreased length of microvilli in the ileum. The collective impact of C. perfringens colonization is to reduce the absorptive surface in the intestinal tract with a consequent reduction in the rate of growth. The most effective antibiotics in enhancing growth performance are active against gram-positive bacteria, especially against clostridia. A vaccine that would induce immunity to the toxins produced by C. perfringens and/or decrease the ability of C. perfringens to colonize poultry, should theoretically enhance growth rates and also have the benefit of eliminating one of most important current beneficial uses of antibiotics in raising poultry.

We will therefore design, construct and evaluate recombinant attenuated Salmonella vaccines to induce anti-toxin antibodies and decrease C. perfringens colonization. The effectiveness of these vaccines in promoting growth of chickens will be evaluated. The objective is therefore to develop a vaccine that would provide economic benefit to the poultry producer because of enhanced feed conversion efficiency and more rapid growth, without the costs of using antibiotics. This vaccine would substantially reduce Salmonella infection of poultry and thus contamination of carcasses and eggs. Therefore, use of such a vaccine for poultry should contribute to reducing the likelihood of transmission of Salmonella and drug-resistant bacteria through the food chain to humans