The use of a bacterin vaccine in broiler breeders in the control of Ornithobacterium rhinotracheale in commercial broilers

Abstract

Respiratory disease complex is a major cause of mortality and economic losses in the commercial broiler industry. In 1991 a previously unidentified bacterium associated with respiratory disease and cranial cellulitis was isolated from broilers in the then Transvaal Province (van Beek, van Empel, van den Bosch, Storm, Bongers, du Preez, 1994. ). In 1994 the organism was named Ornithobacterium rhinotracheale (Vandamme, Segers, Vancanneyt, van Hove, Mutters, Hommez, Dewhirst, Paster, Kersters, Falsen, Devriese, Bisgaard, Hinz, Mannheim, 1994.). Since then Ornithobacterium rhinotracheale has been isolated worldwide from chickens and turkeys showing respiratory signs and has become well established as contributing to the respiratory disease complex in both species (van Empel, Hafez, 1999). In South Africa respiratory disease and Ornithobacterium rhinotracheale in particular is routinely controlled by the inclusion of antibiotics such as Oxtetracycline into the feed of broilers during rearing. Concerns about antibiotic residues in poultry meat for human consumption as well as evidence that suggests that Ornithobacterium rhinotracheale readily develops resistance to antibiotics (Devriese, Hommez, Vandamme, Kersters, Haesebrouck, 1995), make this strategy unsustainable. It was with a view to reducing producers’ dependence on long term prophylactic antibiotic therapy that this study to determine the safety and efficacy of an OR bacterin vaccine was carried out. Injection of the bacterin into broilers was deemed impractical on a commercial scale, so it was applied to broiler breeder parent stock in order that they could protect their progeny through vertically transmitted immunity developed as a result of vaccination. Breeder flocks were vaccinated intramuscularly at nine and 18 weeks with a monovalent bacterin based on OR serotype A with oil adjuvant. Vaccine safety was evaluated by palpation of vaccination sites and clinical observation of breeders for two weeks after vaccination. The serological response of breeders to vaccination was monitored using an ELISA test for Ornithobacterium rhinotracheale optimised for use under South African conditions. Vaccine efficacy was determined by monitoring of broiler progeny of vaccinated breeders raised under commercial conditions as well as through controlled challenge studies with Ornithobacterium rhinotracheale under laboratory conditions. In order to determine the financial consequences of using the test vaccine, a partial farm budget was drawn up from available broiler data and possible outcomes were modelled using a stochastic model. The vaccine proved to be safe for use in commercial broiler breeders and vaccinated birds developed a good humoral response to vaccination. As a result of cross-contamination of isolators with Ornithobacterium rhinotracheale the results of the challenge studies were inconclusive. No evidence of protection of broiler progeny of vaccinated breeder flocks could be detected through the challenge trials. In the absence of in-feed medication, broilers hatched from vaccinated breeders did, however, performed better under commercial conditions than those hatched from unvaccinated breeder flocks. The partial farm budget showed that broilers raised from OR vaccinated breeder flocks were more profitable than the negative control flocks. The quantitative risk analysis showed that the probability of making a relative profit from broilers as a result of OR vaccination of parent stock was 74%, from the use of in-feed medication in broilers from unvaccinated parents was 70% and from a combination of the interventions was 99%. It can be concluded that the last of these options was most profitable.