Novel approaches to control of bacterial infections in animals

Abstract

Bacterial infections of poultry remain of great importance world-wide in terms of economic effects and public health. They include infections caused by Salmonella, Escherichia coli, Campylobacter and Pasteurella. Through the introduction of rigid hygienic measures it is possible to breed and rear poultry free of these pathogens. However, the cost to the industry would be prohibitive and economically disastrous. Biological measures have been introduced albeit in a relatively empirical way. Antibiotic therapy and prophylaxis is used extensively with the associated problems of development of resistance. Killed vaccines are used but are not usually very effective. Live vaccines are increasingly becoming acceptable and studies are under way to increase our understanding of the pathogenesis of these infections so that vaccine development may become less empirical. Work with live vaccines to be used against Salmonella has shown that they may be administered orally to newly-hatched chicks. The vaccine strain colonises the gut extensively and prevents re-infection by other Salmonella strains by a genus-specific mechanism which is similar to that which occurs during down-regulation of bacterial growth in stationary-phase nutrient broth cultures. The mechanism of this phenomenon is currently being studied. This approach may also be applied to control Campylobacter infections. Bacteria of the Pasteurella group and E. coli may produce septicaemic infections in poultry. Recent work with K1+ E. coli infections in mice has shown that virulent bacteriophages may be used to treat or prevent septicaemias and meningitides. This work has been extrapolated to chickens with a similar degree of success and it suggests that some infections of this sort in animals and man may be amenable to this approach. In-bred lines of chickens have been found to vary greatly in their susceptibility to systemic Salmonella infections. This is probably mediated by one gene and the effect is dominant and not linked to sex or MHC. The mouse natural resistance gene (NrampI) does not appear to contribute greatly to this effect. Differences in the extent of gut colonisation by Salmonella in in-bred and out-bred lines can also be detected. These results are very exciting and open up opportunities for disease control for the future.