Are happy chickens safer chickens? Poultry welfare and disease susceptibility.

1. Contaminated chicken meat remains an internationally important vehicle for human infection with Salmonella and Campylobacter spp. In addition, the last 20 years has seen an international pandemic of human salmonellosis caused by the contamination of eggs with Salmonella Enteritidis. 2. It has been a long held scientific view that Campylobacter spp. and most, if not all of the common zoonotic salmonella, are essentially commensal in chickens. They usually form part of the gut flora and contaminate chicken carcases, for example, by faecal spillage at slaughter. Even when certain salmonella serovars like S. Enteritidis are invasive in laying hens overt evidence of clinical disease is rare and the birds appear to behave normally. 3. Are these bacteria just 'passing through' the avian host and only transient members of the bacterial flora or is there a more dynamic perspective to this infection/colonisation process? Chickens mount antibody responses to both pathogens, which indicate something other than commensalism. Such immune responses, however, do not always result in the clearance of the pathogen. 4. Not all animals in a group will carry salmonella or campylobacter, even under experimental conditions, and will vary, especially those that are outbred, in their responses to pathogen challenge. Identifying the reasons behind this could have important implications for disease control. 5. Both salmonella and campylobacter are more likely to be found in animals, which are compromised and this may explain at least part of the variations seen. Animals are more susceptible to infection when they are in a poor environment, fed a poor diet and/or under physical or psychological stress. 6. Work in this area has naturally focused on pathogens of medical significance and has shown that neurotransmitters such as noradrenaline can markedly alter pathogen behaviour. Other host responses like Interferon gamma can also affect host tissues in a way, which facilitates invasion by pathogens and may also interact directly with certain bacteria. 7. From a food safety perspective, there is evidence that egg contents contamination in ovo may be linked to transient stress in the hen. Current work at the University of Bristol on the epidemiology of campylobacter in broiler production is also showing a potential link between gut health and campylobacter colonisation and challenging the concept that these bacteria are common commensals. 8. The poor economic returns received by the egg and poultry industries mean that intensive production methods are common. Is it possible to rear chickens under these conditions in such a way as to exclude zoonotic pathogens like salmonella and campylobacter? Data from the UK strongly suggest that this is possible with the former pathogen. Can similar advances be achieved with campylobacter?