EPIDEMIOLOGY OF SALMONELLOSIS IN THE LIVE SHEEP EXPORT INDUSTRY

Salmonellosis in sheep was studied on 7 commercial voyages from Austra lia to the Middle East and in one animal house study. Faecal excretion of salmonellas was not a good indicator of pathological changes in th e gastrointestinal tract. The ratio of the proportion of sheep excreti ng salmonellas to the proportion with histological lesions of salmonel losis ranged from 1:1 to 23:1. Increasing excretion rates in our studi es indicated that most sheep would be exposed to infection, although d eaths from salmonellosis were not spatially or temporally clustered as would be expected if challenge alone was a sufficient cause. We consi dered that inappetence predisposed to death from salmonellosis, and th at sheep with inappetence were likely to be randomly distributed aboar d ship. In the animal house study, histological lesions of salmonellos is were exclusively in inappetent sheep although almost all sheep excr eted salmonellas in faeces. Lesions of salmonellosis were found only i n sheep that were seriously ill or had died, which suggested that, und er the conditions of lot-feeding and sea transport, most sheep with en teric lesions are likely to die. Lesions were not found in feeding con trols. Adrenal gland weights, an indicator of stress duration and seve rity, were used to examine the temporal sequence of events in the deve lopment of salmonellosis. Weights were greater in sheep that died of i nanition than in controls, suggesting that inappetent sheep were alrea dy stressed for that reason. The presence of enteric lesions of salmon ellosis was associated with further increases in adrenal gland weights . In a small proportion of cases, septicaemic salmonellosis developed rapidly and without adrenal gland hypertrophy suggesting that the shee p succumbed before the adrenal glands had time to enlarge. We conclude d that sheep were predisposed to salmonellosis by inappetence and that stress was Involved in lesion development. We considered that the dev elopment of enteric lesions of salmonellosis, in the live export syste m, inevitably led to death.