Summary
In references published over the last 15-20 years, inanition and salmonellosis have been considered the most important causes of death in exported sheep, together accounting for more than 60% of deaths in both the assembly and the shipping periods, with feedlot non-feeders more likely to die during the voyage than those that eat during the feedlot period. While the overall mortality rates have declined over this time, it appears that these are still the most important causes of death in exported sheep, with deaths due to the persistent inappetence-salmonellosis-inanition complex (PSI) occurring during the shipment phase of the export process. Why some sheep refuse to eat has not been determined.
Normally, short-term control of feed intake is brought about by the integration of peripheral and central sensory pathways related to hunger and satiation, whilst long-term energy balance is accomplished through a highly integrated system that minimizes the impact of short-term fluctuations in energy balance on metabolic reserves. The key areas in the brain, including hypothalamus and brainstem, can receive and integrate signals about the current energy status and satiety from the GIT, and metabolic reserves via metabolic hormones such as leptin and insulin, to result in a decision whether to eat or not. Other hormones, in particular stress hormones, also affect the central regulation of feed intake.
There are many factors that influence feed intake and inappetence, including aspects about the sheep, environment and season, feed offered, and pre-embarkation management. Older, fatter sheep, especially if transported in the second half of the year, are considered at high risk of becoming persistently inappetent, with resultant high mortality rates. This indicates interaction between adiposity and season, which has been investigated to some extent and appears to be related to the inability of such sheep to continually mobilise fat as an alternative energy source when they do stop eating, compared to those sheep in the first half of the year which have a metabolism more tuned to fat mobilisation.
There are many aspects of the pre-shipping management which have been investigated and to date these have not yielded consistent results that could be applied practically as preventative measures. However, further investigation of on-farm factors could yield useful results, given that there is a greater mortality rate of sheep from a small number of farms. Feed type and delivery, and very importantly, the background of animals as to whether they know of pellets as food, can impact on feed intake, and feeding hay or chaff in the initial feedlot period may help to get sheep eating initially.
The interaction of animal factors and the stresses imposed on them is an area that can be further investigated. Mustering, handling, transporting, fasting, mixing with other sheep, altered diet including novel feeds, and shipping itself all constitute many stressors to the sheep, with individual and cumulative effects, that may influence feed intake. While previous work concluded that social dominance did not prevent animals from accessing the feed troughs, the effects of social and other psychological stresses on whether sheep choose to eat remain to be examined.
Work on other species indicates that physical and social stresses can reduce feed intake, and in humans there appears to be an interaction between body fatness and hypersensitivity of the hormonal stress axis, such that with increasing body weight and over-stimulation of the stress axis there is elevated secretion of cortisol, with depression and loss of appetite. Once a sheep becomes inappetent it must mobilise body reserves to sustain essential processes, and the ability to mobilise reserves can differ depending on season and adiposity of the animals. This may also influence the resultant metabolism of the animal, such that fat sheep may be more likely to develop ketosis and hypoglycaemia, which can in turn contribute to further mental depression and loss of appetite, a further explanation for why fatter sheep may be less likely to regain their appetite. The release of cytokines such as occurs in the sickness response can also inhibit feeding.
