horses – Interaction Biology

guest blog by Sarah Giles

‘Cresty neck’ in horses is an abnormally large amount of nuchal neck crest fat, fat along the top of the neck. It can be seen as akin to abdominal adiposity in humans, this region specific adiposity can cause a range of metabolic disorders in both species. In horses, this metabolic changes have been associated with laminitis, a debilitating condition affecting the hoof which can cause debilitating and sometimes fatal lameness. Our paper, recently published in BMC Veterinary Research explores seasonal differences in neck crest adiposity in groups of domestic horses and ponies.

http://www.biomedcentral.com/1746-6148/11/13/figure/F1

It is not yet known why neck crest fat specifically is more strongly associated with metabolic abnormalities, but our study has presented some unusual results which might warrant investigation. Our previous study highlighted the seasonal variation in body condition and obesity present in outdoor living domestic horses and ponies (see previous blog post). This second study was conducted on the same population of animals, yet crucially, showed the exact opposite pattern of seasonal variation!

Unusually, the prevalence of ‘cresty neck’ was highest at the end of winter. This is surprising, firstly because, quite obviously, there is less grass available at the end of winter for outdoor living animals. Then secondly, because it had previously been speculated, arguably quite rightly, that the role of fat stores is to aid survival during winter months when food is scarce. Why then, does cresty neck seem to be more prominent in outdoor living horses at the end of winter?

Supplementary feeding was recorded, and this did not explain the results observed. The paper therefore discusses several other possible explanations. Broadly this includes a physiological explanation, where cresty neck fat is physiologically different to fat stored elsewhere and due to a potentially different physiological role. Or alternatively, we consider whether these results are simply an anomaly with the cresty neck score itself. The score may be difficult to replicate under different conditions, or there may be something about winter conditions, such as a fluffier winter coat on the animals, or less fat elsewhere, which makes the neck crest seem more prominent.

Whatever the explanation, these results were certainly unexpected and are therefore very interesting! This paper was fun to write as it was explorative and allowed for a balanced, speculative and thought provoking discussion. Disproving a hypothesis in this case, was much more interesting than proving one.

Most of all this paper is a reminder that we really don’t know all of the answers with regards to obesity and metabolic pathways in horses. We truly hope that this paper inspires further research into these potentially unusual physiological mechanisms

The usual response to the mention of equine obesity is “I don’t think I’ve ever seen a fat horse”. Followed by a long-winded explanation by me of how horses don’t necessarily ‘look’ fat in the same way as we are used to recognizing fat humans. But they are. Our new study, published yesterday in PeerJ, showed that the prevalence of obesity in outdoor living horses and ponies was a staggering 27% at the end of winter, when we would expect outdoor living animals to be at their thinnest (!) and rising to 35% during the summer months, presumably due to all that lush, green, UK pasture.

So nearly a third of UK leisure horses and ponies could be clinically obese, and other previous studies have had similar findings. That’s a very similar level of obesity to that seen in the human population. In the same way as humans, horses may experience negative health consequences of obesity, including metabolic conditions such as insulin resistance, but also a severe and debilitating hoof condition called laminitis which can render them chronically and even fatally lame.

The risk factors for obesity in any species are fairly straightforward, an energetic intake/exercise imbalance. Eat too much, do too little. But what makes some individuals more susceptible than others? Why do some horses seem to become obese when others do not under the same, outdoor living conditions? The study considered a wide range of food, exercise and management related factors, but by far the biggest risk factor was breed. Different horse breeds appear to have very different levels of obesity susceptibility. Our native UK breeds, including Welsh breeds, such as mountain ponies and cobs, as well as Dartmoor, Exmoor and New Forest ponies all appear to be at a much higher risk than for example the Arabian type lightweight breeds.

It might be that native UK breeds, which have evolved to live on mountains and moorland, are just very efficient at storing fat reserves! They are designed to pile on the pounds during the summer months when food is plentiful, and use these extra stores to survive cold, harsh winters. The problem in domestic animals (which have changed very little physiologically from their wild counterparts) is that this harshness never really occurs in a domesticated environment and horses do not lose their fat reserves during the winter months. Instead they become incrementally fatter and fatter, year-on-year. The study showed that once horses and ponies become obese, natural seasonal fluctuation in body condition reduces and almost disappears. As a result, these animals remain obese, year-round.

The fact that supplementary food and exercise played such a small role in explaining obesity susceptibility in predominantly outdoor living animals is key here. There is clearly a lot of work to be done in investigating risk factors for obesity in these outdoor living animals. Could social and behavioural factors play a role? This is of real interest to us: keep your eyes on the blog for more details.

Tag: horses

Seasonal variation of ‘cresty neck’ in horses

Further reading