“I don’t think I’ve ever seen a fat horse”

guest blog by Sarah Giles

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.

Further reading

Giles SL, Rands SA, Nicol CJ & Harris PA (2014). Obesity prevalence and associated risk factors in outdoor living domestic horses and ponies. PeerJ 2: e299 | full text | pdf