It’s reasonable not to expect your senior horse to look as robust as younger animals, but they should maintain a good covering of flesh over their ribs, back and hip bones. When the older horse starts to look gaunt, there’s a problem.
I’m going to start with the least likely causes first, just because many concerned owners will have their minds rush to the worst case scenario. Although these causes are less common, it’s good for everyone to bear them in mind when going through the process of sorting out the horse’s problem.
Cancer is rare in the equine world, but as the horse ages anything goes. For example, thyroid cancer is limited to older horses and causes dramatic weight loss. Similarly, organ failure involving heart, kidney or liver is also uncommon but prevalence increases with age. The history, constellation of symptoms, physical examination and laboratory work will direct your veterinarian to a diagnosis.
Cushing’s disease, an overactivity of the pars intermedius of the pituitary gland, is a common hormonal disorder in older horses. Clinical signs include muscle wasting and weight loss. Weight loss caused directly by the disease may be compounded by loose teeth due to bone resorption in Cushing’s, and chronic infections in the teeth/oral cavity from depressed immunity in this condition. Cushing’s horses also often have oral ulcerations which can make eating painful.
Dental disorders can easily contribute to weight loss if they interfere with the horse’s ability to thoroughly chew hay. Grass may be handled better in early cases because it is much softer but eventually grass chewing is also compromised. These horses will “quid” – produce half chewed wads of hay that fall out of their mouths.
Another chewing difficulty not as well known is decrease in the force of chewing secondary to changes in the angle of pull along the chewing surface as the horse ages (the curve of Spee). Dental examination may show no abnormalities, leading to the false conclusion that chewing is not an issue. The horse may or may not quid but by not chewing forcefully enough the food particles are not broken down as efficiently so not exposed as well to digestive juices and enzymes.
The solution for chewing difficulties is to change the diet. Replacing hay with chopped hay may be enough in some cases. This can be purchased bagged or some people make and bag their own by passing hay through a chipper. If chopped hay does not lead to sufficient weight gain, a switch to soaked cubes, pellets or hay meal is necessary. An alternative is to use a high fiber complete feed, preferably soaked, to provide all the needed calories. Regular hay can still be provided to keep the horse busy but do not count on it for any calories.
Moderate amounts of fat can be a good addition for calories but you should first be sure the horse is also eating the required amount of protein, vitamins and minerals since fat is otherwise empty calories. Older horses will also sometimes hold muscle mass better if supplemented with a high quality protein source such as a soy and whey mix, or the three commonly scarce amino acids – lysine, methionine and threonine.
Senior horse weight loss is common but not inevitable. Work with your veterinarian to rule out serious illness and identify the cause. Diet modification is often the solution.
Eleanor Kellon, VMD