Written By Dr. Kris Hiney
Similar to people, horses can develop allergies to environmental contaminants that lead to asthma-like symptoms. In the equine world, this syndrome is referred to as recurrent airway obstruction (RAO). It was previously referred to as chronic obstructive pulmonary disease or (COPD) but due to dissimilarities with the syndrome observed in humans, RAO is now the preferred term. When some horses are exposed to dusts and molds, they develop allergies. Why some horses do and others do not develop allergies is unknown. There is some evidence that this disease may be genetic. Horses born to affected parents are three times more likely to develop RAO than horses born to non-affected parents. Therefore, if you know your horse is predisposed to RAO, it is even more important to identify the symptoms and to properly manage its environment.
When the horse is exposed to an allergen, the body responds by increasing inflammation through the bronchioles coupled with bronchoconstriction. The lungs increase in mucus production which the horse may not be able to clear easily. Frequently nasal discharge is seen, along with coughing, an increase in lung sounds and more distress during breathing. Lung sounds are typically heard with expiration, or the horse will “wheeze” when breathing out. Continual exposure to allergens may lead to fibrotic changes within the lungs. This will result in a horse which cannot take in oxygen to the same extent as an unaffected horse. You may notice that the horse has an increase in respiration rate over what is normally experienced during exercise, or he may become more exercise intolerant or fatigue easier. Horses with this syndrome which have had it more severely or more chronically may even develop a “heave line”. This is due to the hypertrophy of the abdominal muscles which must be used to assist the animal in breathing, rather than just solely using their diaphragm.
Management of the RAO horse
As pollen and mold counts increase in the environment, horses with RAO will experience more symptoms or episodes of RAO. Often barns and stables are not designed with proper ventilation in mind. This contributes to a continual exposure of the horse’s respiratory system to particulate matter. Hay and straw bedding ideally should always be stored in a separate building from where horses are housed, and certainly not overhead. Additionally, the stalling area should be separate from the riding arena. All the dust that is kicked up while horses are exercising can greatly exacerbate the problem. Ever think about how much dust is stirred up into the air while the barn aisles are being swept? Ideally all of this dust exposure should be minimized and RAO affected horses removed from the barn whenever dust is stirred up. One of the best changes to management practices of the RAO horse is simply to house them outdoors. Often an improvement in airway function is seen within days.
While horses stabled inside are clearly more likely to be exposed to particulate matter, horses on pasture during the summer can also have trouble. This even has its own nomenclature, summer pasture associated obstructive pulmonary disease or SPAOPD. Horses with this syndrome should be kept off pasture during the summer months, but can be housed outdoors during the rest of the year. If horses must be housed inside, whether they are SPAOPD or perhaps competitive horses that must be stalled, it is imperative that the environment is as dust free as possible. Straw bedding may not be a great choice for RAO horses but shavings can also contain molds similar to those in hay and straw. High quality straw may actually be lower in spore count that some shavings. Alternative beddings may yield the best results such as cardboard or newspaper pellets. If straw or shavings are used, remove any RAO affected horses while bedding is introduced into the stalling area. Essentially, let the dust settle before the horse is put back into the stall!
Ideally the RAO horse should graze fresh pastures as much as possible, but obviously this is not always possible. One of the most immediate dietary changes for the affected horse is to absolutely eliminate any moldy hay or straw that may be in the horse’s environment. While moldy hay should never be fed to horses, it is more critical with RAO horses. One of the difficulties in finding suitable hays for RAO horses is that humans may not always be able to detect the presence of mold if it is not obvious. Try to find hay sources from a knowledgeable producer who bales high quality hay. The type of fungus which produces the most damaging fungal spores prefers relatively hot temperatures. This would be seen in poorly cured hay, or hay that is baled at too high of a moisture content. The heating which occurs during spoilage is a haven for these fungi. Round bales may not be an ideal choice, unless they are stored completely inside and are never subject to any sort of spoilage. Additionally, round bales encourage the horse to almost bury their head within the bale, making the immediate breathing area of the horse very dusty. Completely pelleted diets might be a good choice for these horses at it greatly eliminates the dust exposure to the horse while feeding. The quality of the pellet is also critical. Uncoated pellets may break down more easily and have a substantial dust component. Hay cubes and haylage are also alternative feeding strategies. Moistening the feed can also help in dust suppression prior to feeding. Hay only needs to be soaked for 30 minutes to achieve optimal dust suppression. Beyond that time no additional benefits are seen. However, the down side to soaking the hay is that some of the nutrients are leached out into the water, including P, K, Mg and Cu.
Beyond eliminating dust and molds other dietary therapy may be aid the RAO horse. Supplementing RAO horses with additional anti-oxidants in the diet may be helpful. There is an increase in free radical formation or reactive oxygen species (ROS) in horses with lung inflammation. In fact, the increase in RAO may increase the upregulation of genes which produce inflammatory factors such as interleukins. Horses with RAO given a supplement of vitamin C and E plus Se resulted in less airway inflammation and an increase in exercise tolerance. Other natural sources of anti-oxidants such as sorghum and omega 3 fatty acids have not yet been studied for their effectiveness in RAO horses. Additionally, due to the increased energy requirement just to breath, RAO horses are often underweight. The sustained stress to the horse’s system may also contribute to this weight loss. You may need to find a feed with an increase in energy density, such as a fat added feed to help maintain its body weight as well as additional anti-oxidants.
If one follows these management guidelines carefully, RAO horses may be symptom free for years to come. While pasture is ideal for RAO horses, performance horses which need to be stalled can be kept healthy with a rigorous adherence to maintaining a dust and mold free environment and proper dietary management.