Question:I have done some reading on heaves, and I am wondering if my horse has this respiratory condition. One fall, we had forest fires with smoke so severe that people were asked to stay indoors and wear respirators outside. That same fall I switched to a different hay supplier.

My horse’s “loud” breathing started the following spring, when he was 20 and only when I galloped him. I thought he was out of shape at first but soon found that after a summer of climbing mountains, he sounded the same. He sounded like he was going to keel over, yet he had such energy under the hood, I could tell he was getting enough oxygen.

Then the next winter I noticed him sounding wheezy when eating his hay. In spring he had a bad reaction that caused him to cough and show some nasal discharge. It took about four weeks to fully get over the coughing/allergic attack. I believed it to be heaves, but my horse does not have a heave line and seems to be otherwise normal. 

My veterinarian says that the only way to be certain of this diagnosis is by using an endoscope to examine my horse’s airways, but where I live, I would have to haul him over the mountains, and the expense is not something I can afford right now.

I have found that it helps to soak hay and keep my horse away from dust. Can a horse get heaves and not have a heave line? Also, can horses simply be allergic and not have heaves?

Name withheld by request 

Answer: “Heaves” is the common term for equine asthma, also called recurrent airway obstruction (RAO), a chronic respiratory disease most often found in older horses. Asthma affects the deep airways of the horse’s lungs and makes breathing difficult, due to three factors: bronchoconstriction, which is the narrowing of the passageways within the lungs; inflammation, which causes tissue swelling and fluid accumulation; and the buildup of mucus, which is also thicker and stickier than in normal horses.

Affected horses are allergic to dusts that they inhale—in particular, the molds commonly present in hay. Hay that is obviously moldy contains very high levels of dusts that can trigger an asthma attack in susceptible horses within hours to a couple of days. However, good-quality hay contains the same types of molds in much lower quantities. Therefore, feeding even good- quality hay to very sensitive horses can also trigger an asthma attack. Once a horse has developed this sensitivity to molds, he will remain susceptible to further heaves flare-ups when exposed to them for the rest of his life.

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Most affected horses develop signs during winter or early spring when they tend to be kept in the barn because of cold weather. If susceptible horses are fed hay year-round (indoors or outdoors), they will show signs continuously, although they may have good days and bad days. Another form of heaves occurs in horses who develop sensitivities to pollens and molds from trees and grasses common in pastures in the summertime. Inhalation of smoke from fires can injure the lungs in horses as it can in people; however, if the horse recovers from this insult, he would not necessarily be more likely to develop heaves.

As with human asthma patients, some heavey horses are only mildly affected while others become very sensitive. Milder signs may include intermittent coughing and decreased performance. Horses with the more severe form of the disease may exhibit signs of difficult breathing (nostril flaring and visible “heaving”) while at rest as well as frequent coughing, wheezing and exercise intolerance (that is, they may not be able to move any faster than a walk).

Horses with severe heaves have to contract their abdominal muscles harder with each effort to exhale. Over time—weeks to months—these muscles may become overdeveloped and form a visible, raised “heaves line.” Horses can also develop other types of respiratory allergies, such as allergic rhinitis to dust and molds, but these generally produce different signs, such as head-shaking, nose rubbing and sneezing.

In practice, equine asthma is usually suspected in horses who show respiratory signs for at least several weeks but continue to feel good otherwise with a normal appetite (unless heaving is severe) and no evidence of infection (such as fever). Listening to the chest with a stethoscope may reveal wheezing and sounds of breathing occurring over a wider area than normal, indicating that the lungs have expanded.

Confirmation of the diagnosis requires an endoscope for two purposes: to look for excess mucus and swelling of the small airways within the lungs, and to collect mucus samples to examine under a microscope. This second test will demonstrate the presence of inflammation in the lungs and rule out infections. Administering a drug that opens the airways (a bronchodilator like inhaled albuterol or injectable Buscopan™) can produce a rapid improvement in a heavey horse’s breathing; however, in some horses the effects may be minimal. Finally, diagnosis may also be confirmed by treating horses appropriately and monitoring them for a response after two to four weeks.

The most important part of heaves therapy is management changes that limit the horse’s exposure to dust and molds. The best approach is to remove hay from the diet and keep the horse on pasture. If pasture is not available, hay can be replaced with a complete pelleted feed or hay pellets/cubes. Soaking hay in water helps decrease dust levels, but the humidity it creates may cause mold growth in the stall. Haylage —grass that is baled with a higher moisture content and sealed in plastic—is a good low-dust alternative to feeding hay, but it is best to purchase this from commercial manufacturers that implement quality-control measures to reduce the risk of botulismtoxins. Horses with the summer form of asthma do better in the barn during periods of high pollen and mold levels outdoors.

In a double-blind study here at Purdue University, we found that administering a dietary supplement with omega-3 fatty acids significantly improved the cough, lung function and other signs of heaves in horses compared to untreated controls. If all other efforts fail, medical therapy with corticosteroids and bronchodilators can also help horses breathe more comfortably. Unfortunately, there is no known cure for the disease at this point.

To sum up, it’s impossible to diagnose your horse definitively without examining him. However, based on your description, a mild case of heaves does seem like a possibility. The good news is that the two measures you are already taking—soaking your hay and limiting your horse’s exposure to dust— are helping to control his signs. If you remain vigilant in protecting your horse from allergens, he ought to be able to live a long and healthy life.

Laurent L. Couetil, DVM,  PhD, DACVIM
Purdue University
West Lafayette, Indiana 

This article was originally published EQUUS #475 

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