You take your horse out of the stall or paddock, and he starts to cough. You tack him up and head into the indoor arena. Even at the walk, he coughs a couple more times. You assume you’re probably stirring up a bit of dust, so you’re not overly concerned, but as you move up to a trot, his coughing increases. You decide to just cool him down and try for a longer ride another day.

When you dismount, you notice your horse’s nostrils are flaring and his breathing is rapid, as though you had a much harder workout.

“I think he’s got heaves,” another rider says, as you remove the saddle.

Heaves is probably the oldest name for this disease. It’s also been called, depending on symptoms and timing, Inflammatory Airway Disease (IAD), equine Chronic Obstructive Pulmonary Disease (COPD) and Recurrent Airway Obstruction (RAO). Some researchers and veterinarians treated these as separate diseases, depending on various indicators or the time of year when the symptoms were seen.

In 2015, an article by Dr. Jean-Pierre Lavoie, of the University of Montreal, recommended that all of these conditions be described as equine asthma, and that has become the commonly-used term among veterinarians. Asthma in humans is a condition that can have multiple causes and symptoms that are more or less severe; equine asthma is much the same.

Dr. Lavoie explained that the names given to this disease were initially based on the symptoms owners and veterinarians observed. Newer names were based on research discoveries about the cells and signalling pathways involved. “That was not only complicated for veterinarians, but confusing to clients, which is why I recommend using equine asthma,” he said. “People are familiar with asthma, so it is clearer to them. But I would add that using a single term doesn’t mean it is a single disease.”

What Does It Look Like?

Dr. Lavoie classifies the type of asthma based on the horse’s symptoms:

  • A horse that has periods of respiratory distress when at rest, not related to exercise, has severe equine asthma.
  • A horse that has no symptoms at rest, but has a decrease in performance due to inflammation of the lungs, or coughs while exercising or while eating has moderate equine asthma.
  • A horse that has no symptoms at rest, but has some exercise intolerance and is not able to work to full capacity, and may cough occasionally has mild equine asthma.

The mild or moderate types do not necessarily progress to the more severe form, he added, although some can. “Two horses may have similar symptoms, but the pathway that leads to the illness may be quite different, so one may be progressive and one may not,” he said.

Dr. Lavoie said this is not strictly an allergic reaction, as asthma often is in humans, because a true allergy involves certain antibodies (IgE). In horses, the asthmatic reaction is usually not an immediate, acute response and tends not to involve IgE. But the process is similar.

The triggers for equine asthma seem to be quite specific. “In one study, horses from one U.S. state were moved to another state and their symptoms improved dramatically. We think that was because the hay was different. If you moved a horse with equine asthma from Ontario to Alberta, for example, he might not react to the new hay although the Ontario hay caused him a lot of problems,” Dr. Lavoie said.


Equine asthma can be difficult to study because the symptoms can be transient. Dr. Lavoie is working with a herd of horses with severe asthma that allow him and his team to study all aspects of the condition.

The biggest focus of his work is on airway remodelling. With equine asthma, something in the horse’s lungs – dust or mold, for example – causes inflammation, which leads to swelling and the production of mucus. Over time, these changes to the lungs become permanent and breathing difficulties are more severe. Dr. Lavoie is studying the causes of these changes and looking for ways that they might be reversed.

Another part of his research program looks at the role of neutrophils (white blood cells). Neutrophils are normally present in the lungs and are involved in fighting bacteria. They are not usually associated with allergies. When horses have severe equine asthma, however, neutrophils become predominant even though other factors present in the lungs indicate allergic responses.

Dr. Lavoie initially thought this was one difference between asthma in humans and equine asthma, but ongoing research detected the presence of neutrophils in the lungs of humans with severe asthma as well, especially those who are older. Cases of milder asthma have lower levels of the neutrophils.

Current methods of diagnosis are fairly invasive – from washing cells out of the affected horse’s lungs to biopsies of the lung tissues. Dr. Lavoie is working on identifying blood biomarkers to help diagnose the disease in a less invasive way. By following up with horses with various biomarkers he hopes to be able to predict which horses are likely to become worse and which will not.

What Causes Equine Asthma?

Dr. Lavoie said there seems to be a strong genetic component. If both parents have it, there is a high probability that the foal will too, although research has not identified a single gene that explains the disease in all horses. He recommends that this be considered when planning to breed your horse – if you have a mare with equine asthma, look for a stallion that is clear of the disease.

The environment has a major impact on whether asthma actually develops in those horses that are genetically at risk, though. Dr. Lavoie said about 15 per cent of adult horses in Eastern Canada have severe asthma, and a much larger number have mild or moderate asthma. “But if you look at horses in California, for example, where the weather is dry and sunny most of the time, the rates are much lower,” he pointed out.

Another difference is that in Canadian horses, asthma is associated with being stabled. In Australia, where the weather is hot and humid, asthma is associated with being out on pasture.

Researchers have also discovered that horses living in an urban setting (more common in England, where there are stables in some large cities such as London) are more likely to develop asthma, and horses sharing a barn with chickens are also at higher risk.

“The best way to prevent asthma, here in Canada, is to keep the horse on pasture and feed the horse a hay alternative when needed,” Dr. Lavoie said. “Equine asthma is rarely seen in outdoor horses that are not fed hay.” While hay is the biggest environmental cause, he pointed out that there are many other antigens in a typical barn including gas from urine and dust. Hay is particularly problematic, though, because the horse is inhaling particles of dust, pollen and possibly mold while he eats.

Dr. Lavoie acknowledges that keeping a horse outdoors is not practical in many situations, and recommends that feeding hay alternatives (such as pelleted feeds) to ease asthma symptoms. While wetting hay is a common approach, he said, “Most horses we are treating had previously been fed with wet hay, and therefore, this is not a recommendation that we commonly make. Pasteurized [steamed] hay is very popular, and, anecdotally, many horses appear to be doing well with it.” He noted that these two treatments have not yet been independently studied with regards to equine asthma. It is also critically important to manage wet and steamed hay correctly to prevent bacterial and mold growth.

And while being kept inside is a triggering factor for most horses with equine asthma in Canada, he said being outside during hot and humid weather can cause worsening symptoms in some. “We initially thought it was a problem of thermoregulation,” he explained. Horses dissipate heat by breathing, so a horse with asthma that was having trouble breathing might have trouble cooling down during hot weather, increasing the inflammation.

Further studies on these horses, however, showed that the hot, humid air actually caused the small tubes (bronchioles) in the lungs to constrict, as well as increasing the production of mucus in the lungs. So, the asthma was actually worsening as a result.

There might be another factor, too, Dr. Lavoie said: “We also looked at the amount of pollen in the air, and found that it was higher during hot and humid weather.”

He advises moving horses that have these worsening symptoms to a sheltered area where the temperature can be better controlled.

Managing Symptoms

Dr. Lavoie said the cough is usually the first thing an owner notices, and this is typically connected with a decrease in performance or reluctance to exercise. If the disease gets worse, the horse will have more difficulty exercising and will take longer to recover. In the more severe cases, the horse may lose weight because he is using up so much of his energy just breathing.

The gold standard for confirming equine asthma is for the veterinarian to introduce an endoscope into the horse’s lung, pour in some fluid and then remove it, along with some cells from the walls of the lung. These cells can be examined under a microscope to see if they are the types associated with equine asthma.

Often though, a veterinarian will make the diagnosis without doing this procedure, just based on the signs and symptoms reported by the owner and the vet’s examination of the horse, which rules out other possibilities such as pneumonia.

Asthma currently can’t be cured, so the treatment focus is on managing the symptoms.

“The best treatment is changing the environment,” said Dr. Lavoie. “Having the horse full-time on pasture is ideal. Many Canadians feed their pastured horses from large round hay bales in winter, though, and that increases the risk.” Pelleted feeds or hay cubes can be a better alternative.

For horses that react during hot and humid weather, a sheltered area that can be kept cool will help. These horses also may benefit from being fed hay alternatives.

Despite considerable research, there are still only two main types of medication used to treat these horses, just as in humans with asthma. Bronchodilators work to expand the constricted tubes in the lungs, and can be helpful if taken right before exercise or during a severe attack. They should always be used in conjunction with corticosteroids, said Dr. Lavoie, in addition to steps taken to change the environment.

The corticosteroids are important to actually reduce the inflammation, but because they have significant side effects they are not recommended for long-term use. The inhaled forms of both drugs are generally well tolerated by horses, and can be given with an aerosol chamber and canister filled with a concentrated form of the drug. “The problem with this is that it is quite expensive, given the quantity that you need to administer,” said Dr. Lavoie.

A 2017 study from Italy performed a meta-analysis of the effects of the medications on lung function in equine asthma. The researchers looked at 32 studies and found that the inhaled bronchodilators and inhaled corticosteroids had the best results, rather than other medications such as chromone or furosemide. They also found that long-term treatments were more effective than short-term and early and consistent treatment with these medications could prevent the airway remodelling that creates more serious problems.

Changing the horse’s environment shouldn’t be seen as a quick fix, Dr. Lavoie added. Many people tell him that they tried moving the horse outdoors, for example, but saw no improvement in symptoms. He pointed out that it can take three or four months to really see the horse getting better. Adding treatment with corticosteroids and bronchodilators can speed up the reduction of symptoms though, and with that regimen many horses will be better within two or three weeks of moving to a better environment.

“We also looked at the amount of pollen in the air, and found that it was higher during hot and humid weather.”