Is  Your  Horse  at  Risk?


From  "Canadian Horse"  -  Oct/Nov  2015

Heaves is a chronic, non-infectious lung disease that primarily affects mature horses and can have a significant effect on a horses well-being and performance ability. 

Heaves is also referred to as recurrent airway obstruction (RAO) and is often compared to human asthma. 

The primary pathologic mechanisms leading to clinical signs in affected horses are bronchospasm, inflammation and thickening of the lower airways (small bronchi), and accumulation of mucus and inflammatory exudates in the airway lumen. The term RAO indicates that the disease is chronic and recurrent, although "remission" from clinical signs can be achieved through treatment and proper management of affected horses.

Heaves is considered an allergic condition that is set off by inhalation of "respirable" particles. Mould spores, bacterial endotoxin and dust particles are the most commonly recognized inciting allergens. The primary sources of these allergens are feed (especially hay) and bedding materials.

Clinical Signs of Heaves

The typical clinical signs a horse owner will notice are an increased respiratory effort and coughing. Nasal discharge can also be seen in some horses but horses do not appear sick and do not have a fever. Signs can vary considerably depending on the severity of disease; in mild or early cases, coughing at the beginning of exercise may be the only noticeable sign. Horses with advanced disease typically show an obvious "abdominal lift" when


More commonly known as heaves, RAO can be triggered by something as simple as eating poor quality hay. Feeding of hay is a major environmental risk factor due to the wide variation in hay quality and dust content. Round bales in particular should be avoided because horses tend to bury their heads in the bales when eating, increasing the number of inhaled respirable particles.

Symptoms of RAO or heaves commonly include frequent coughing spells and nasal discharge. A diagnosis of RAO may seem daunting to horse owners, but most people can successfully adapt their management regimes to ensure their horse's continued health and well-being, although it can end a high-performance animal's career.

breathing (especially at the end of exhalation), and may develop a "heave line." A heave line indicates hypertrophy of the abdominal muscles due to continued increased breathing effort. Severely affected horses may also lose weight and will generally have significantly reduced performance ability.

Additional signs that can be noticed on veterinary examination are an increased respiratory rate, a tracheal rattle due to accumulation of fluid in the trachea, abnormal lung sounds on auscultation (especially wheezes), and intolerance to a rebreathing examination.

Due to its nature as a chronic recurrent disease, clinical signs of heaves typically occur in a seasonal pattern. Many horses will have exacerbation of the disease in the winter months when they are stalled, and will improve when they are turned out to pasture. A syndrome of a "summer pasture associated obstructive pulmonary disease" occurs in the Southern United States, and may be very similar to heaves, although the inciting causes appear to differ. Based on my personal experience, some horses in Western Canada appear to have exacerbations of heaves in late winter/early spring as well.


Heaves is diagnosed based on historical information (especially the recurrent pattern and seasonality of disease), clinical examination findings, and laboratory diagnostics. Laboratory tests typically include the analysis of one or more lung lavage samples, which may include a tracheal aspirate and/or a broncho-alveolar lavage (BAL) sample. A BAL is a saline wash of the airways and air sacs, performed by passing a long tube through the horse's nostrils and into the lungs through which sterile saline is infused and then retrieved for recovery of inflammatory cells. A BAL sample is generally preferred as it is more indicative of changes in the deeper tissues of the lung. Blood work and chest radiographs or ultrasound may also be used, primarily to rule out other possible disease conditions such as pneumonia.

Tests of lung function are also useful in the diagnosis of heaves. In practice, a simple response test to an injectable bronchodilator such as atropine may be done to demonstrate the presence of bronchospasm. More advanced lung function testing requires specialized equipment and is therefore usually reserved for advanced diagnostic testing or research studies concerning heaves. Challenge tests are also common in research studies and may be done with natural substances such as mouldy hay, or with individual stimulants such as histamine or endotoxin.

Environmental management to reduce exposure to inciting allergens is the only true treatment for heaves. 

The goal of management is allergen reduction in the horse's "breathing zone" and, ideally, in the entire living space of the horse. Most horses with heaves do better in pasture than when stalled, and many research studies have shown that remission from heaves can be achieved by environmental changes alone. As hay has been implicated as the main trigger of heaves, alternatives to hay feeding should be considered for affected horses. These may include cubes, complete pelleted rations, or silage or haylage (use caution when feeding haylage or silage). Hay treatments to reduce dust, spores, and moulds are also offered by several companies. If hay feeding cannot be avoided, hay quality should be optimized. Soaking hay, to reduce dust can be helpful in the short term but is not recommended as a long-term management option. Feeding of round bales should be strictly avoided.

In addition to feeding considerations, management should aim to optimize air quality and ventilation, and reduce overall dust exposure. Some options include the use of alternative bedding materials, reorganization of barns to move horses away from storage sites for hay and bedding, removal of horses during barn cleaning, and spraying of barn isles. Heaves is inherently a chronic progressive disease but optimal management in the early stages may delay the onset or reduce the severity of disease long term.

Medical therapy of heaves is necessary with acute exacerbations and to provide relief until environmental changes can be implemented and take effect. 

Antiinflammatory corticosteroids and bronchodilators are the mainstay of medical treatment and these medications maybe administered systemically (i.e., by injection or orally) or locally by inhalation therapy. Medical treatment should only be regarded as a short-term solution and should not be considered an alternative to environmental management.



Sucking air through a straw.

Those are the words that many people with asthma use to describe their frightening struggle to breathe during an asthma attack.

Now imagine a thousand-pound animal experiencing the same panic-stricken feeling.

For a horse, that's what it's like during an acute episode of recurrent airway obstruction, or heaves. As the horse's airways narrow, it increases its breathing effort until it starts to cough.

This debilitating disease detracts from horses' quality of life and limits their careers. And while RAO affects many horses in Western Canada, veterinarians still don't fully understand the problem.

"Like many veterinarians, I have struggled with understanding the disease process of RAO and treatment of severe cases," says Dr. Michelle Husulak, a resident in equine field service at the Western College of Veterinary Medicine (WCVM).She and her supervisor, Dr. Julia Montgomery, are conducting a study investigating treatment options for horses with RAO that will hopefully answer some questions about the disease process.

In human medicine, current research in asthma suggests bacteria may play a role in the disease in people. As well, horses with RAO often have bacteria in their lungs - but researchers still don't understand the importance of the bacteria in relation to the disease.

"Often we find bacteria that are not causing a pneumonia and may or may not be normal flora. In these cases we are unsure if treatment with antibiotics is required," explains Husulak.

She wants to determine if targeting these bacteria with antibiotics - on top of traditional therapy for heaves - helps the horses improve. Answering that question may also shed light on the role of bacteria in the disease process.

Traditional therapy for RAO consists of environment management - reducing exposure to dust and mould by keeping the horse outdoors on pasture, soaking hay or feeding hay cubes, and using a low-dust bedding such as wood chips. As well, veterinarians often prescribe drug therapy: corticosteroids such as Dexacort® (dexamethasone) to reduce lung inflammation along with bronchodilators such as Ventipulmin* (clenbuterol) to open up the horse's airways.

Husulak wants to know if adding in antibiotic therapy will further help the horses improve from an acute episode of heaves.

"Do these horses need to be treated with antibiotics to help resolve their clinical symptoms, or is treating with antibiotics considered inappropriate use of antibiotics?" says Husulak.'This is a question I would like to answer."

For her research, Husulak is recruiting horses with heaves in the Saskatoon area. Before the horses can be enrolled in the study, the diagnosis of RAO has to be confirmed and bacteria must be present in their trachea.

Once they're part of the study, Husulak and members of her research team give horses a thorough physical examination with special attention paid to the lungs. The clinicians use a technique called "rebreathing" where they place a plastic bag over the horse's muzzle for a few minutes so the animal rebreathes the air it expires. The horse will naturally breathe deeper and allow Husulak to hear more lung sounds with her stethoscope.

The research team also performs two tests - the tracheal wash and bronchoalveolar lavage - to retrieve samples from each horse's upper and lower airways.

Once the veterinarians can confirm an RAO diagnosis and bacteria are found in the trachea, they start the horses on a treatment protocol. They also ask owners to keep their horses outdoors as much as possible and to soak their hay. As well, all of the RAO-positive horses receive steroids and bronchodilators.

To determine if the antibiotic therapy is effective, half the horses receive antibiotics (the treatment group) while the other half receive saline (the control group). Neither Husulak nor the horse's owner knows whether the horse has received antibiotics or saline - ensuring that there isn't any bias when assessing the horse's improvement.

After the treatment course is finished, Husulak visits each horse a second time to re-perform the physical exam, tracheal wash, and bronchoalveolar lavage, and to assess its improvement.

How does she determine if the horses have improved?

"We are looking at the clinical signs such as respiratory rate and effort, nasal flaring, nasal discharge and lung sounds. We are also looking at the tracheal wash bacterial culture, and the inflammatory cells and proteins on bronchoalveolar lavage," says Husulak, who has tested two horses so far.

She and her colleagues will continue testing horses for the study over the next few months. And ultimately, if results of the WCVM project show that antibiotic therapy can help horses with RAO to recover from acute episodes of the disease, veterinarians would have something that could help their equine patients - as well as their clients - breathe easier.

Hayley Kosolofski of Sherwood Park, AS, is a third-year veterinary student who was part of the WCVM's Undergraduate Summer Research and Leadership program in 2014. Haytey's story is part of a series of articles written by WCVM summer research students.

Risk  Factor  for  the  Development  of  Heaves

There is now convincing evidence that heaves has a genetic basis which makes certain horses susceptible to development of the disease once they are exposed to the inciting allergens. Several studies have investigated the genetics of heaves and have found a strong predisposition in certain horse families. One study found genetic predisposition only to severe forms of heaves, and it is quite possible that the different clinical severities represent different forms (rather than different stages) of the disease. There does not appear to be one gene that determines susceptibility to heaves but, rather, the genetics are very complex and different genes may be implicated in different families of horses. The inheritance pattern may also vary among equine families. In general terms, studies to date suggest that presence of heaves in one or both parents significantly increases the risk that an individual horse will develop heaves. 

There is currently no genetic test for heaves and the complexity of the genetic predisposition makes it necessary that more research is done.

Age is a risk factor for heaves and the disease generally develops in middle-aged to older horses. It appears reasonable to assume that optimal management in susceptible horses may delay the onset of clinical signs; however, evidence to this effect is lacking to date. Similarly, the potential relationship between inflammatory airway disease, a condition primarily of young racehorses, and heaves is not understood.

Hay feeding is the strongest environmental risk factor for heaves. As hay varies greatly in dust content, hay source and quality are important and feeding of round bales poses the highest risk. In addition to the frequently lower hygienic quality of round bales, horses tend to bury their heads in the bales when eating, thereby increasing the number of respirable particles they inhale.

Indoor housing is often named as a risk factor for heaves and many studies have shown that remission of heaves can be achieved by keeping horses in pasture full time. Hay feeding may be the component of indoor housing that most significantly increases the risk for heaves and one study showed that horses kept indoors in a well-ventilated barn and on a hay-free diet fared as well as horses in pasture. Similarly, the evidence for the relative risk associated with different bedding materials is not consistent.

In partial contradiction to the protective effect of pasture housing, outdoor housing in the winter was found to increase evidence of airway inflammation in one study. Exercise in cold, dry weather may also increase the risk of heaves exacerbation as it leads to inflammation associated with airway cooling.

The potential for heaves development or exacerbation subsequent to respiratory infections (such as viral infections) requires further investigation. 

Viral respiratory infections can increase airway hyper-responsiveness, and sufficient rest periods following viral infection are necessary to allow for complete airway healing. There is interesting evidence concerning the relationship between the airway microbial flora and chronic airway diseases in humans, but research in this direction has not been undertaken in horses to date.

Consider moving

sensitive horses out of

the stable during times

of high activity such as

stall cleaning, which

affects the air quality in

adjoining stalls.

Exposure to dust, mouldy

hay, ammonia fumes,

and fungal spores in the

environment can

aggravate the condition.


Heaves is an important and common chronic respiratory disease of horses. Susceptible horses develop clinical signs in response to environmental allergen challenge, and environmental management is the only true treatment for heaves. Further research is needed to understand the genetic basis of heaves, investigate the potential for different forms and triggers of the disease, and to elucidate the potential relationship between heaves and other conditions such as respiratory infections.

Dr. Katharina Lohmann is a board-certified specialist in large animal internal medicine at the Veterinary Medical Centre and an associate professor at the Western College of Veterinary Medicine in Saskatoon, SK. Dr. Lohmann works with all species of large animals and has a special interest in neonatal medicine. Her main area of expertise is with horses.

Studying  the  Airway  Microbiome  in  Horses


Veterinary researchers at the Western College of Veterinary Medicine (WCVM) in Saskatoon, Saskatchewan are investigating whether certain bacterial populations in a horse's windpipe can contribute to RAO, or heaves.

Motivated by human research on asthma and chronic obstructive pulmonary disease (COPD), large animal internal medicine specialist Dr. Katharina Lohmann has developed the airway microbiome project. She and associate professor Dr. Julia Montgomery are investigating the bacterial populations (microbiomes) that inhabit the trachea (windpipe) of both healthy horses and horses with RAO.

Dr. Katharina Lohmann of the Western College of Veterinary Medicine is working

with associate professor Dr.lulia Montgomery to investigate the bacterial

populations that inhabit the windpipe of both healthy horses and horses with RAO.

It's the first study of its kind and builds on an earlier research project by Montgomery in which she performed airway diagnostics in healthy horses. She was able to establish normal reference ranges for the tests currently used to diagnose equine airway disease, particularly RAO.

"This is really the first step," says Montgomery. "No one has ever looked at these basic questions before. What kind of bacteria are there in the airway of horses? Are there different profiles that we can discern for healthy and sick horses?

"If there are distinct groups, then that might help us down the road to identify at-risk horses sooner, and potentially improve their clinical outcome."

The study includes results from 12 healthy horses and 12 horses diagnosed with RAO. To obtain samples, the WCVM researchers sedated each horse before inserting a video endoscope into the animal's windpipe. The endoscope allowed team members to visualize the upper respiratory tract for conformation defects, grade the amount of mucus in the windpipe, and facilitate sample collection.

Next, the researchers injected fluid through a small port in the endoscope and then used a syringe to collect fluid, mucus, and bacteria from the windpipe. This sample is used to establish profiles of bacterial populations living in the airways for the microbtome analysis.

Finally, researchers passed a second longer tube through the horse's nose into its lungs. They injected a larger amount of fluid into the lungs and then used multiple syringes to draw back a sample. The research team analyzed this sample for its cellular composition so they could establish or confirm a diagnosis of RAO - or determine airway health in the healthy horses that were enrolled in the study.

In her earlier project, Montgomery was intrigued to see that almost all of the samples showed the presence of bacteria in the airway. Like any other part of the body that has direct contact to the outside world,the airway is not sterile, and bacteria are part of the normal flora, just as they are in the gut. However, no one in human or veterinary medicine has yet determined exactly how these bacteria contribute to both airway health and disease.

The researchers hope they can identify more effective treatments and management strategies by unravelling the potential role that bacteria play in airway disease and gaining a better understanding of how horses develop RAO.

"One thing that has always interested me is the different outcomes we see in horses with heaves," says Montgomery."Why do some respond to treatment and others do not? Why do some respond to environmental management and others do not?"

A diagnosis of RAO may seem daunting to horse owners, but Montgomery finds that most people can successfully adapt their management regimes to ensure their horse's continued health and well-being.

"The biggest challenge that people with newly diagnosed animals face is to implement the recommendations that we give them," says Montgomery. "Once they can control the horse, maybe initially with drugs, and then with environmental management, most horse owners do really well. So long as it is within their means, they're willing to do whatever is necessary to ensure the comfort of their animal."