Heaves: Diagnosis, Treatment, Management, and Research of the Disease

The equine respiratory condition, heaves (recurrent airway obstruction, or RAO) is often likened to asthma in humans. It is chronic, incurable and can be debilitating.

Heaves is believed to be caused by a combination of environmental and genetic factors. A range of organic particles from hay, bedding materials and other farm-related effects – mould spores, dust, bacterial molecules (endotoxins) and mites, for example – are present in the air of every barn. In many horses, these particles pass through the upper and lower airways without causing a problem. But, in horses who are predisposed to heaves, the immune system kicks in, creating an allergic response to these irritants. The immune system produces inflammation and an accumulation of mucous in the lower respiratory tract, which contracts the airways, causing what is known as a bronchospasm.

After repeated spells of inflammation, the mucous becomes thicker and the amount increases. The airway walls become permanently thickened in response to the repetitive injury created by the inflammation in a process called remodeling. Another result of repeated heaves episodes is smooth muscle hypertrophy in which the muscle becomes thicker and the lungs lose their elasticity.

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All of these factors make breathing difficult for the horse, especially expelling air, so he has to breathe harder and faster, even at rest, pushing – or ‘heaving’ – the air out.

The airways in affected horses also become hypersensitive and tend to narrow in response to even slight exposure to allergens or other stimuli (i.e. citric acid has been used in experiments).

“Clinically, airway hyperresponsiveness is important because it means that RAO-susceptible horses are prone to develop bronchospasm in response to levels of stimuli that would not affect a normal horse,” reports N.E. Robinson from Michigan State University’s Veterinary Medical Centre in a 2001 paper on heaves.

Diagnosis

The onset of heaves generally occurs in horses aged seven or older who have spent a lot of time in stables eating hay. It is important to note, however, that not all horses with heaves are affected by allergens in the barn environment. Instead, some respond to allergens found in pastures and are typically affected during the warm, humid summer months. Summer-pasture associated obstructive pulmonary disease (SPAOD) is the same disease as heaves, just with different triggers.

Horses with mild heaves might suffer coughing episodes in conjunction with exercise or when exposed to dust. They might show less tolerance for exercise and/or be less able to recover from work.

Unmanaged, mild heaves usually becomes serious, with symptoms including flaring nostrils, sometimes with a mucous discharge; an increased respiratory rate accompanied by wheezing; and long, deep coughs. The horse can become agitated and lose its appetite with an eventual loss of weight and condition. Because the horse has to use its abdominal muscles to help exhale, eventually heave lines develop, which are a prominence of the muscle at the bottom of the rib cage.

If heaves is suspected, a veterinarian must be called in to examine the horse. The veterinarian will want to rule out other possible causes of the respiratory distress including viral and bacterial infection and inflammatory airway disease (see sidebar – Inflammatory Airway Disease: A Different Beast).

The veterinarian will examine the horse’s lungs with an endoscope to check for increased mucous and will likely perform a bronchoalveolar lavage (lung wash). This technique was developed in 1979 by Dr. Laurent Viel, professor of large animal internal medicine at the University of Guelph’s Ontario Veterinary College, and is now the gold-standard for diagnosing heaves. Fluid that is present in the lower airway is collected by endoscope using a saline solution wash while the horse is standing and sedated. The recovered cells are then analyzed at a laboratory. A horse with heaves will have a higher-than-normal white blood cell count than a horse without the condition, confirming the diagnosis.

Treatment

The veterinarian’s main initial goal is to reduce inflammation in the airways. They will use two main drug types, often in combination.

Corticosteroids are anti-inflammatory drugs that help to open the airways. Some of the more common corticosteroids include prednisone and dexamethasone. They are powerful medications and long-term corticosteroid use can produce side effects including laminitis due to the drug’s suppression of the immune system. Their use must, therefore, be reduced gradually once the desired control of the condition has occurred.

For horses in severe distress, bronchodilators such as clenbuterol, atropine and albuterol dilate the airways and relax the smooth muscle to allow increased airflow in and out of the lungs and reduce respiratory distress.

Corticosteroids and bronchodilators can be administered orally, by injection or inhalation.

Inhalers have been on the market for about 20 years. (The original AeroMask was another invention by Viel and his team at OVC.) They can offer a more targeted delivery of drugs to the respiratory system, which also has the benefit of fewer systemic side effects.

Aerosolizing masks that fit over the muzzle deliver the medications directly into the horse’s lungs from a metered-dose aerosol canister. The masks can, however, be distressing to some horses. Now available are smaller, portable, single-nostril equine aerosol metered-dose inhalers that are quite similar to human ‘puffers.’

Management

Beyond drug treatment, it is essential to modify the horse’s environment to minimize possible factors that are contributing to heaves.

Provide as much turnout as possible – 24/7 is best, with adequate shelter from wind and precipitation. Although, horses with summer-pasture associated obstructive pulmonary disease should be kept inside during seasonal heaves episodes. SPAOD-affected horses, as well as those for which permanent turnout isn’t an option, should be placed far removed from the dustiest areas of the barn. Ensure the stall and barn have excellent ventilation and keep doors and windows open as much as possible.
Clean the stall at least once a day to remove ammonia from urine and manure. Move the horse away from the area when mucking out and don’t return him until the dust settles. Also avoid sweeping around a horse with heaves.
Avoid using straw as bedding, as it tends to be dusty and can be filled with mould particles. Use minimal-dust bedding such as paper, wood shavings or wood pellets. Even shredded paper or cardboard are said to be excellent bedding for heaves horses if you can source it.
Store hay in a separate building from horses. When feeding hay, it can be helpful to soak it for about five minutes prior to feeding to decrease the amount of dust and mould spores that will be released and inhaled while eating. Soak with nets and buckets or, if expense isn’t a barrier, try a commercially available hay steamer.
When the horse is on turnout, don’t offer him a round bale. Most horses will stick their heads in the bale’s centre where they will inhale all sorts of nasty particles. Instead consider spreading hay on the ground to encourage mucous drainage from the head-down position. Alternatively, some owners feed silage, a fermented hay product, but it can be susceptible to botulism if not used quickly enough.
Some owners might even find it best to offer pelleted or cubed feeds – including hay – instead of high-dust feeds such as rolled oats. Occasionally, pellets are dusty too, in which case adding a sprinkle of water a few minutes before feeding will help prevent airbourne particles from reaching the horse’s breathing zone.

Other tips: watch the dust that flies during grooming sessions; stay out of dusty rings or arenas; avoid riding in cold weather, which can exacerbate symptoms; and don’t idle farm machinery or vehicles near a horse with heaves.

Ongoing Heaves Research

Veterinary institutions worldwide are busy researching this chronic disease that, so far, has no cure. One of the most recently published – and fascinating – studies comes out of Munich, Germany, and examines heaves at its teeny-tiny core. In February, researchers at Ludwig Maximilians University’s Department of Veterinary Medicine announced they have had success using nanoparticle research – the current scientific hot topic – to help in the fight against heaves. A nanoscale molecule, called a nanoparticle, is a microscopic particle that has one or more dimensions measuring 100 nanometres or less (a nanometre is one billionth of a metre. The nanoparticle in this case is used to deliver the DNA, CpG-ODN, that is shown to suppress allergies in humans, directly into cells to target the inflammation caused by the body’s immune response.

Each of Canada’s veterinary schools is actively involved in heaves and respiratory disorder research as well. Just one of the many ongoing investigations, for example, at the Ontario Veterinary College (OVC), is how levels of a certain protein could lead to diagnosis and detection of heaves.

Clara cells, which line the bronchi of the lung, produce a protein which reduces the inflammation caused by allergens. OVC researchers felt that with repeated exposure to allergens, these cells become depleted, produce less protein and, therefore, are less able to prevent the allergic response. Testing of lung fluids obtained by bronchial lavage, have indicated depletion of the protein is, in fact, a key feature of heaves. They can tell how advanced the disease has become based on the levels, and have also discovered the protein leaks into the blood of horses with heaves. This means a blood test for heaves could be developed that would indicate the severity of the disease.

Out west, Dr. Renaud Leguillette heads the Faculty of Veterinary Medicine’s Equine Respiratory Laboratory at the University of Calgary. Leguillette, an equine internal medicine specialist with interests in chronic respiratory disease in horses and biology of airway smooth muscle in both horses and humans, has researched inhaled medications and is overseeing an epidemiologic study on horse health and heaves in Alberta.

A reason heaves is one of the more heavily researched equine conditions is, as respiratory specialist Dr. Jean-Pierre Lavoie of the University of Montreal’s Faculty of Veterinary Medicine suggests, human asthma sufferers and horses with heaves share similar disease characteristics.

An article co-authored by Lavoie, Heaves, an asthma-like disease of horses, published in the 2011 edition of the journal Respirology states: “The naturally occurring nature of heaves, the duration of the disease that can last decades and the long lifespan (30-35 years) of horses make this disease closer to human asthma than any other animal models. This is particularly important for the study of airway wall remodeling, that is likely the result of complex immune response and bouts of inflammation and bronchoconstriction occurring over many years.”

Earlier this year, Lavoie reported that he and fellow researchers identified genes associated with heaves. Lung biopsies were taken of 11 horses – six with heaves and five control horses – while the horses with heaves were in clinical remission out at pasture, and when exposed to allergens during 30 days of stabling. Researchers undertook genetic analysis of the biopsies and identified genes previously associated with human asthma. “The findings of genes previously associated with asthma validate this equine model for gene expression studies,” said the study, which also added the discovery could lead to anti-inflammatory and anti-remodeling therapies for horses and humans.

Among Lavoie’s many other respiratory research projects, he and a multi-disciplinary team are researching whether remodeling – airway restructuring – is actually reversible by removing environmental allergens or by using anti-inflammatory drugs. They have examined horses’ lung tissue samples before and during bouts of heaves and after a course of drug treatment with a synthetic corticosteroid. ‘We have confirmed that horses with heaves have approximately twice as much smooth muscle surrounding their smallest airways,’ said Lavoie, adding that they also found an increase of collagen (found in scar tissue) in the airways of these horses.

Furthermore, preliminary results indicate that treatment for 11 months with inhaled corticosteroids or improvement in the environment, while controlling the clinical signs at rest, only partially decrease the airway smooth muscle. ‘We believe that these results are very important as they indicate the need to develop early markers of the disease that would allow us to intervene early before susceptible horses develop the lung lesions observed in heaves,’ said Lavoie. ‘We hope that our findings will contribute to improve both human and equine health.’

INFLAMMATORY AIRWAY DISEASE: A DIFFERENT BEAST

Inflammatory airway disease (IAD) and heaves are both are non-infectious ailments of the horse’s lower airway. And while they both produce cough, nasal discharge, increased mucous in the lungs, and intolerance to exercise, IAD generally affects younger performance horses and has several causes including bacterial and viral infections; allergens; bleeding into the lungs; and environmental factors.

Generally, affected horses continue to eat, drink and behave normally, and their breathing does not appear laboured at rest as it does in horses with heaves.

Treatment consists of rest, changes in the horse’s environment similar to those recommended for heaves, and medications such as corticosteroids and bronchodilators.

Dr. Maureen Wichtel, assistant professor of large animal science at the Atlantic Veterinary College in Prince Edward Island has learned IAD is common in both racehorses and sport horses, but, “the primary complaints by the owners differ,” she said. “Racehorses are performing poorly; most other horses have a chronic cough, or behavioural complaints. Nasal discharges are less common, but may occur. Also, the type of inflammation in the lower airways differs between the poor performing racehorses and the coughing horses, for reasons that we don’t understand. Horses with a chronic cough seem to have a larger degree of inflammation than exercise-intolerant racehorses.”

Wichtel also said that while genetics appears to be a factor in heaves, “there has been no such documentation for IAD. Much of what we see in IAD horses may simply reflects increased levels of particulate matter in the lower airways, rather than a hypersensitivity to allergens in dust, as is incriminated as the cause of the inflammation in horses with heaves. Since there seems to be many more horses with IAD than with heaves, other factors (perhaps genetics) must play a role in the development of heaves.”

Some researchers suspect that if left untreated, IAD could lead to heaves. Wichtel said those horses with IAD that develop heaves later in life probably had “(at least) a genetic predisposition to do so.”

In 2008, Wichtel and her colleagues acquired a portable pulmonary function testing (PFT) system that helps provide early diagnosis so horses can be treated promptly, thereby decreasing the likelihood of permanent damage to the lower airways, and in susceptible horses, preventing progression of the disease to heaves. This non-invasive system has allowed the ability to compare lung function to inflammation levels. And she and her colleagues have been working to determine whether IAD can be diagnosed solely with this system or in conjunction with other traditional techniques and technologies.