Heaves in Horses: Causes, Symptoms and Treatment Explained

Heaves in horses is the oldest name for what vets now call severe equine asthma, and it remains the commonest long-term breathing problem of mature horses in Britain. It is an allergic airway disease driven almost entirely by what the horse breathes in, which is both the bad news and the good news: the dust and mould that cause it are things you control. With the right care, the great majority of affected horses go back to breathing comfortably and working normally. This guide explains what heaves is, how to recognise the heave line and the earlier signs that come before it, and exactly what the evidence supports doing about it.

What Are Heaves in Horses?

Heaves is a chronic, non-infectious inflammatory disease of the lower airways. In a susceptible horse, inhaled dust, mould spores and other small particles provoke an allergic-type reaction deep in the lungs. The airway lining becomes inflamed, the smooth muscle around the airways tightens, and the airways fill with mucus. All three narrow the tubes the horse has to move air through, and the horse compensates by working much harder to breathe out.

The effort matters more than it sounds. Breathing in is an active movement in any horse, but breathing out is normally largely passive. A horse with heaves has to push air out against narrowed airways, using the abdominal muscles to do it. That extra work is what produces every classic sign of the disease, from the flared nostril to the heave line itself.

Heaves is a lower airway disease, which is worth stressing because owners often lump all noisy breathing together. A horse that makes an abnormal roaring or whistling noise only at fast work, and breathes silently and easily at rest, is far more likely to have an upper airway obstruction such as laryngeal hemiplegia, covered in our guide to wind surgery in horses. Heaves does the opposite: it is at its most obvious when the horse is standing still.

It is also not contagious. One horse cannot catch heaves from another. Where several horses on the same yard start coughing together, the shared cause is usually the shared air, the shared hay or an infection passing round, not the heaves itself.

Heaves, COPD, Broken Wind and RAO: One Disease, Many Names

Few equine conditions carry as many names as this one, and the confusion is a genuine obstacle for owners trying to research it. Heaves, broken wind, COPD, RAO and severe equine asthma are all the same disease. The names are historical layers, not different diagnoses.

• Heaves and broken wind are the traditional stable-yard names, both describing the visible breathing effort rather than the underlying pathology.
• COPD (chronic obstructive pulmonary disease) was borrowed from human medicine and is now considered a poor fit, because the human disease is largely irreversible and the equine one is not. You may still meet it in older books and on our own glossary entry for equine COPD and heaves.
• RAO (recurrent airway obstruction) replaced COPD and is still widely used, including by the MSD Veterinary Manual and in our equine RAO glossary entry.
• Severe equine asthma is the current term, and the one your vet is most likely to write on the invoice.

The change was deliberate. In the revised consensus statement of the American College of Veterinary Internal Medicine (Couëtil and colleagues, Journal of Veterinary Internal Medicine 2016;30(2):503-515, PMID 26806374), the expert panel adopted the umbrella term "equine asthma" to cover horses with mild through to severe airway disease, on the grounds that the conditions previously called inflammatory airway disease and RAO form one spectrum rather than two separate diseases. The British Horse Society uses the same framing, describing equine asthma as an umbrella term for the diseases previously known as inflammatory airway disease, COPD, RAO and heaves.

In practice the spectrum has two ends. Mild to moderate equine asthma shows up as an occasional cough and a drop in performance in a horse that breathes normally at rest. Severe equine asthma, which is what "heaves" has always meant, is the end where the horse struggles to breathe standing in its stable. The BHS draws the line in exactly that place: difficulty breathing at rest is typically seen only in severe cases. The milder end matters to owners of horses that have never heaved in their lives, which is why it is covered in full in its own section below; the equine asthma glossary entry gives the short version of the terminology.

How Common Are Heaves in UK Horses?

Heaves is common enough that most yards have a horse with it, whether or not it has been diagnosed. The best British figure comes from a survey of horse owners across Great Britain by Hotchkiss, Reid and Christley (Equine Veterinary Journal 2007;39(4):301-308, PMID 17722720), which used a validated screening questionnaire with two-stage cluster sampling of veterinary practices and their clients. It put the estimated true prevalence of RAO in the general British horse population at 14.0%, with a 95% confidence interval of 10.7 to 17.4%. The British Horse Society quotes the same 14% figure for severe equine asthma in UK horses.

That study also identified the risk factors, and they are informative in themselves. Risk rose with increasing age and with exposure to an urbanised environment. A second model linked exposure to hay and respiratory infection in early life to a horse having RAO later on, which suggests that challenges to a young horse's airways may help set the disease up years before it appears. Heaves is strongly associated with mature horses rather than youngstock, but the research does not support quoting a specific age at which it starts, so treat any tidy cut-off you read with suspicion.

The Heave Line in Horses: What It Is and How to Spot One

The heave line is the single most recognisable sign of heaves in horses, and the one owners most often ask about. It is a visible ridge of muscle running along the lower edge of the abdomen, angling up and back towards the flank. It is not swelling, fluid or fat. It is genuine muscle development, built up exactly the way any muscle builds up under repeated work.

Why a Heave Line Forms

In a healthy horse, exhaling costs very little effort: the chest wall and lungs recoil and the air leaves. When the small airways are narrowed by inflammation, mucus and muscle spasm, that passive recoil is no longer enough to empty the lungs. The horse recruits its abdominal muscles to squeeze air out on every breath, hundreds of times an hour, day and night. Over weeks and months those muscles hypertrophy, and the external oblique becomes prominent enough to cast its own shadow. The line marks the lower border of that overdeveloped muscle.

How to Check Your Horse for a Heave Line

1. Stand the horse square on level ground and look from the side, ideally with the light coming across the body rather than straight at it. Low, raking light shows the contour far better than flat midday sun.
2. Look along the bottom of the ribcage and follow the line back towards the flank. A heave line runs diagonally upwards and backwards from behind the elbow.
3. Watch several full breaths rather than taking a single glance. A heave line is most obvious at the end of exhalation, when the muscle is contracted.
4. Compare the two sides, and compare against a photograph of the same horse taken months earlier if you have one. Muscle development is easier to judge against your own horse than against a textbook picture.
5. Check the nostrils and the flank movement at the same time. A heave line rarely appears on its own, and the other signs of effort give you the context.

What a Heave Line Does and Does Not Tell You

A heave line is strong evidence of sustained, increased expiratory effort, and in an adult horse that is usually severe equine asthma. But it is a late sign, not an early one. Muscle does not hypertrophy in a week. By the time the ridge is clearly visible, the horse has been working hard to breathe for a long stretch, and the airway inflammation behind it is established rather than new. Finding one is a reason to get the vet out promptly, not a reason to feel you caught it early.

The reverse also holds, and matters more: the absence of a heave line does not rule heaves out. Plenty of horses with genuine, treatable airway disease never develop a visible line, particularly those in the milder part of the spectrum or those whose flare-ups are seasonal and short. A well-covered horse can hide a good deal of muscle definition under condition, and a clipped horse in winter light may show a line that a hairy one conceals. Judge the horse's breathing, not just its outline.

One practical point: a heave line does not vanish overnight when treatment works. The airway inflammation settles first and the breathing eases; the muscle takes far longer to reduce, and in a horse that has had heaves for years it may never fully disappear. Do not use the line as your measure of whether management is working. Use the cough, the respiratory effort at rest and the horse's exercise tolerance.

Signs and Symptoms of Heaves

Heaves is a spectrum, and the earliest signs are the ones most easily explained away as the horse being unfit, getting older or having "a bit of a cough". Catching it at that stage is the difference between a management change and a crisis.

Early Signs That Are Easy to Miss

• An occasional cough, characteristically at the start of exercise or as soon as the horse puts its head into a haynet.
• A drop in performance that does not match the horse's fitness, or taking noticeably longer to get its breath back after work.
• A small amount of white or cream nasal discharge, often noticed on the stable wall or the rug rather than on the horse.
• Reluctance to go forward, or a horse that feels like it is working with the handbrake on.
• Signs that are clearly worse after a night in and better after a day out, or worse in one particular stable.

A cough is the sign most owners notice first, and it is worth remembering that it is not specific to heaves. Our guide to coughing in horses works through the other causes and when each becomes likely.

Signs of Established or Severe Heaves

• A persistent cough, often in runs rather than single coughs.
• Visible effort to breathe while standing still, with a double lift of the flank on each breath out.
• Flared nostrils at rest.
• An increased breathing rate at rest, faster than that horse's own normal.
• Audible wheezing, and crackles or wheezes that the vet can hear through a stethoscope.
• A visible heave line along the lower abdomen.
• Weight loss in long-standing cases, because breathing this hard burns a surprising amount of energy.

The BHS is unambiguous that difficulty breathing at rest points to the severe end of the disease, and that any horse showing signs of equine asthma warrants veterinary advice. Our own advice is simpler still: a horse that is visibly struggling to breathe while standing in its stable is an emergency, and the fact that heaves is a chronic disease does not make a bad flare-up any less urgent. Phone the vet, and while you wait, get the horse out of the stable into fresh air and away from hay and bedding if it is safe to move it.

What Mild to Moderate Equine Asthma Looks Like

Everything above describes the severe end. The milder end of the same spectrum is where most horses are, and it is far harder to spot, because the one sign everybody watches for is absent: a horse with mild to moderate equine asthma breathes normally standing in its stable. The ACVIM consensus panel treats that absence of laboured breathing at rest as the practical line between the two.

What shows instead is performance. The horse works, but not quite as it should: it fades in the last part of a gallop, it is ridden off in a chukka it would normally win, it resists coming onto the bit or changes leg for no reason, or it simply lacks the quality it had a season ago. Alongside that you may notice a breathing rate that takes longer than usual to settle after work, a little more mucus at the nostrils than you would expect, and one or two coughs at the start of every ride.

That last one is worth stating plainly, because it is the single most commonly dismissed sign in the yard. A cough at the beginning of work is common. It is not normal. It means something in the lower airway is irritated, and a horse whose mild signs are left unmanaged can move along the spectrum towards the severe, heave-line end - the consensus panel notes explicitly that horses showing mild respiratory signs may progress to the severe form. Catching it here, before years of airway remodelling, is what gives the best long-term outlook, and the environmental changes further down this page are the same ones that work at this end.

What Causes Heaves in Horses?

Heaves is caused by inhaled particles in a horse whose immune system overreacts to them. The BHS groups these triggers as aeroallergens: dust, mould, bacteria, mites, plant debris, pollen, and irritant gases such as ammonia. Almost all of them are generated by ordinary stable management.

Dust and Mould in Hay

Hay is the single biggest trigger for most affected horses, and the exposure route is brutal in its simplicity. Hay that looks and smells perfectly acceptable still carries respirable particles and mould spores too small to see. When a horse buries its muzzle in a haynet, it inhales them directly into the deep lung, in a confined space, for hours at a time. A horse eating dry hay from a net in a closed stable is in about the worst position its airways can be in.

The Stable Environment

A barn or stable concentrates every trigger at once: dust from bedding, dust from hay and feed, mould in both, ammonia rising from wet bedding, and fine particles thrown into the air every time someone mucks out or sweeps. Poor ventilation then keeps those particles suspended for hours rather than letting them disperse. This is why so many horses with heaves are worse in winter, when doors and windows are shut and horses spend far longer inside. Our guide to how to design a stable covers airflow in more detail.

Summer Pasture-Associated Severe Equine Asthma

A smaller group reacts to something outdoors instead. This form, which the BHS calls summer pasture-associated severe equine asthma, flares in warm weather in horses kept at grass, and pollen is the usual suspect. It matters because it inverts the standard advice: for these horses, more turnout makes things worse, and the management is the reverse of the stabled case. If your horse is consistently worse in summer and better when stabled, say so clearly to your vet rather than assuming the usual pattern. Our guide to hay fever in horses covers the overlapping pollen-driven signs.

Why This Horse and Not the One Next Door

Two horses can share a stable block, eat the same hay and only one develops heaves. Susceptibility varies between individuals and there is a familial tendency, so genetics plays a part. Early-life airway challenge appears to matter too, given the association Hotchkiss and colleagues found between hay exposure and respiratory infection in early life and RAO in later life. What is not in doubt is that the trigger is environmental. The genetics load the gun; the dust pulls the trigger, and the dust is the part you can do something about.

How Heaves Is Diagnosed

Diagnosis starts with history and clinical examination, and in a classic case an experienced vet will have a strong suspicion before reaching for any equipment. The pattern of a mature horse that coughs, works hard to breathe at rest, improves at grass and worsens when stabled is close to diagnostic on its own.

What Your Vet Will Do

• Clinical examination. Listening to the chest with a stethoscope for wheezes and crackles, and assessing the pattern and effort of breathing at rest.
• A rebreathing examination. A bag is held over the nostrils briefly so the horse takes deeper breaths, which makes abnormal lung sounds much easier to hear. It also often provokes the characteristic cough.
• Endoscopy. The BHS notes that respiratory tract endoscopy is the most used diagnostic tool for equine asthma. A camera passed up the nose and down the windpipe shows mucus accumulation directly, and rules out upper airway problems at the same time. The mucus is graded on a scale, and the consensus panel treats a tracheal mucus grade above 2 out of 5, alongside marked exercise intolerance, as pointing to the severe end of the disease.
• Bronchoalveolar lavage (BAL). A small volume of fluid is flushed into the deep lung and retrieved, and the cells in it are examined. This is what distinguishes the asthma phenotypes from one another and from infection, and it is the test the consensus definitions are built on. It is also the test that tells your vet which end of the spectrum your horse is at. Against healthy reference values of no more than 5% neutrophils and 1% eosinophils, Couëtil and colleagues record pronounced neutrophilia above 25% as the usual picture in severe equine asthma, while more modest rises - above 10% neutrophils, or above 5% mast cells or eosinophils - fit the mild to moderate form. BAL is not the test for a horse in active respiratory distress; the priority there is settling the flare-up first.
• Blood tests and, occasionally, allergy testing. Mainly to exclude infection and build a fuller picture.

Ruling Out the Other Causes of Coughing

This step is not a formality. Several conditions produce a coughing horse with nasal discharge, and the management of each is completely different. A vet will be working to exclude equine influenza and other viral infections such as equine rhinitis virus, which are contagious and spread through a yard; equine sinusitis, which typically gives one-sided, often smelly discharge; bacterial lower airway infection; lungworm in horses grazed with donkeys; and the upper airway obstructions such as roaring that cause noise at exercise rather than effort at rest. A fever is the most useful early discriminator, because heaves itself does not cause one. Any coughing horse with a raised temperature should be treated as a possible infection until your vet says otherwise, and kept away from other horses in the meantime.

Managing Heaves Through the Environment

Environmental management is not the supporting act for heaves treatment. It is the treatment. Drugs relieve the inflammation and open the airways, but they do nothing about the particles causing the problem, and a horse returned to the same dusty stable relapses. The BHS puts it plainly: management changes such as more turnout, soaking hay and reducing aeroallergen exposure can often be enough on their own to reduce the signs.

The research supports that hierarchy in a striking way. Leclere and colleagues (American Journal of Respiratory Cell and Molecular Biology 2012;47(5):589-596, PMID 22721832) followed heaves-affected horses for a year on either inhaled fluticasone or antigen avoidance alone, with peripheral lung biopsies at six and twelve months. Lung function improved more quickly with the inhaled steroid, but normalised in both groups by the end. Airway smooth muscle remodelling fell by roughly 30% in both. And airway inflammation was better controlled by antigen avoidance than by the drug. Taking the dust away is not the cheap option; it is the one that treats the cause.

Hay: Soaking, Steaming and the Alternatives

Since hay is the main source, hay is where the biggest gain is. There are three approaches, and they are not equivalent.

Soaking wets the dust and spores so they cling to the hay instead of becoming airborne. The BHS is specific that the hay must be fully immersed, because hosing it down is often insufficient, and advises soaking for no more than 30 minutes to avoid excessive loss of nutrients. It is worth knowing that the trial which actually measured lung function used a longer soak: Westerfeld and colleagues (Journal of Veterinary Internal Medicine 2024;38(1):469-476, PMID 37930110) immersed hay for 45 minutes and discarded any that dried out between meals, and over six weeks lung resistance in the soaked-hay group fell significantly. The authors themselves noted that owners may find that protocol too inconvenient to sustain. The honest summary is that soaking works, that the BHS ceiling is about protecting nutrition rather than about dust, and that if you soak you should also think about what you are washing out. Our guide to the nutrients in hay covers what soaking costs you.

Steaming is the better-evidenced option and loses fewer nutrients. The BHS states that steaming kills 99% of mould and bacteria when the hay reaches 90°C to 100°C, and warns that home-made devices that fail to reach those temperatures can actually increase mould and bacteria. A UK study from the Royal Agricultural University (Daniels, Hepworth and Moore-Colyer, PLoS ONE 2020;15(11):e0242373, PMID 33201929) compared dry, soaked and high-temperature steamed hay and found steaming produced the greatest reduction in viable bacteria associated with respiratory disease, while soaking reduced bacterial diversity and increased Gram-negative bacteria. In fairness to the reader, that study was part-funded by a commercial hay steamer manufacturer and one author acts as its scientific adviser, which is declared in the paper; the finding is consistent with the wider evidence, but it is not independent.

Replacing hay altogether is the most reliable route and the one most often overlooked. Haylage carries a higher moisture content and therefore far fewer airborne particles, and our guide to haylage for horses covers feeding and storage. Soaked or unsoaked hay replacers, short-chopped forages and complete pelleted feeds remove the problem at source, and our round-up of hay alternatives for horses compares them. Avoid round bales: horses push their faces deep into them, and the dust exposure is correspondingly high.

Whichever you choose, feed from the ground rather than from a haynet. The BHS advises this specifically, noting that head-lowering helps clear the airways and that haynets have been found to increase inflammatory responses in the respiratory tract. Dampen bucket feeds too.

Bedding

• Move off straw, which is among the dustiest options, onto a low-dust bedding. The BHS lists dust-extracted wood shavings, paper, cardboard and pelleted straw; our guide to choosing shavings for bedding covers the practical differences.
• Avoid deep litter. The BHS describes it as the least suitable bedding system for respiratory health, because of the ammonia that builds up beneath it.
• Muck out with the horse elsewhere, and let the dust settle before it comes back in. Mucking out around a horse with heaves undoes the rest of your effort in ten minutes.
• Ensure the floor drains properly and lift and wash rubber matting regularly, so urine and ammonia are not trapped underneath.

Air, Turnout and Where the Horse Lives

• Maximise turnout. For every horse except the summer pasture-associated cases, outside air carries a fraction of the particle load of stable air, and time at grass is the most effective single change most owners can make.
• Ventilate rather than seal. Open doors, windows and vents, including in winter. A horse with heaves copes far better with cold air than with still, dusty, warm air; rug it more heavily rather than shutting it in.
• Never store hay or straw above or immediately beside the stable of an affected horse. Dust drifts down continuously.
• Think about the neighbours. Shared airspace means a dusty horse two boxes down is still your horse's problem. Position an affected horse at the end of a row, nearest the door, and away from the muck heap.
• Sweep and groom outside where you can, and deal with cobwebs, which are a good visual indicator of how much dust the airflow is depositing.

Feeding Horses with Heaves

Beyond cutting dust out of the ration, diet can support the lungs directly, and this is one area where there is decent controlled evidence rather than marketing.

Nogradi and colleagues (Journal of Veterinary Internal Medicine 2015;29(1):299-306, PMID 25307169) ran a randomised, controlled trial in horses with RAO and inflammatory airway disease. All horses were moved onto a complete pelleted diet with no hay, then randomised to a daily omega-3 supplement or a placebo for two months. Every group improved, which is itself a clean demonstration of what taking the hay away does. But the supplemented horses improved more: cough scores improved 60% against 33% on placebo, respiratory effort fell 48% against 27%, and bronchoalveolar lavage neutrophils dropped from 23% to 9% while the placebo group's rose from 11% to 17%. The supplement supplied 1.5 to 3 g of DHA per day. That is the dose the benefit was shown at, and it is worth checking against the label of anything you are asked to pay for.

The rest of the feeding advice is straightforward. Feed at ground level. If you have moved the horse onto soaked hay, remember that soaking leaches out water-soluble nutrients as well as dust, so a broad-spectrum vitamin and mineral balancer becomes more important, not less. Horses breathing hard burn energy doing it, so monitor condition in long-standing cases. And antioxidants, particularly vitamin E, are worth discussing with your vet or a nutritionist for a horse with ongoing airway inflammation.

Medical Treatment for Heaves

Medication earns its place when environmental change alone is not enough, when a horse is in an acute flare-up and needs relief now, or as a bridge while you reorganise the yard. All of the drugs used are prescription-only and must come from your vet, who will choose the type, dose and duration for the individual horse. The descriptions below are here so you can follow the reasoning, not so you can medicate a horse.

Bronchodilators

Bronchodilators relax the smooth muscle around the airways, opening them mechanically. They work fast, which makes them valuable in a crisis and sometimes before exercise, and they can be given before an inhaled steroid so the steroid reaches deeper into the lung. What they do not do is treat the underlying inflammation. The BHS notes that their effects are short-lived, lasting up to 12 hours, and that they should therefore be used alongside management changes rather than instead of them. A horse kept comfortable on bronchodilators alone in an unchanged environment is being masked, not treated.

Corticosteroids

Corticosteroids are the mainstay, because they act on the inflammation that drives everything else. They can be given by mouth, by injection, or inhaled through a mask or spacer fitted to the horse's nose. Inhaled delivery puts the drug where it is needed with less systemic exposure, which matters in horses at risk of laminitis or with other metabolic problems. As the Leclere study above showed, inhaled steroids get lung function back faster than environmental change alone, so they are genuinely useful; they simply are not a substitute for removing the trigger. Your vet will also weigh timing against competition rules if the horse is affiliated.

What About Natural Remedies?

"How to treat heaves naturally" is one of the most searched questions on this topic, and it deserves a straight answer. There is no herb, oil or supplement with evidence comparable to getting the dust out of the air, and nothing that will hold a horse in a severe flare-up. Omega-3 supplementation is the one nutritional intervention above with a controlled trial behind it, and even there the benefit was measured on top of a low-dust diet rather than instead of one. Some owners find respiratory herbs helpful as part of a wider plan, and our guide to herbs for horses covers what is commonly used, but they belong alongside veterinary treatment and a clean environment rather than in place of either. Steam inhalation and essential oils carry a specific caution: anything that irritates already inflamed airways can make a horse worse, so clear it with your vet first.

Can You Ride a Horse with Heaves?

Yes, in most cases, and this is where owners are often more pessimistic than the evidence warrants. A horse whose heaves is properly controlled can usually work normally, and many compete. Exercise is not harmful in itself; if anything, light work helps clear mucus from the airways, and the Horse & Hound vet library takes the same line on keeping controlled cases in work.

The sensible boundaries are these. During an active flare-up, rest the horse until its breathing has settled rather than asking it to work through laboured breathing. Warm up and cool down for longer than you would with a healthy horse, since narrowed airways take longer to adapt to a change in demand. Ride outdoors where you can: an indoor school with a dusty surface is a significant airway challenge, and watering the surface before you ride is worth the effort. And pay attention to recovery rate after work, because a horse taking longer than usual to get its breath back is often the earliest warning that a flare-up is building, well before the cough returns.

The Outlook for Horses with Heaves

Heaves cannot be cured, and it is worth being honest about that, but the prognosis for a well-managed horse is genuinely good. The airway narrowing is largely reversible, which is exactly why the human term COPD was abandoned for this disease. Leclere's twelve-month study found lung function normalised in both the antigen-avoidance and the inhaled-steroid groups, and that the structural remodelling of the airway smooth muscle itself reduced by around 30% over the year. The lungs are not simply held steady by good management; they recover.

The condition that does not change is the horse's sensitivity. Put a well-managed heaves horse back into a closed stable on dry hay and straw and the signs return, sometimes within days. The management is therefore permanent, and it needs to survive holidays, loan arrangements, moving yards and whoever is on the rota at the weekend. Write the horse's regime down and make sure the yard knows it, because the commonest cause of a sudden relapse is someone helpfully giving a haynet.

Horses diagnosed early, before years of remodelling, do best, and horses whose owners act on the environment rather than relying on repeat prescriptions do better still. With clean air, low-dust forage and veterinary support, the large majority of horses with heaves go on working comfortably for many years.

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