Equine asthma is caused by inhalation of dust and toxins from the environment, usually when a horse is stabled. Hay and straw contain fungal spores and organic material which trigger an allergic response causing inflammation and narrowing of the lower airways. The inhaled particles that cause the allergic reactions in susceptible horses are known as allergens.

When hay is baled with high moisture content (above 20%), the bales can heat up and the growth of moulds (such as Aspergillus Fumigatus, Faenia Rectivirgula and Thermoactinomyces Vulgaris) occur. All hay made in the UK can have mould growth, even good quality hay. Inhalation of such spores triggers an allergic reaction in some horses and the airways become hypersensitive. There is also a similar disease seen in some horses that live out at pasture. This is another variation or type of asthma often called 'summer pasture associated obstructive pulmonary disease' (SPAOPD). It produces the same clinical signs and may be linked to pollens. It is thought that some horses have a genetic susceptibility to equine asthma and are more prone to this sort of problem.

Free radicals and oxidative stress:-

The horse’s respiratory system is constantly exposed to external irritants. Researchers have shown that this can cause the release of abnormal amounts of ‘free radicals’ which can attack cell membranes. (A free radical is a molecule or atom that has one or more unpaired electrons. It is unstable and tries to attract electrons from other molecules to pair up with these. This can start a chain reaction with increasing amounts of free radicals being released).

If the production and removal of these is not controlled, tissue damage may occur leading to inflammation. This is known as ‘oxidative stress’ and may be an important factor in the development of equine asthma. In a normal horse, the production of free radicals is controlled and those produced are ‘neutralised’ by well-developed antioxidant defence mechanisms. Experimental work has shown that the major antioxidant in the fluid lining of the lungs is ascorbic acid (vitamin C) and horses suffering from  airway inflammation often have reduced levels.

Predisposing factors:-

• repeated exposure to hay and straw dust containing moulds, forage mites, plant/tree pollens, endotoxins and inorganic material

• dusty feeds

• long hours in the stable or, in the case of SPAOPD, out at pasture

• poor stable hygiene

• inadequate ventilation

• a respiratory virus e.g. equine influenza or strangles

The disease usually develops over a period of time. Affected horses do not have a temperature and they appear well in themselves, but they may have reduced exercise tolerance and not be able to perform to the best of their ability.

The first clinical signs include:

• reduced exercise tolerance

• increased respiratory rate

• increased expiratory effort (the abdominal muscles are used to force the air from the lungs)

• an occasional cough, usually at the start of exercise

• milky white nasal discharge from both nostrils, especially first thing in the morning and after exercise.

In performance horses (especially racehorses and eventers) mild equine asthma may produce no overt clinical signs, and the only indication of airway disease may be a loss of exercise tolerance.

As the condition progresses, breathing out (expiration) requires even more effort and there is considerable movement of the abdominal muscles; this is known as ‘heaving’;

• the abdominal muscles become overdeveloped and a ‘heave line’ is seen

• the respiratory rate increased further

• the nostrils flare as the horse or pony struggles to breathe

• the horse’s anus moves in and out in time with the respirations

• the nasal discharge may become thick and yellow

• the horse coughs at rest in the stable

• lumps of mucus are coughed up and can be seen outside the stable door and in water/feed buckets

Once a horse has become sensitised, it may suffer acute attacks of the disease and develop severe respiratory difficulties in a short period of time. The animal breathes with flared nostrils and heaving flanks, and has spells of continuous coughing. Wheezing and crackling sounds may be heard at the nostrils and the incidence of the disease increases with age.

Why do these signs develop?

The expiratory difficulty is caused by obstruction of airflow in both the large and, especially, small airways. This is due to spasm of the smooth muscle (bronchospasm) in the larger airways, and bronchospasm, inflammation and the accumulation of mucus in the small airways (bronchioles). Both lead to a reduction in airway diameter so breathing requires more effort. These changes occur within a few hours of a susceptible horse being placed in a stable or dusty environment

If the disease is not controlled, structural changes occur in the lungs over a period of time. The chronic inflammation leads to thickening of the mucosal lining of the proliferation of the smooth muscle. In severe, long term disease, the alveoli (air sacs) become over-inflated and emphysema (i.e. structural destruction of the alveoli) can develop. This end stage lung disease is what is traditionally called “broken wind”.

When to call the vet?

Early diagnosis, treatment and management changes often prevent it from developing into a serious problem, so contact Bell Equine as soon as you have any concerns.

Diagnosis is made based on the history and the clinical signs. In mild cases the vet may not hear any abnormal sounds with the stethoscope. In severe cases, a wide range of lung sounds including crackles and wheezes may be heard.

In some cases the upper respiratory tract is examined with an endoscope, and a stream of mucopus may be seen in the trachea. Examination of airway secretions at the laboratory can reveal large numbers of neutrophils (pus cells) when compared with secretions obtained from an unaffected horse. 

At Bell Equine we make take samples to investigate the disease using either a tracheal wash or a bronchoalveolar lavage.

TREATMENTS AND CONTROL:

Once a horse has developed a hypersensitivity to stable dust, there is no cure. If treated promptly, the changes are reversible but the horse remains more sensitive to respiratory allergens than normal.

- Treatments:

• environmental control

• administration of bronchodilators to ease breathing

• corticosteroids to reduce the inflammation

- Management:

The key to both treatment and prevention of the disease is environmental control by good stable management and stable design to minimize exposure to dust.

- Turning out:

All horses should have plenty of fresh air with minimal exposure to dust and fungal spores. Whenever possible, the first step in treatment of horses with equine asthma is to turn them out completely for a period of at least two to three weeks (preferably longer).

The field should be well away from the muck heap and the hay store. Bringing the horse into a stable for even a short period, e.g. for grooming or for the farrier, will cause further inflammatory changes to take place in the lungs and prolong the period of recovery. The best management for a horse with equine asthma is to keep it permanently out at grass.

- Stable design:

Where it is not possible for horses to live out, changes in stable management and design may be beneficial.

Good stable ventilation is essential for the horse’s health. Many modern boxes have low roofs and insufficient air vents to achieve the recommended ventilation rate of 8-10 complete air changes per hour.

Talk to our vets at for practical advice on what is best for your horse.

Some suggestions could include:

• Additional air inlets and outlets. To avoid draughts, air inlets should be positioned at the same height as the eaves. Ideally, each stable should have an outlet at the highest point in the roof

• An extra window at the back of the box can improve the air quality and considerably reduce the incidence of the disease

• Top doors should routinely be left open

• Wherever possible, each horse should have its own air space and the side walls should reach the roof of the stable.

- Bedding:

Straw should be avoided as it has higher levels of fungal contamination than a well managed peat; dust-extracted wood shavings; cardboard or paper bedding. All beds need to be kept clean and dry; the urine and droppings must be removed each day to prevent mould growth. Deep litter is not recommended as high levels of noxious gases such as ammonia and hydrogen sulphide may be produced (mould will also grow in the bedding when it becomes damp and soiled); these gases are irritant to the respiratory tract (as are some disinfectants, pesticides and preservatives).

Rubber matting may be helpful if used with a small amount of bedding material. However, good drainage and management are required with rubber mats to prevent pooling of urine and an unhealthy atmosphere.

- Mucking out:

It is essential that all the bed is removed and replaced at regular intervals. Wood shavings and paper beds that look clean can develop high levels of fungal growth after several months in the stable especially if the environment is damp.

During normal mucking out, the number of fungal spores and the dust levels in the stable is increased three to six fold, and can remain airborne for many hours. It therefore makes sense to muck out as soon as the horse is turned out so the spores and dust have a chance to settle before the horse is brought back in again.

Mucking out while the horse is in the box should be avoided in all cases. Ledges, window sills and ceilings should regularly be cleared of dust and cobwebs to remove dust, fungal spores, bacteria endotoxins and general dirt.

- The muck heap:

The muck heap should be sited as far from the stables as practical and preferably downwind.

- Transport:

When travelling, horses are often exposed to high dust levels. Straw and shavings in lorries and trailers quickly become musty and mixed up with old hay.

The best solution is to use rubber matting and be careful if feeding hay inside the vehicle. Lorries and trailers should also be well-ventilated.

- Respiratory infections:

Horses with equine asthma should avoid contact with others suffering from respiratory viruses.

The signs may be exacerbated following a respiratory infection. Remember that all horses that catch a respiratory virus infection are at increased risk of developing respiratory difficulties at this time, so they should be kept in a 'clean air' and 'minimum dust' environment.

- Hay:

Wherever possible, hay should be excluded from the diet as even well made, good quality hay has very high levels of dust and fungal spores. Grass, haylage or vacuum-packed forage such as Horsehage are suitable alternatives.

- Buying and storing hay:

It is always worth buying the best quality hay that is available. When a bale is opened it should have a fresh, sweet smell with no visible mould or dust. Hay should be stored in a separate building from the horse because millions of fungal spores are released into the atmosphere when hay nets are filled. In order to minimise fungal growth, the bales should be raised from the floor on wooden pallets.

- Soaking hay:

The area within 30 centimetres around the horse’s nose is called the ‘breathing zone’. When a horse pulls dry hay from a net, large numbers of fungal spores and dust particles (up to 63,000 per litre of air) become airborne and are inhaled.

Ideally hay should be soaked for 30 minutes to one hour in clean water just prior to feeding to significantly reduce the amount of dust inhaled. If using a net, it should be positioned by the door or a window. If the stable is large enough, it should be tied so there is minimal mixing with the bedding and any that is not eaten can be swept up and removed.

- Vacuum-packed forage (Haylage):

Vacuum-packed forage has been developed as an alternative to hay. Grass is cut and allowed to wilt before being baled and compressed. The bales are sealed in bags to exclude air and a mild fermentation process begins. Under these conditions mould growth is inhibited and the feed will keep for up to 18 months.

Vacuum-packed forage has a higher nutritional content than most hay. It should be introduced to the diet over a period of 2 – 3 weeks and concentrates may need to be reduced. Opened packs should be used within 5 days and if the bag is accidentally punctured it should be fed immediately. Horsehage High Fibre is lower in sugar and starch and is suitable for horses and ponies prone to laminitis.

A feeding guide can be obtained from the manufacturers. Hay nets with small holes slow down the intake of this forage.

- Concentrates and grains:

A complete cubed diet or molassed mix has considerably less dust and fungal spores than rolled grains such as oats or barley.

- Antioxidant supplementation:

Antioxidants are sometimes known as ‘free radical scavengers’ and they are able to neutralize free radicals. Some antioxidants occur naturally in the body but their capacity may be overwhelmed in the face of challenge.

Horses and ponies suffering from equine asthma often have low natural antioxidant defences. Dietary supplementation with a balanced antioxidant mix has been shown to improve lung function and reduce inflammation . Vitamin C is the most important antioxidant in the fluid lining of the lungs.

- Exercise:

Horses with severe breathing difficulties should not be worked. Mildly affected horses should have their exercise restricted to a level they can manage comfortably, but exercise can be beneficial to help clear the lungs. During a bout of coughing, the horse should be allowed to extend its head and return to walk.

With good management, a definite improvement should be seen in mildly affected horses within 3 – 4 weeks. However, moderate and severely affected horses require medication to alleviate the respiratory distress and aid recovery.

The most useful medications are bronchodilators and corticosteroids.

Bronchodilators:

Bronchodilators are used to relieve the respiratory distress experienced by horses during acute episodes. They relax the smooth muscle in the airways. Bronchodilators are useful in the short term to relieve bronchospasm, but they do not address the underlying problem of inflammation of the small airways. Using bronchodilators as a sole treatment in the long run is usually contraindicated, as it opens up the airways and allows dust and allergend to access the lower airways, which can make the problem worrse in the long run.

The commonly used ones include:

• Clenbuterol (Dilaterol or Ventipulmin) relieves bronchospasm and increases the speed of clearance of mucus from the airways. The drug can be given intravenously or orally. Advice should be sought from Bell Equine regarding the dose if your horse fails to show a satisfactory response to the standard dose – any increase in dose rate must be undertaken gradually. At higher doses, side effects of sweating, trembling and raised heart rate may occur. With long term use, horses can become resistant to the effects of Clenbuterol, so it is generally best used in acute episodes or “flare ups”.

• Inhaled bronchodilators such as Salbutamol (Ventolin) are occasionally administered using an Equine Aeromask or other inhalation devices but their duration of action is relatively short.

• Hyoscine (Buscopan) may be given once by intravenous injection at the start of treatment to relieve acute respiratory distress. If bronchospasm is a contributory factor, the drug will be effective and provide relief within 15-20 minutes.

Corticosteroids:

Corticosteroids are the most effective drugs for reducing inflammation in the lungs of horses with equine asthma. They can be administered systemically (ie by injection or orally) or by inhalation. A horse with severe disease may need corticosteroids to be given by injection or by mouth because to start with they are unable to breathe in enough of the drug in by inhalation. Alternatively, bronchodilators may be given first, to allow better distribution of the inhaled drug. Dexamethasone and prednisolone are commonly used injectable and oral forms.

Once the symptoms are controlled, inhaled medication is preferable as there is less risk of side effects which include laminitis, Cushings-type signs and suppression of the immune system. Immunosuppression increases the risk of the horse succumbing to bacterial infection. A number of systems for delivering the drug are now available. Examples are the 'Equine Haler' inhalation spacer or in smaller ponies, pediatric spacing chambers can be used. Flexineb is a facemask equine nebuliser and the 'Aservo Equihaler' is a Soft Mist inhaler, providing better deposition of the steroids into th edeep lung. 

Other Medications:

Mucolytics are medicines that help to break up the mucus so it is more easily cleared from the airways. They can be useful in cases where there is a lot of thick, tenacious mucus present in the airways. An example is sputolosin.

If managed correctly, the changes in the lungs of most affected horses are reversible.

However, once a horse or pony has been sensitised, the symptoms will recur if it is exposed to the offending environmental allergens. The airways also become hyper-responsive to other irritants in the atmosphere such as noxious gases.

The condition tends to become worse with age and causes reduced exercise tolerance in performance horses. The prognosis is therefore guarded.