Scientific and Equine Consultant

Respiratory problems in horses: How to recognise, avoid and manage them

Respiratory problems in horses: How to recognise, avoid and manage them

Dr David Marlin


The importance and limitations of the respiratory system of the horse

In the unfit/untrained horse, it is the heart that is the main limitation to exercise performance followed by the muscles. As a result of training both the heart (which is made of a special type of muscle) and the muscles change and become more able to cope with exercise. However, the respiratory system cannot and does not change as the horse gets fitter with training and so in trained horses, the respiratory system is the main limitation to exercise performance. Some studies have also shown that the effects of respiratory disease may extend to adverse effects on other areas of the body, such as the muscles and the heart (Gehlen et al. 2008a & Gehlen et al. 2008b).


How common is respiratory disease in horses?

Many survey type studies have found that after lameness, the most common reason for a vet to be called out or for horses to not be able to be exercised or competed is respiratory disease. In a study of 3000 horses in Germany, it was observed that respiratory disease occurred in 420 horses over one year (Knubben et al. 2008).

In a study of horses referred for a veterinary examination because of poor performance, 81% (4 out of 5) had respiratory disease (Nolen-Walston et al, 2013).

In a study in Germany (Herholz 1994), vets examined 112 sport horses (dressage and showjumpers) who the owners thought were perfectly healthy because they were not showing any of the classic signs of respiratory disease (see below under How to Recognise Respiratory Disease in Horses). When examined by vets using endoscopy (putting a ‘scope into the trachea [windpipe]) and auscultation (listening to the lung sounds with a stethoscope), over half the horses were found to have respiratory disease. So, even if a horse is not showing any signs of respiratory disease, the only way to be sure is to have a proper clinical examination by a vet. This is especially important for horses that are competing.

Even horses that live outside 24/7 can have sub-clinical respiratory disease (i.e. disease not apparent from simply looking at the horse). In a group of 14 elite endurance horses which all lived out, on veterinary clinical examination, 12 had evidence of respiratory disease.

Recurrent airway obstruction (RAO or “heaves” or equine COPD or “broken wind”) is the most common chronic (essentially there for life) respiratory disease affecting horses in the UK. It is a disease affecting older horses (predominantly 8 years plus) and has been estimated to affect 10-17% of the total horse population in the UK (Hotchkiss et al. 2007). This may mean that in horses over 8 years old, the prevalence is nearer to 1 in 2 or 1 in 3 horses with RAO.


What are the causes of respiratory disease in horses?

The causes of respiratory disease in horses can be divided up as follows:

  1. Infectious: respiratory disease caused by infectious agents
    1. viruses (e.g. equine influenza virus, equine rhinovirus, equine herpesvirus types 1 & 4)
    2. bacteria (e.g. Streptococcus zooepidemicus, Streptocioccus pneumoniaePasteurella specis, Actinobacillus species)
    3. mycoplasma (e.g. Mycoplasma equirhinus)


  1. Allergic: respiratory disease caused by allergic reactions to inhaled materials
    1. pollen (grass, tree, shrub, crop)
    2. mould/fungi
    3. dust/forage mites
    4. bacterial toxins


  1. Irritant: respiratory disease caused by inhaled material that is irritant (but not allergic)
    1. ammonia (produced by bacteria in the bedding from protein in urine)
    2. dust (e.g. fine sand)
    3. cold air (similar to “ski asthma” in people)


  1. Other
    1. Anatomical dysfunction (e.g. upper airway obstruction: “roaring”, “gurgling”)
    2. Lung bleeding during exercise (exercise-induced pulmonary haemorrhage EIPH)
    3. Reactions to medications or treatments
    4. Pulmonary embolism or thrombosis (air bubbles and blood clots that trap in the lungs)


In younger horses, respiratory disease due to bacterial infection is more common than due to viruses or allergy. In older horses, allergic disease is more common. In young children when they go to playgroup or school they initially suffer a lot of “colds” due to viral infections. It has been shown that this is a necessary part of the development of the immune system and in fact that these repeated early infections whilst unpleasant for children and inconvenient for parents, may actually be protective against certain immune mediated disease such as asthma and leukaemia. Foal and young horses also suffer repeated respiratory infections, but these are more commonly due to bacteria than viruses. Whilst severely affected horses may be treated with antibiotics, it is not possible to “protect” a horse from this phase of development by continuously treating them with antibiotics.  As in children this is simply a phase they must go through. For most sport horses this is not a problem as this phase usually occurs between 1-3 years of age, but it can be a significant problem for two year old racehorses.


How to recognise respiratory disease in horses

The most common external (i.e. ones that can be seen without the need for a veterinary examination with an endoscope, stethoscope, blood test, x-ray or ultrasound) clinical signs of respiratory disease in horses can include any combination of one or more of the following:

  • Cough
  • Nasal discharge (clear, white, yellow, green, think and runny or thick or foamy)
  • Increased respiratory rate at rest
  • Increased respiratory effort at rest
  • Increased temperature
  • Abnormal breathing during exercise (e.g. abnormal noise or irregular breathing)
  • Slow recovery after exercise
  • Poor exercise performance
  • Frequent swallowing during exercise
  • Blood at the nostrils after exercise

A horse with respiratory disease may have one or more of the above symptoms. Some combinations of symptoms may be more likely to indicate a problem with the upper airway (essentially the nostrils and airway in the head) whilst other symptoms may be more indicative of lower airway disease (essentially disease in the lungs themselves).

It is a myth that a horse must be healthy if it doesn’t cough. Compared to people horses are much less likely to cough even when they have quite severe respiratory disease. In one study it was shown that cough has a 84% specificity but only a 36% sensitivity for detecting the presence of respiratory disease or infection in horses. What this means in simple terms is that if a horse is coughing there is a very high (84%) chance that it has a respiratory tract infection or some other form of respiratory disease (high specificity), but if it is not coughing there is a smaller chance that its healthy (36% – low sensitivity).

When people have respiratory disease they cough frequently and more or less constantly e.g. every few minutes. Horses have been shown to be different and may cough a few times and then not cough for several hours and then cough for 10 minutes and then not cough for a further 5 hours. This can make it less likely for an owner to hear a horse coughing.

It is also often said that it is “ok for a horse to cough a few times” during warm-up as this is simply the horse clearing its throat. This is NOT true and the horse that coughs a few times during warm-up is likely to have some respiratory disease that should be investigated by a vet.

Respiratory disease may be apparent in horses at rest in the stable or during or after exercise. Symptoms of respiratory disease is also often seen in horses after travelling due to a combination of stress, enclosed space with reduced ventilation and air quality, dehydration and the fact the head is kept in an elevated position which reduces the natural clearance of inhaled material.


Treatments for respiratory disease in horses

Common treatments prescribed by vets for the treatment of respiratory disease include:

  • Antibiotics: when there is evidence of bacterial infection these may be given orally or by injection.
  • Bronchodilators: drugs which open up the airways (e.g. Ventipulmin) to allow the horse to breathe easier and to clear mucus. These may be given in feed, intravenously or by inhalation.
  • Anti-inflammatories: drugs that reduce inflammation (e.g. dexamethasone). These may be intravenously, into the muscle, in feed or by inhalation.
  • Antioxidants: High dose antioxidant supplements have been shown in several controlled studies to be beneficial in reducing respiratory symptoms.


How can you reduce the risk of respiratory disease in your horse?

Many studies have shown that a major factor in respiratory disease in horses is the air quality in the stable although even horses that live out 24/7 can develop respiratory disease. The main factors that relate to air quality in the stable are the hygienic quality of bedding and forage, the flooring and the ventilation. Several studies have shown that people working with horses in stables for 6-8h each day have increased respiratory symptoms compared with those who don’t work in stables, so it is clear that if a horse is breathing the same air for 3-4 times longer each day that the effect on respiratory health will be even greater.

Flooring: The ideal flooring is a sealed rubber floor with a built-in fall towards a drain or the stable door that is easy to clean with a pressure washer or hose and brush.  A sealed rubber floor allows the amount of bedding in the stable to be reduced which in turn reduces the amount of dust in the stable. Rubber floors can be sterilised reducing the amount of bacteria on the floor and reducing the production of ammonia. As a guide, if you can smell ammonia in a stable then it is already at a level that will cause irritation of the horses’ airways.

Bedding: The ideal bedding is dust free, absorbent, inexpensive and will encourage the horse to lie down. In the past 25 years in professional racing and sport horse yards, large-chip shavings have essentially replaced straw as the first choice bedding. However, recent studies showed that horses actually lay-down more on straw than on shavings. Good straw may in some cases contain less fungi and bacterial toxins than ordinary low-quality shavings.

Stable Management: Don’t be tempted to much-out with you horse in the stable. The process of mucking out raises a tremendous amount of respirable dust into the air which will hang around for at least 30 minutes. Try to make sure all windows and doors are open when you muck-out and wait at least 30 minutes before putting your horse back in the stable.

Ventilation: Many stables have poor ventilation and this can be made worse when owners shut windows and stable top-doors during  poor weather. Horses with rugs on are perfectly able to cope with draughty well-ventilated stables. The dislike and concern over draughts is a human problem not a horse one.  Closing the stable top-door and window over a single night may be enough to cause a problem in more susceptible horses.

Feeding: Feeding forage and hard-feed (wetted) from the floor will encourage head lowering.

Forage: The ideal forage will have a low level of respirable dust (dust of a size that can be breathed into the lungs – only dust particles of a particular size reach the lower lungs. Large particles are trapped in the upper airways and very small particles are breathed in and then back out). Soaking hay has been shown to reduce respirable dust but takes up a fair amount of time and effort and soaking tubs need regular cleaning or else they become unhygienic. Soaking also reduces the nutritional value of the hay as water soluble vitamins, minerals and carbohydrates (sugars) are lost into the water. This can be an advantage in managing some horses (e.g. laminitics). Steaming hay is a good option. Not all steamers perform as well, but good ones significantly improve the hygienic quality of hay, are quick and simple to use and result in less loss of nutrients. Another option is to feed haylage.

Turnout: Time out of the stable is extremely beneficial in reducing respiratory disease, especially if horses can perform grazing behaviour. If there is concern about the amount of fresh grass horses will eat, then rather than shorten or avoid grazing, the use of grazing muzzles should be considered. These restrict the amount of grass the horse can actually eat but at the same time the horse is out of the stable (which recues exposure to stable “dust”) and has its head lowered (which aids clearance of material from the airways).  Even turnout in bare paddocks with hay/haylage fed from the floor is beneficial to maintain respiratory health compared with stabling.

Vaccination: Keeping your horse up to date with relevant vaccinations. Vaccination may or may not prevent your horse from infectious disease. However, in most cases, if a vaccinated horse does contract an infection then the severity and duration are usually reduced compared with unvaccinated horses.

Disinfectants: By all means use disinfectants perhaps once a week when you give the stable a thorough clean-out but be careful what you use and follow the manufacturers instructions. Using certain disinfectants (e.g. Jeyes Fluid) in confined spaces such as stables can damage the horses airways.

Travelling: Travelling can cause respiratory disease or worsen symptoms in horses with existing problems such as RAO. Always check your horses temperature before travelling. If your horse has a temperature then it is unwise to travel and compete as the risk of respiratory disease will be increased. Travel with good ventilation and make sure forage is soaked or feed steamed hay or haylage and use a low dust bedding. The longer the journey the more important it is to think about respiratory disease.

Scoping: Because horses can often have mild-moderate respiratory disease and not show any signs, if you are competing then it’s advisable to ask your vet to ‘scope your horse at the start of the season, mid-way through the season and at least 2-3 weeks before any major or important competitions or for any event where you might have to travel more than 6 hours. The reason for ‘scoping 2-3 weeks in advance is to allow time for laboratory analysis of samples your vet may take from the airways, time for treatment and time for withdrawal of medication before competition.

Date: February 09, 2015