Why Does My Horse Cough?

A cough, in one form or another, is something we hear a lot. Whether it’s us, a family member, our horse, our cat, a dog. We all recognise a cough. And during COVID, anyone coughing gets a very wide berth. But what is a cough and what’s its function?

At the most basic level, coughing is a reflex that occurs when the air passage (airways) are irritated. Cough is a natural defence mechanism to help clear the air passages of mucus and virus and or bacteria or dust or allergens such as mould spores or pollen or irritants such as smoke or pollutants. Certain drugs can also initiate coughing, including ACE inhibitors, which is the most common, but also beta blockers, NSAIDS and aspirin.

Cough starts when cough receptors, specialised nerve cells, in the airways are stimulated. These send nerve impulses via the Vagus nerve to a region of the brain known as the caudal nucleus tractus solitarii (cNTS) and then into the respiratory centre in the brain, the medulla oblongata.

There are actually three phases to a cough. Firstly, a rapid inhalation, secondly a closure of the upper airways in the head at the same time as the abdominal muscles and chest expiratory muscles contract. This increases pressure within the lungs. The final phase is when the upper airway opens rapidly causing a rush of air out – the cough. And that air really does rush out – reaching speeds of around 100mph in a horse!

Interestingly, cough can be both voluntary i.e. we can decide to cough or involuntary – i.e. you can’t stop it. It just happens.

So, is cough normal? Well, there is no doubt it’s a very important protection mechanism but the bottom line is that healthy horses should not cough. Even occasionally. Even when warming up. The horse is a little different to us in terms of what hearing the odd cough means and I’ll now explain this in detail, but starting with using ourselves as an example.

In people, if we are coughing then there is a very high chance that we have some degree of respiratory irritation or disease. If we are not coughing then there is a very high chance, we are healthy. And when we have some disease, whether it’s allergic or infectious or another type, we tend to cough fairly regularly e.g. every few minutes. So, absence of cough means healthy and presence of cough means unhealthy. Horses are different. Of course, they are.

If a horse is coughing then there is a very high chance, they have some form of respiratory disease. BUT horses don’t tend to cough regularly. In one study where they placed students outside horses’ stables to record coughing, they found horses might not cough for 3h, then cough 5 times, then not cough for another 4h and so on. And even worse, if a horse is not coughing, you still cannot be sure it does not have respiratory disease. The only way to be absolutely sure is for a vet to carry out an endoscopy – placing a thin tube with a camera up the nose and into the airways. There are two basic types of respiratory examination – one in which the endoscope or ‘scope is only advanced down the trachea – the windpipe and not into the deeper parts of the lung.

In this examination, a small amount of saline may also be instilled into the trachea to “wash” the trachea and collect a sample of the mucus and cells and any bacteria. These samples can be processed in the lab to get a better idea of whether there is inflammation and/or infection. For this type of examination, often referred to as a tracheal wash, it is often not necessary to sedate horses.

The second type of respiratory examination is a BAL or broncho-alveolar lavage. This is a little more invasive as the ‘scope is passed past the trachea and into the deeper parts of the lung. This requires sedation and the use of local anaesthetic as passing the scope deep into the lung often triggers coughing. A lung wash can also be taken using saline solutions and analysed in a similar way to the tracheal wash.

Horse coughing — Why does my horse cough?

In Winter, at least outside in the fields, there is usually little pollution, dust levels are low and mould and pollen levels are very low. So, if your horse does cough it’s most likely due to something in the stable. This could be physical dust or mould growing on surfaces within the stable. Or it could be due to dust, bacteria and allergens brought into the stable in bedding, forage and feeds. Of course, you can manage this by choosing low dust and clean beddings and soaking or steaming hay or feeding haylage and damping down hard feeds.

As we move from Winter into Spring, things can change very rapidly. The first warm days of Spring as we come out of Winter can often trigger moulds to produce spores. People often think of summer as being the pollen season, but February and March can see very high levels of tree pollens such as Alder, Hazel, Yew, Elm and Willow combined with moderately high levels of grass pollens. And the amount of pollen depends obviously and not only on the trees around your fields, but it varies from region to region and year to year.

And Spring often coincides with increased training intensity, so your horse is moving more air in and out as the harder it works the harder it breathes, which also means more dust, pollen, etc being deposited in the lungs which the lungs defence system has to deal with.

And for many horses, long term exposure to these respiratory allergens leads to a lifelong respiratory disease – equine asthma. Equine asthma, which was previously called broken wind, then equine COPD and more recently RAO – recurrent airway obstruction, is very common in horses in temperate climates and the onset is typically around 6-7 years of age. It can result in reduced quality of life and reduced performance, combined with high ongoing veterinary costs.

One other thing to be aware of is that we have good evidence to show that fast canter and galloping exercise in cold air can lead to airway inflammation and cough in horses. Something to be aware of on frosty mornings, especially if you do have a horse prone to coughing.

Of course, cough is not the only sign of respiratory disease in horses. Nasal discharge either at rest or often only after exercise, increased respiratory rate and or increased respiratory effort are clear signs of respiratory disease.

So the bottom line?
It’s not normal for healthy horses to cough at any time. Don’t ignore the odd cough. Remember you are only with your horse a few hours a day. If you hear even a few coughs, then it’s possible that if you were there all day and all night you would hear a lot more coughing. It’s also not ok for horses to cough during warm-up or exercise. This is not the horse “clearing its throat”, its mucus moving in the airways or the increased airflow irritating the airways. And remember, even if your horse doesn’t cough, it might still have significant respiratory disease, even if it lives out 24/7. The only way to know the state of your horses’ lungs is to have your vet do an examination.