Written by: Dr. David Marlin

At this time of year as we start to get a few warmer days and horses are back in work or being turned out for longer, ironically many owners see signs of respiratory disease. These are most likely to be the occasional cough or a slight nasal discharge or both together. There are potentially a number of reasons which might explain why we tend to see this every year around this time!

day for 3-4 months. Even a “clean” stable is likely to generate some dust and the longer the horse is exposed to this the greater the likelihood of a cumulative effect on respiratory health. The risk of a horse developing equine asthma (RAO, equine COPD) increases with age. Generally speaking it is fairly rare below the age of 5-6 years. But we know that exposure to stable allergens, primarily from forage and bedding plays a central role in equine asthma.

Also there is the possibility that hay not stored in good conditions over winter, especially any hay that has become damp, may have reduced in hygienic quality and have increased numbers of moulds. The few warmer days that we have had recently will also lead to an increase in mould growth. Its also likely that at the start of the winter we chose the best quality hay bales to feed but by the time its come to late winter we may be feeding the lower quality bales we avoided at the start of the winter.

Many people may attribute the coughs and nasal discharge to respiratory infections, especially in view of the recent equine flu outbreak. Respiratory infections whether caused by viruses or bacteria are more common in younger horses, especially less than 4 years of age. And stress increases susceptibility to infection, so its common for young 2 year old flat racehorses in their first season of training to develop airway inflammation and bacterial upper airway infections (in the trachea as opposed to deeper in the lungs).

Winter can actually be a good time for horses and bad time for people. For people, the reason we get more viral infections (colds, flu), and to a lesser extent bacterial respiratory infections, in Winter is because we spend more time with other people in enclosed spaces i.e. we have more close contact with more people than in Summer. However, a recent study in people also suggested that cold air decreases mucosal immunity – our defence against infections. Mucosal immunity is the bodies system for defending against bacteria and viruses in the nose, mouth, airways and urinary and intestinal tracts – the places where most viruses and bacteria enter the body. For horses, the risk of infection is greatest when they are leaving the yard and going to mix with other horses or when new horses come to the same yard. So the risk for horses with respect to infectious disease may actually be less in Winter and greater in the competition season. However, we do also know that exercising in cold air (0-5°C) also causes airway inflammation in horses.

Many people might imagine that a mild winter can lead to an increased risk of bacterial and viral infections in horses in the same way that mild winters often fail to kill off wasps and other undesirable insects however this is unlikely to be the case directly. However a harsh winter might result in horses being travelled and competed less and so horse to horse interactions are reduced which would reduce the risk of infection.

Exercise also makes it more likely that we will be able to detect the signs of respiratory disease. If a horse is out of work during the winter it may well have a degree of ongoing airway inflammation with increased mucus production but this isn’t apparent. We don’t hear the horse cough and we don’t see mucus at the nostrils as when it comes up the trachea it is swallowed. However, when we start to work horses, exercise is very good at shifting mucus from the lungs and its not uncommon to hear horses cough when warming up or to show some nasal discharge after exercise even though these horses previously showed no signs when out of work. Remember that even the occasional cough should not be present in a healthy horse. Cough and nasal discharge are clear signs of respiratory disease. Left unmanaged this can progress to equine asthma which is a lifelong disease.

One of the main reasons that horses start to show respiratory symptoms around this time of year is that whilst pollen and mould levels are very low over Winter (November to February), things start to pick up for both pollen and moulds in February/March and this is often triggered as you might expect by the odd warm early Spring day. February-March starts to see Hazel, Yew, Elm and Alder and to a lesser extent Willow, Poplar, Ash and Birch all producing pollen. Grass pollens don’t really come in until May-June-July. The peak months for worst air quality with respect to Mould (total fungal spores) in the UK is June-September, with the peak in July. For Pollen, there are plants and trees producing pollen from February to October! Many websites publish a pollen index e.g. the Met Office and there is also a monitoring station in the East Midlands called MAARA which publishes UK mould spore reports. 
The most common signs of respiratory disease are cough, either at rest or during or after exercise and nasal discharge at rest or after exercise of any colour and down one or both nostrils. Other signs which may not be so specific include: increased respiratory rate, increased respiratory effort, flaring of the nostrils, respiratory noise at rest or during exercise, poor recovery after exercise and generally lower performance than expected.

What can you do to reduce the risk of your horse suffering from respiratory disease or if your horse starts to develop respiratory symptoms in the Spring as the pollen and moulds increase and air pollution worsens? If your horse is stabled overnight or most of the time, make sure you provide them with good air quality – steamed hay, sealed rubber floor and minimal low dust bedding (e.g. large woodchip). Keep windows and ventilation grills open at all times – you can always add more rugs. If your horse lives out and is being fed hay, then it’s still worth steaming to reduce inhalation of dust, pollen, bacteria and moulds.

Soaking of hay is also popular both as a way to reduce inhaled dust but also as a way to reduce energy/sugars for horses prone to obesity and laminitis. Some recent research shows that whilst soaking hay is effective for reducing energy content of water-soluble carbohydrates, this can result in a decrease in hygienic quality of hay and that it is beneficial to steam hay after soaking.

The horses’ airways and lung tissue contain high levels of the antioxidant Vitamins, Vitamin C (mainly in the fluid lining the airways) and Vitamin E (in the lung tissue itself). These reduce the effect of pollution which can generate free radicals and cause or worsen inflammation. Vitamin E and Vitamin C also help to control “damage” to the lung caused by its own defence mechanism which are activated by moulds, pollens, viruses and bacteria. Feeding a supplement high in Vitamin E and Vitamin C has been shown to be effective in reducing the severity of respiratory symptoms in horses whether they are affected by RAO or not. If your horse is at risk of developing respiratory symptoms in Spring based on previous experience, then starting to feed now and increasing the level if symptoms start to develop is recommended. With reference to Vitamin C, supplements that contain L-ascorbic acid are very poorly absorbed by horses so you should be looking for a supplement which includes Ascorbyl Monophosphate as the source of Vitamin C (Deaton, Marlin, Smith et al., 2003). Ascorbyl palmitate is also well absorbed by horses but is not particularly stable in supplements.

Don't ignore even a single cough- of all of these it’s cough that most horse owners ignore. It’s commonly believed that it’s OK for a horse to cough a few times when warming up. It’s not. It indicates the horse has respiratory disease. When people have respiratory disease they cough regularly and continuously. When people are not coughing they are highly likely to be healthy. Horses are different…..Who would have guessed? If horses cough they almost certainly have respiratory disease (even if you only here the odd cough). But horses do not cough regularly. Studies have shown that horses may cough once and then 3 hours later cough 6 times, then 12 hours later cough twice, etc. They do not cough regularly like people. So unless you fit a video camera or put a microphone in the stable or spend 24h with your horse you won’t know how much he coughs.

If your horse is not coughing, that unfortunately does not necessarily mean he is healthy and does not have respiratory disease. The only way to be sure your horse does not have respiratory disease is to have them ‘scoped.

Summary

• Respiratory disease can be caused by infection or allergens or irritants.
• Infection is common in younger horses (less than ~5 years of age) and allergy is more common in older horses (over 6-7 years of age)
• Allergic respiratory disease is very common in the riding/competing age group of horses.
• If a horse coughs, even only occasionally or when warming-up, it should not be ignored and you should speak to your vet about ‘scoping.
• I recommend having your horse ‘scoped around a month into bringing them back into work.
• It’s always a good idea to get your horse ‘scoped at least 2 weeks before a major competition or 3-4 weeks if your horses is travelling more than 10h by road or flying
• Use the smallest amount of the lowest dust bedding you can
• Rubber floors make a big difference to stable air quality
• Never compromise stable ventilation – more rugs if needed
• Ensuring your horse has access to low dust, clean forage is an essential part of maintaining health airways