The healthy respiratory system
The horse’s respiratory system is quite different to ours and has a staggering capacity to move air in and out of the lungs, in order to supply the body with oxygen. A galloping horse will take over 150 breaths per minute, having less than half a second to inhale and exhale about 15 litres of air.
Air is breathed through large nostrils, into the nasal passages – where it is warmed and debris filtered out – and goes on through a cavity called the nasopharynx, through the larynx and down into the trachea. The soft palate – which, in the horse, is very long, separates the nasopharynx from the oropharynx (part of the mouth), and together with the larynx (the voice box), control swallowing and breathing. The larynx consists of several cartilages, including the arytenoids and the epiglottis, as well as the vocal cords. During swallowing, the arytenoids close, the epiglottis flips up and the soft palate moves upwards, all of which close the airway and allow feed and water to travel down the oesophagus. Due mostly to the very long soft palate, horses cannot breath in air through their mouths – they are obligate nasal breathers.
The larynx is the dividing point between the upper and lower respiratory tract. The trachea begins at the larynx and travels down into the chest (thorax). Within the thorax, it divides into two tubes, or bronchi, each of which leads to a lung. Within the lungs, the bronchi divide and subdivide into smaller and smaller tubes, eventually called bronchioles. At their endpoint are tiny grape like clusters of air sacs called alveoli, where the gas exchange takes place.
At the alveoli, oxygen in inhaled air diffuses into tiny blood vessels called capillaries, on into the red blood cells, and carbon dioxide diffuses from the blood in these capillaries out into the alveoli. Since the concentration of oxygen is so much higher in the inhaled air compared to the deoxygenated blood in the capillaries, oxygen simply diffuses from an area of high, to an area of low concentration. The oxygenated blood then flows back to the heart where it can be pumped around the body, supplying oxygen to all body tissues.
Horses can move up to 80 litres of air per second during treadmill exercise, during which time huge pressures are present in the airways. This capacity for air intake, alongside a tremendous cardiac function (a sprinting horse can move 200 litres of blood through the heart per minute) means horses can take up about 40 times the amount of oxygen at maximal exercise compared to rest, whereas the best elite human athletes take up about 7 times the amount of oxygen at maximal exercise intensities compared to resting.
When it goes wrong
Respiratory related health problems are the second most common cause for poor performance in athletic horses, and it’s not surprising, considering how much air needs to be delivered into the lungs to ensure optimum cardiovascular function.
Allergic airway disease (RAO), respiratory tract infections, soft palate and larynx problems and exercise-induced pulmonary haemorrhage (EIPH) are the most common disorders of the airways.
In the upper airways, anything that interrupts airflow will have huge implications on exercise performance, and problems with the soft palate and larynx are relatively common, especially in young racehorses.
Conditions of this area include:
- Dorsal displacement of the soft palate (DDSP) – which might be linked to inflammatory conditions and/or immaturity of tissue development
- Laryngeal hemiplegia (roaring) – damage to the left recurrent laryngeal nerve, which causes a sagging of the left arytenoid cartilage and reduced airflow. Could be inherited.
- Epiglottic entrapment – mucus membrane gets trapped over the epiglottis, cause laryngeal dislocation and reduced airflow during exercise
These conditions are only apparent in horses exercising at high intensities, and can be difficult to diagnose. Endoscopy – enabling observation of the upper airways during exercise – whilst galloping on a treadmill, or with a mobile endoscope out in the field is usually required. A variety of treatments can help, including higher levels of fitness, surgery, and treatment of underlying inflammation. There is some controversy about performing surgery on cases of DDSP, where conservative treatment may be as or more effective.
Lower respiratory disease can be difficult to diagnose and simply listening to breathing is unreliable. Coughing is believed to be the most reliable clinical indicator of lower airway disease in the horse, although some horses with advanced lung disease do not cough. Endoscopy is the most useful diagnostic technique. The airways and the first parts of the lungs can be viewed, samples of secretions taken and even small foreign bodies removed using an endoscope.
Conditions of the lower airways include:
- Recurrent airway obstruction (RAO) – allergic airway disease causing inflammation, narrowing of bronchioles and mucus production; coughing may be present, forced expiration (heave line)
- Inflammatory airway disease – a condition of young racehorses in training which is not understood; may involve bacterial infection, inhaled toxins, allergens or poor air quality
- Infections – viral and bacterial (which may follow viral)
- Exercise induced pulmonary haemorrhage (EIPH) – affects most racehorses, but more severe cases will impair performance
Many horses with airway disease will not show nasal discharge because all the secretions may be swallowed. Any amount of coughing from a horse should be taken seriously and a vet consulted, because significant airway disease can be present even before a horse coughs.
Ensuring good respiratory health from the outside
Considering how susceptible horse’s airways are to environmental irritants and dust, it is very important to maintain as hygienic an environment as possible. The two most common culprits for affecting the airways are forage and bedding, and both can cause airway disease.
Once a horse is allergic to bedding and/or hay, then both need to be changed to a non-allergenic form that doesn’t expose the horse to the dust, insects, mould spores and other potential allergens. All performance horses should be in an environment suitable for a horse with RAO. Rubber matting with a small amount of dust-extracted bedding – ideally paper – is recommended, and either well-soaked hay or haylage. Some horses will need haylage rather than soaked hay to ensure respiratory health. Some vets state that all UK-made hay should be soaked before feeding to horses because all contain so much dust and other airway irritants. Hay need only be soaked for up to half an hour for respiratory health, because longer and you will lose valuable nutrients. For overweight horses and ponies or those with low calorie and/or low sugar requirements, this can be useful, but for those with higher energy requirements, soaking for just long enough to dampen dust and swell mould spores is recommended.
Environmental changes are even more important than medical treatment because these remove the cause of the allergic airway disease, and help the horse breath freer. Medical treatment usually involves inhaled corticosteroids to dampen the inflammatory allergic response, inhaled bronchodilators, and drugs to help loosen and expel mucus. More information on natural remedies to help soothe the airways and expel mucus is given in the next section.
Even if a horse doesn’t have RAO and isn’t allergic to bedding or forage, the environment should be kept as clean as possible if peak performance is expected from the horse. Stables should be regularly dusted (ideally with a vacuum), hay should be soaked (or haylage fed instead) and bedding dust-free, mucked out at least twice daily and never left deep-littered.
Deep littering bedding exposes the horse to a huge load of mould spores and mycotoxins and is not recommended for athletic horses or ponies.
Management to reduce the risk of infection is also important to avoid respiratory disease. Keeping horses new to a yard separated in a quarantine period is recommended, as is good hygiene when out at competition e.g. not allowing horses to touch noses with others, or touch noses with other’s stables, buckets and other kit.
Ensuring good respiratory health from the inside
Feeding horses to ensure good respiratory health relies on dust-free forage, and this is the key factor. The next consideration is a balanced diet that supplies enough antioxidants, which are important for good airway and lung function. Diseased lungs have lower levels of antioxidants and so higher levels are required in the diet to compensate. Adequate intake of vitamins A, C and E are important, and extra vitamin E and vitamin C should be added to the diet of any horse with respiratory disease or dysfunction. Antioxidant bioflavonoids from dark green vegetables, purple fruits and other natural sources can also be added to help boost respiratory health and function.
The essential micromineral zinc is often short in horse diets and supplementing a little extra to horses with respiratory challenges is worthwhile because it is involved in the health and integrity of lung cells when exposed to inflammation. Furthermore, increased intakes have been linked to reduced risk of lower respiratory infections in humans.
For horses with challenged airways, certain herbs are very useful to help soothe inflammation and expel mucus. Traditionally, herbs such as garlic, thyme and elderflower are at the top of the list for respiratory support. Other useful herbs include liquorice, elecampane and marshmallow.
The horse’s airways can move a tremendous amount of air in and out of the lungs, giving the horse a superior ability to deliver oxygen to exercising muscles. However, the respiratory system of the horse is subject to huge pressures and it is very susceptible to dysfunction and allergic disease. Keeping the horse’s respiratory tract healthy relies on taking action early when any symptoms are noticed, and providing a clean, hygienic environment. Understanding how to ensure peak respiratory health from both the inside via nutrition and the outside via environment – especially forage and bedding – will help to ensure easy breathing throughout the seasons.