Dr David Marlin and Dr Kirstie Pickles
The importance and limitations of the equine respiratory system
In the unfit/untrained horse, the heart 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 to become better able to cope with exercise demands. However, the respiratory system (upper airways and lungs) cannot adapt as the horse gets fitter and so, in trained horses, the respiratory system is the main limitation of exercise performance. This means that a respiratory problem is one of the most common causes of poor performance.
How common is equine respiratory disease?
After lameness, the most common reason for a vet to be called out, or for horses to be unable to be exercised or competed, is respiratory disease. Respiratory disease occurred in 420 of 3000 German horses (14%) followed over a one-year observation period. In another study of horses referred for a veterinary examination because of poor performance, 81% (4 out of 5) had respiratory disease.
Over half of 112 sport horses (dressage and showjumpers), thought to be healthy by their owners, were found to have respiratory disease upon veterinary examination using auscultation (listening to the lung sounds with a stethoscope) and endoscopy (putting a camera into the trachea [windpipe]). So, even if a horse is not showing any overt signs of respiratory disease, the only way to be sure the respiratory tract is healthy is to have a clinical examination by a vet. This is especially important for horses that are competing and/or travelling frequently. 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, 12 had evidence of respiratory disease on veterinary clinical examination.
Equine asthma (formerly known as recurrent airway obstruction, “heaves” or equine COPD) is the most common chronic (long-term) respiratory disease affecting horses in the UK. It is seen in older horses (predominantly 8 years plus) and has been estimated to affect 10-17% of the total horse population (all ages) in the UK. In horses over 8 years old, the prevalence of equine asthma is nearer to 1 in 3 horses.
How to recognise equine respiratory disease
Respiratory disease is classified as occurring in the upper (nasal passages and larynx) or the lower (windpipe and lungs) respiratory tract. Table 1 shows common external (i.e. can be seen without the need for a veterinary examination or equipment) clinical signs of equine upper and lower respiratory tract disease.
Table 1: Common clinical signs of equine upper and lower respiratory disease.
A small amount (few drops) of clear, watery nasal discharge is normal. Increased volumes of watery discharge or white (Fig. 1), yellow, or bloody coloured, or thick and runny or foamy secretions are abnormal.
Fig. 1: White nasal discharge from a horse with equine asthma.
In general, coughing in horses indicates lung disease. Horses tend to cough very intermittently which can make it difficult for owners to hear their horse coughing. It is often said that “it is ok for a horse to cough a few times during warm-up”. This is NOT true and a horse that coughs a few times during warm-up is likely to have lung disease that should be investigated by a vet.
Respiratory disease may be apparent in horses at rest in the stable, during, or after exercise. Signs of respiratory disease can be exacerbated by 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.
NEXT WEEK: Common Causes of Equine Respiratory Disease