Heaves - Recurrent Airway Obstruction (RAO)
OverviewMany horses are often heard coughing in dusty stables which is often dismiss as something that will resolve itself. Yet, if ignored, in this environment a horse could develop a chronic breathing problem called heaves. Heaves results from breathing allergens, irritants and microbial products which are found in a horse's environment.
Noxious gases (ammonia,methane, hydrogen sulphide)
Endotoxin (Gram negatives)
Peptidoglycan Gram positives
Mould and fungal products ( toxins and beta glucans)
It is possible for the breathing zone around a horse to reach 17.5mg/m3 ( the maximum level permitted for humns in a workplace is 2.5mg/m3).
Amongst the equestrian community it has been believed that horses with heaves suffered from chronic obstructive pulmonary disease (COPD). COPD is a common lung disease in humans that causes difficulty breathing. Heaves is similar in that it is a chronic lung disease, and it does cause obstruction, but that is the limit of the similarity. COPD in humans is very different from heaves in horses. COPD in humans is caused by cigarette smoking, which causes airway inflammation and damage and produces airway-blocking mucus. Ultimately, COPD involves permanent scarring. Long-term exposure to other irritants can cause the same. Veterinary research has revealed that heaves has more similarities to human asthma, in which the airways tighten and make respiration difficult. Yet, the most common case of heaves that we treat is simply an airway constriction and once the airways open again, the horse does fine. So asthma isn't a perfect comparison either. While asthma sufferers have allergic airway disease, heaves is generally not an allergic response, even if the trigger is hay dust or hay mould.
Research has also identified a specific gene that is present in certain equine family lines which make them more likely to develop heaves when placed in a susceptible environment.
To address this confusion, a veterinary research workshop was held and it was decided that the term 'COPD' should be eliminated, and recommended the term 'heaves' be used to describe the severe but reversible airway obstruction seen in mature horses. It was also agreed that the recurrence and reversibility with medication of the airway obstruction with heaves meant that recurrent airway obstruction (RAO) has also been deemed an accepted term.
Many horses have a chronic, lower grade, diffuse form of airway inflammation that is not accompanied by clinical signs of airway obstruction, and it has been agreed to name this syndrome inflammatory airway disease (IAD).
|RAO (Recurrent Airway Obstruction)||IAD (Inflammatory Airway Disease)|
|Age||Mature to Older Horses||All ages|
Crackles & Wheezes heard
Rapid breathing at rest
Usually normal noises heard
- Coughing - initially at the start of exercise, and increases in frequency.
- Nasal discharge.
- Exercise intolerance - taking longer to recover after exercise.
- Abnormal breathing pattern at rest - an increased respiratory rate with a normal inspiration and prolonged expiration requiring an extra push from the abdominal muscles.
- Heave line - seen along the lower abdomen due to the chronic recruitment and effort of abdominal muscles to aid expiration.
- Reduced appetite - difficulty eating while trying to breathe.
DiagnosisDespite the clinical signs being highly suggestive for heaves it is sometimes necessary to perform further diagnostic tests. In addition to a complete physical examination, the vet may suggest endoscopy (scoping) to visualise the airways and may suggest taking a wash sample from the airways to aid diagnosis.
TreatmentThe aims of treating heaves are:
- Relieve bronchospasm to open up the airways
- Reduce inflammation
- Eliminate trigger factors - improve environmental conditions.
It is important to start with treatment to improve the horse's, ability to breathe, and lung function. The use of bronchodilators, such as clenbuterol, will not treat the underlying cause yet they will improve lung function providing rapid relief until the trigger factors can be identified. Emergency treatment can be achieved by injection and then maintenance therapy can be given by either oral medication or inhalatory medication.
After administrating bronchodilators to manage the clinical signs, it is necessary to address the Inflammation driving the bronchospasm. As inflammation of the airways is central to the disease anti-inflammatory therapy is also vital to improve lung function. Anti-inflammatory therapy is given in the form of corticosteroids. Corticosteroids can also be administered via injection, oral medication or inhalatory medication.
Eliminate trigger factors
Poor environmental air hygiene will exacerbate airway inflammation. Once the clinical signs have been controlled by medication, prevention measures must be put in place to remove the trigger factors and potentially stave off heaves re-occuring altogether.
Initially, ventilation around the horse needs to be addressed:
- Try to keep the horses outside as much as possible.
- Muck out while the horse is turned out. Research shows dust in the stable is at its highest for two hours after mucking out.
- Feed haylage rather than hay. If haylage isn't possible then totally immerse the hay net in water.
- If possible, do not bed these horses on straw. Straw has an enormous amount of dust-more fine particles than hay dust.
Horse owners should think of heaves as "horse asthma", a performance and quality of life-limiting condition. While veterinarians can turn to treatments such as bronchodilator and anti-inflammatory therapy to relieve clinical signs when necessary, it's even more important to try to avoid the inhaled triggers-such as barn/hay dust and moulds-in the first place. With correct management heaves can be prevented.
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