RAO is caused by exposure to allergens, such as moulds and dust. Environments in which there is high exposure result in an increased likelihood of RAO occurring, e.g. horses and ponies stabled for long periods in dusty surroundings with poor ventilation. This triggers inflammation of the airways, constriction of the bronchi and mucus secretion in the lungs. This progressively causes damage to the lungs causing an insidious onset of clinical signs although acute episodes may occur.
Horses may also develop summer pasture associated recurrent airway obstruction (SPARAO), where the allergens that cause lung damage are pollens. For this condition the horse will have to be stabled to avoid allergen exopsure during the high risk summer months. Some horses can suffer from RAO and SPARAO which can be very difficult to manage.
Early signs may be very mild or only seen at exercise with occasional coughing. If untreated this may progress over time to a more persistent chronic cough. A thick white-yellow nasal discharge and sluggishness at exercise may also be present. Over time a ‘heave-line’ (over-developed abdominal muscles) may develop due to the increased expiratory breathing effort – see picture.
Acute flare ups can present dramatically with a markedly elevated breathing rate and effort, flared nostrils and standing with the head down and elbows out. The horse may also appear very distressed. If this occurs you should call your vet immediately.
You should call your vet immediately, who will carry out a thorough examination.
The examination may include listening to the lungs after exercise or ‘rebreathing’, this is when your horse breathes into a large plastic bag which stimulates the effects of exercise by limiting oxygen supply.
Crackles or wheezes may be evident and the horse may find it difficult and have to breathe much faster to compensate. If further diagnostics are required, endoscopy (a fibreoptic camera is passed up the nostrils) can reveal evidence of mucus in the trachea. By placing small amounts of fluids into the lungs the vet may also perform a ‘bronchoalveolar lavage’ or ‘tracheal wash’. The fluid is retrieved in order to look at the types of cells present within the lung airways. A high percentage of neutrophils (a type of white blood cell) are suggestive of RAO.
The most important method of treating RAO is through management changes. Ideally the horse should be turned out to pasture as much as possible and spend minimal time stabled. If stabled, horses should be in clean, well ventilated stables. Shavings or cardboard bedding are preferred as bedding material as they contain fewer mould spores and dust than straw. All the stables in the vicinity need to be similarly maintained or the environment will remain high risk for the horse. Forage (hay or haylage) should be soaked and the horse should be fed from the ground to aid drainage of mucus out of the lungs.
If management changes alone are insufficient to control the RAO, your vet may prescribe some medications. The most effective medications available are inhaled drugs using inhalers identical to those used by human asthmatics. Inhalation of the drugs allows high concentrations to reach the lungs. The most commonly used are bronchodilators and steroid anti-inflammatories. This may be required for long-term use or only for flare-ups, depending on the severity of the disease.
This picture shows an equine inhaler being used to administer medication to a horse with RAO.