Recurrent Airway Obstruction

Recurrent Airway Obstruction (RAO), also known as Chronic Obstructive Pulmonary Disorder (COPD) or "broken wind" is a respiratory disease in horses. Also known as "heaves".

This is a chronic condition of horses involving an allergic bronchitis characterised by wheezing, coughing and laboured breathing.
 
CAUSES

An allergic reaction to certain otherwise innocuous substances - allergens, typically dust and mold spores (e.g. Aspergillus spp.). It is therefore most common in horses fed on hay and bedded on straw. Endotoxins from organisms in the bedding and feedstuffs may also play a role.
 

The condition is most common in the Northern Hemisphere - it is rare in the South. This is probably because northern horses are more likely to be overwintered in stables or barn, and therefore become sensitised more readily. In contrast to human asthma (which it otherwise resembles), RAO is associated primarily with neutrophil mediated inflammation, and IgE and mast cells are much less important in the pathophysiology.
 

NOTE: A similar condition, Summer Pasture Associated RAO also exists. In this case, the allergens are derived from fodder and pasture. This is more common is summer, and management is reversed: horses should be stabled in well ventilated areas.

SYMPTOMS
Increased respiratory effort and dyspnea (shortness of breath, especially in response to strenuous exercise. In some cases, the horse may present acutely with severe dyspnea, such that it really struggles to inspire sufficient air. This is a veterinary emergency.

In addition, a soft, moist cough may be seen, most commonly in association with exercise or eating. This cough may be productive, with expectoration of thick, viscous sputum.

There may also be an audible wheeze.

In chronic cases a "heave line" may be visible on the ventral abdomen. This is caused by hypertrophy of the extrinsic respiratory muscles.

DIAGNOSIS
The vet will auscultate (listen to the horse's chest with a stethoscope) to attempt to detect abnormal lung sounds. If these are very quiet, a rebreathing bag may be used - a plastic bag over the nose to intensify the horse's respiratory effort and hence the sounds. Blood gas analysis may also be carried out (although it is rarely required).

In some cases, a bronchoalveolar lavage may be carried out - an endoscope is used to look down the trachea and mucus and cells are washed out of the lower airways for analysis. Typically, a BAL recovers an abnormally high percentage of neutrophils from an RAO horse - up to 50 or 70% in severe cases.

First Aid
If a horse is severely dyspnoeic (i.e. really struggling to breathe), the most important measure to take is to REMOVE IT FROM THE STABLE OR BARN into FRESH AIR. This can be a life-saving measure in an serious attack. Seek veterinary advice immediately.

TREATMENT

As RAO is primarily a management condition, primary treatment is necessary. Where possible, the horse should be turned out to pasture. When removed from the allergens in the stable, the symptoms will usually subside, although they will recur if the horse is stabled again at a later date.
 

If this is not practical, certain alterations to stable routine may be effective.

• Minimise dust and maximise air quality in the stable.
• Soak hay or feed a low dust alternative such as haylage. The length of time to soak hay for the maximum benefit is debated; however, approximately 30 minutes appears to be the current consensus.
• Feeding from the ground is often considered to be more beneficial than in a haynet, as this allows any mucous to drain out of the lungs.
• Bed on a dust free bedding. Those horses that must continue to be stabled are normally bedded on paper, or a combination of rubber matting and paper, or low dust wood shavings. Straw contains dust and may irritate the condition further.
• Make sure the stable is well ventilated.
• Don't muck out or brush up while your horse is in the stable to minimise the dust in the air.

• Bronchodilators: Often the mainstay of therapy. One of the most common is clenbuterol, either as an oral medication administered twice daily in feed, or via a nebulizer. Clenbuterol also has anti-inflammatory actions, and is therefore often preferred. In an emergency, intravenous clenbuterol or atropine may be used, but care must be exercised with atropine as it may predispose to adverse systemic side effects such as mydriasis, excitement, GI stasis and colic.
• Corticosteroids: Oral steroids such as prednisolone are commonly used; however, side effects are common, and the horse may be predisposed to laminitis. Therefore, the use of aerosolised steroids via an equine inhaler are becoming more common. This route of administration reduces the dose required, and the risk of side effects.
• Mast cell stabilisers: Cromoglycate has been used, but this is may or may not be fully effective.

Care should be taken with these drugs in competition horses, as many of them are forbidden substances under racing and FEI rules.

PROGNOSIS

RAO often limits the horses' ability to work, and it may find strenuous activity difficult. However, with prompt diagnosis and treatment the condition can be managed successfully.