Approximately 40-50 people attended the "Heavey Horse" table topic, which took place at the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif. The discussion covered a number of areas, including the pathophysiology of recurrent airway obstruction (RAO, also called heaves) and its diagnosis, treatment, and prevention.

The causes of RAO are known to be inhaled allergens and irritants from the horse's diet and environment, especially mold spores from straw bedding and hay, but they can also include other materials, such as dust containing endotoxin and other irritant materials. In fact, it has recently been suggested that we should term this disease as "inflammergic" rather than allergic due to the variety of stimuli that can set off an attack.


Dr. Harold McKenzie discusses the source and management of heaves. (5:54 min)
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Researchers have been working to understand the exact nature of the inflammatory response in RAO, and it appears that it does not fit well into the established paradigms of allergic and inflammatory responses. It falls somewhere in the middle, with characteristics of both an innate inflammatory response and an allergic response. Scientists hope this knowledge will help them design better treatments in the future.

A good bit of discussion revolved around the diagnosis of RAO, and facilitators and attendees acknowledged that this can range from the simple clinical recognition of an older horse with intermittent episodes of respiratory difficulties and the monitoring of the response to empiric treatment (relying on practical experience), to more detailed diagnostic work-ups. These detailed evaluations rely primarily on the assessment of lower respiratory inflammation using bronchoalveolar lavage (BAL). The facilitators encouraged all in attendance to consider adding this diagnostic tool to their practice, as it is simple to perform and safe, while providing critical information regarding the severity and character of lower respiratory inflammation. Monitoring of BAL results over time also is very useful in assessing the response to treatment.

Treatment of RAO ultimately hinges on dietary and environmental management. This typically consists of trying to lower the individual's exposure to inhaled irritant material, and it is mostly simply achieved by placing the horse on full-time pasture turnout and not feeding him hay.

The difficulty is that few owners/farms can achieve this type of management year-round, necessitating further management changes. If horses must be stabled, they (and the other horses in the barn) should be managed on low-dust bedding, such as quality shavings. Hay storage within the stable must be avoided, and the affected horse should be fed a hay-free diet. This most often consists of chopped forage and a complete feed pellet.

If the management is not optimal then pharmaceutical therapies will be required, with steroids representing the cornerstone of therapy. Corticosteroids are most safely administered by the inhaled route, but cost considerations often require that they be delivered systemically, as this is equally efficacious and far more economic. The most commonly used corticosteroid is dexamethasone, which is highly effective. This drug can be administered by injection or orally, although veterinarians must increase the dose by 30-60% when giving the drug orally, due to variable absorption. The goal with steroid therapy is to find the lowest effective dose, and with dexamethasone the dose can often be very small and given every other day. Bronchodilator therapies can be very helpful in the treatment of RAO, but they should not represent the primary therapy, and their use is best restricted to an as-needed basis and pre-exercise medication.

Preventing RAO requires that horses be managed in a low-dust environment for life in order to alleviate the stimulus for RAO to develop. Unfortunately, there is now evidence that there might be some genetic predisposition to RAO, meaning that some horses are more likely to develop this condition regardless of management.

This table topic was facilitated by Harold C. McKenzie III, DVM, MS, Dipl. ACVIM, assistant professor of Equine Medicine, Marion duPont Scott Equine Medical Center, Leesburg, Va.

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