Inflammatory Airway Disease in Performance Horses
Dr. Melissa Mazan
There are three objectives to treatment for IAD: Bronchodilators. Initially, your horse is often suffering from spasms of constricted airways, known as bronchospasm. In order to open up these constricted airways, your horse may be given drugs known as bronchodilators, which are best given in the inhaled form.
It is extremely important to remember that bronchodilator drugs do not help to fix the underlying problem – which is inflammation of the airways. Bronchodilators are best thought of as 'rescue drugs' – best used for quick relief, but not for long-term therapy. As a matter of fact, one study showed that human asthmatic athletes who used bronchodilators twice a day for two weeks (without using any anti-inflammatory drugs) had worse lung function at the end of the study than they did at the beginning.
Typical bronchodilator drugs include a group called beta-2 agonists, such as and another group called parasympatholytics. The most commonly used beta-2 agonists are albuterol (a short-acting drug) and salmeterol (a long-acting drug). Drugs like albuterol and salmeterol can affect other body systems if they are not targeted directly at the lungs. For instance, if your horse is given enough albuterol that it is detected in the blood stream, he might also have a high heart rate, and feel very jittery.
The most commonly used parasympatholytic drugs are atropine and ipratropium bromide. These drugs take a little longer to work than do albuterol and salmeterol.
Atropine, unlike the other drugs we have discussed, must be given by injection. It can be extremely useful in reversing signs in a horse with severe heaves, but it can also have the effect of massively slowing down normal intestinal activity, and can result in colic (gastrointestinal pain). For this reason, it must be given with great care.
Remember – IAD stands for small airway inflammatory disease. So, it is extremely important to decrease the amount of inflammation in the lungs. If we can do this, we are really doing something to fix the horse, not just hide the symptoms.
The best agents for reducing airway inflammation are a group of drugs known as corticosteroids. Oftentimes, we initially treat with corticosteroids given orally or by injection, to make sure that we get a quick and effective offensive against the inflammation.
As with the other drugs, however, corticosteroids can have considerable undesirable side effects. These include (rarely) laminitis, and suppression of the immune system. In order to avoid these side effects, we continue long-term treatment with inhaled corticosteroids.
Unlike the relief drugs, inhaled corticosteroids such as fluticasone are usually part of a long-term treatment plan for horses with IAD. Depending on your horse's response, he may need treatment every day or every few days, or he may only need treatment during a particular allergy season.
In order to understand the importance of long-term control, we must keep in mind that the airway inflammation that characterizes IAD is due to allergies. There are substances in your horse's environment that trigger what is termed as an inflammatory cascade – and the end result is obstructed, irritated, hyperreactive airways – and a horse that no longer performs at her peak.
If you have a horse with signs of IAD, of course you want to know what triggers this condition. Unfortunately, this is not an easy question to answer, because many horses are allergic to many things. Skin testing or a blood test (RAST test) may provide some answers; however (and this is a big however), these tests are not very specific. They don't identify which allergens are important in causing airway disease. For instance, when we do blood tests for allergies, we find that many horses are allergic to molasses, corn or wheat. BUT, if we expose the horse to these substances, we find that it doesn't, after all, trigger an episode of IAD.
The best that we can do is to be aware of the substances that we know cause allergic airway disease in many horses – the molds and spores in even the best of hay and straw, plant pollens at certain times of year, molds and fungi that may grow in damp environments, and the contaminants in dusts that are in some grain. We also know that dust and air pollution can irritate the airways even if they don't themselves cause and allergic reaction.
It may also help to keep your horse well-vaccinated against respiratory viruses, especially Equine Influenza and Rhinopneumonitis (Equine Herpes Virus). These viruses can make the respiratory system more sensitive to other triggers of inflammation.
Good environmental control can be difficult to achieve. Allergens can cause a profound inflammatory response even when your horse is exposed to minute quantities of the offending substance.
The long and short of it is that you do the best you can to maintain environmental control. Reality tells us that most horses will eventually need to use some drug therapy to beat IAD.
Administering Inhaled Drugs
Most of us have seen people with asthma using inhaled drugs – they come in the form of metered dose inhalers (MDIs, or puffers). They are easy to use, and keep the drugs in the respiratory system, thus avoiding most of the side effects associated with giving these drugs systemically.
It is important for aerosolized drugs to reach deep within the airways. When we compare horses with people, we see that horses have much longer noses and throats. This means that you can't just hold the puffer up to a horse's nose and puff. If we give the drugs that way, the very small particles end up just sitting in the nose and upper airway, without reaching their target, the small airways.
We can help the drugs reach their target destination by using a spacer device and mask (Aeromask™), that work to direct the particles down into the airways. We hope that there will soon be new and improved methods of delivery available to the horse owner in the near future.