Inflammatory Airway Disease in Performance Horses
Dr. Melissa Mazan
Everyone knows a horse or two that coughs at the start of work. Many of us have ridden such a horse, that practically hauls us out of the saddle as he lowers his head readying himself for a good, satisfying cough – once, twice, three times – and then, after a few minutes, stops and goes on his way. If you ask most horse people, they'll say that such a cough is acceptable – as long as the horse stops coughing before too long. Cough
For many years, veterinarians also accepted some coughing even in performance horses as normal, primarily because we lacked the diagnostic techniques necessary to probe further and diagnose the subtle cause, which is a significant abnormality of the lung. This abnormality can now be readily detected.
We now know that coughing horses with lung disease are extremely common, but they should not be viewed as normal. We also know that the coughing, otherwise healthy horse, has a lower airway disease, known variously as inflammatory airway disease (IAD), small airway inflammatory disease (SAID), small airway disease, and chronic obstructive pulmonary disease (COPD).
What distinguishes IAD from heaves (late-stage COPD) is the lack of obvious labored or increased breathing effort of horses at rest. In fact, IAD is usually not visible in horses at rest.
Many of us also failed to realize over the years, that some horses that cough intermittently will go on to develop heaves if improperly managed. Indeed, most horses with heaves have a history of more low-grade signs of coughing and exercise intolerance (signs of IAD), making the connection between IAD (early on) and heaves (with age) likely.
What to Watch For
Increased respiratory rate
Nasal discharge that is usually clear.
However, if your horse has an early stage of IAD, he may not show any of these classic signs.
Anatomy and Physiology
IAD is a disease of the airways which disturbs normal lung function (causing exercise intolerance) in addition to acting as a source of irritation (causing cough). To understand the impact of IAD on the lung, we need to review some physiology.
Oxygen is the essential fuel for the body – if O2 can't get to the bloodstream, then effectively, the horse is like a car out of gas. No matter how willing the horse is, if he can't get enough oxygen to the bloodstream and thus to the muscles, he won't be able to do his job. The job of the respiratory system is to bring adequate amounts of oxygen into the body.
First, the horse draws air in through his nostrils. Unlike humans, horses can't breath through their mouths – no matter how stuffy their noses may be. Their anatomy simply doesn't allow it.
Next, the air must go through the larynx (voicebox) and down the trachea (windpipe).
The trachea then branches into two mainstem bronchi (large airways). Each of these bronchi then branches into smaller bronchi, and then into smaller and smaller bronchioles (small airways). There are hundreds of thousands of these small airways.
Eventually, the smallest of the bronchioles end in alveoli (air sacs), where the oxygen (O2) in the air diffuses into the blood, and the gaseous waste product in the blood (carbon dioxide, or CO2) diffuses out of the blood and is transferred back into the outside air.
IAD is a blockage of the bronchioles. In fact, the bronchioles are swollen, sometimes shut, and this prevents air from reaching the maximum number of alveoli. Since the alveoli are the gas exchangers of the lung, IAD restricts oxygen uptake from inspired air, and therefore reduces exercise tolerance.
When a horse suffers from IAD, then his small airways are narrower than normal because of inflammatory secretions, excessive mucus and fibrous tissue. Just as a kink in a garden hose will constrict the flow of water, so too will this narrowing of the airways constrict the flow of air. With a kinked garden hose, you can, to a certain degree, keep delivering the same amount of water as long as you turn up the water pressure. After a certain point, however, even this approach won't get you enough water for your lawn.
With the airways, the equivalent of turning up the pressure is to breath harder. This increase in pressure results in a higher airflow, but it also increases the amount of work that the horse has to do to achieve each breath. Eventually, this mechanism also fails to result in a high enough airflow. The end result? Your horse is breathing harder and harder to get less and less fuel to his muscles.
Horses develop inflamed airways in response to allergens in their environments. Some animals (we don't know why some are affected and others are not) develop an immunologic response to common substances in their environments, such as plant pollens, and the molds and spores found in even the best of hay. The lung, thinking that it is repelling foreign invaders, mounts an attack. This attack takes the form of inflammation of the lung, with its full complement of cells and debris.
IAD may be triggered by a viral disease such as equine influenza. Certainly, the flu can cause your horse to have reactive airways for months after the actual disease is gone – you may note a persistent cough, especially during exercise.
A phenomenon known as airway reactivity exacerbates this whole process. Inflamed airways are more reactive, or 'twitchier' than the airways in normal horses. This means that the airways, which are already narrowed, constrict even more in response to a number of stimuli – these stimuli may be the same allergens that caused the airway inflammation in the first place, such as molds and pollens, or the stimuli may be cold air, or the gases and particles found in the air in highly polluted areas. It is truly a losing situation for the horse.