Poor Performance in The Sport Horse
Dr. Melissa Mazan
Inflammatory Airway Disease (IAD)
In addition to being great athletes, horses are greatly allergic. Who doesn't know a horse with hives or with heaves? An early form of respiratory disease causing poor performance stems from an allergic condition as well. In certain horses that are predisposed, allergens set up inflammation in the small airways (hence the name, inflammatory airway disease, IAD). Some of the allergens we think are particularly instrumental in causing IAD include fungal spores, bacteria and their toxins, and air pollutants. The fungal spores can come from the cleanest looking hay, so proving to someone that hay was the source is a hard sell. This inflammation smolders for a long time and is generally invisible to the even the keenest observer, until the horse slows down or can't work as hard. At this time, further diagnostic tests are pursued that reveal the problem.
In horses with IAD, there are more profound changes than just inflammation. Inflamed airways transform, becoming hyperreactive, or twitchy. Essentially, they constrict more readily and to a greater degree than do the airways in normal horses. The symptoms of constriction (bronchoconstriction) are coughing and exercise intolerance.
Because horses with allergies (IAD) have endured bouts of inflammation and constriction over and over again, their airways are also thicker due to development of excessive tissue and inflammatory secretions, and thus have a narrower lumen (passageway). The narrower airways cause a bottleneck in airflow, especially during hard work, which in turn decreases the available oxygen that is needed for work.
Because horses have an enormous respiratory reserve (the amount of excess lung not called upon during rest), the effects of IAD are often not noted until the horse is asked to exercise strenuously, breath deeply, or work at higher temperatures. This is why IAD was first recognized in racehorses. However, by the time IAD takes hold and causes the horse to slow down or a long-term cough to develop, the damage done to the lungs, while reversible, may be profound. It is absolutely imperative that IAD is recognized as early as possible, or the condition could go on to cause heaves.
Left Recurrent Laryngeal Neuropathy (LRLN, Laryngeal Paralysis, Roaring)
The larynx or voicebox forms the passageway to the trachea (windpipe). For effective function, the larynx must be able to close while the animal is swallowing, and it must become very wide during maximal exercise to afford the greatest airflow.
Laryngeal paralysis, which is most common in horses taller than 16 hands, primarily affects the left side. For reasons that are not well understood, the nerves (the left recurrent laryngeal nerve) that serve the muscle that opens the larynx begin to die. With the decrease in stimulation of the muscle, it atrophies, and the larynx becomes paralyzed. The left side no longer opens normally, preventing the full potential of the horse to breath freely during intense exercise.
The degree of laryngeal paralysis is very minor in up to 80 percent of horses, and severe in up to 10 percent of horses. It's a problem that worsens with age, so the percentage goes up in the higher age brackets. This is one reason why older horses can't work as hard as young racehorses.
Owners may suspect that their horses have LRLN because the collapse of the larynx during inspiration causes them to make a distinctive roaring noise. In some cases, the sound is more like whistling.
LRLN is more of an aesthetic problem in horses that do not compete at maximum capacity. It is not until horses are competing in races greater than one mile, high goal polo, or upper level three-day events, that laryngeal paralysis will actually interfere with performance.
In elderly horses, the right side can get bad in addition to the left, and the larynx can collapse. These horses can no longer perform without surgical intervention.
Exercise Induced Pulmonary Hemorrhage (EIPH)
EIPH is very common in racing thoroughbreds and standardbreds (80-90 percent of all horses in this category). However, very few horses bleed visibly (less than 5 percent).
The causes of EIPH are widely debated. One of the most accepted theories is that the pressure in the vessels of the lungs becomes so great in racing horses, that capillaries (very small blood vessels) in the lungs actually rupture.
Another theory is that horses have a low-grade blockage somewhere in their respiratory tract (nose, throat, lungs) and they need to pull air in harder. This creates a huge vacuum in the lungs with furthers explodes vessels.
Still another theory contends that a "shock wave" travels up from the weight-bearing forefeet through the chest wall to the lungs, which shake so violently that there is rupture of vessels. Of all these theories, only the first theory has a lot of support form research studies, and from the fact that lasix, which decreases vessel pressure, seems to reduce bleeding in some studies. But he debate about the cause of EIPH and the effects of lasix go on.
Although EIPH has caused great concern to many trainers, owners and spectators, and poor performance is often ascribed to EIPH, it likely does not cause poor performance except in the few horses that bleed extensively on race day. How extensive the bleeding must be to affect performance is unknown.
Horses have enormous hearts in comparison to other species, and with training, they become even larger. Intrinsic neurologic (vagal) input to the horse's heart also ensures that the horse has a low resting heart rate (32 to 44 beats per minute). This is known as having high vagal tone.
Both the size of the horse's heart and the high vagal tone contribute to the development of atrial fibrillation. In horses, atrial fibrillation is usually benign, meaning that there is no underlying cardiac disease; this is different from cats, dogs, and humans. Occasionally, horses may develop atrial fibrillation because of problems such as leaky heart valves.
With atrial fibrillation, the electrical signals that ordinarily pass from the atria, the first set of filling and pumping chambers of the heart, to the ventricles, the second set of chambers, becomes disorganized. This causes the atria to beat in a very erratic fashion.
Because the ventricles still beat normally despite the erratic signals sent to them, the heart is able to pump adequate amount of blood at rest. But the extra blood pumped in by the atria, while insignificant during rest, becomes important for the horse to perform strenuous exercise. For this reason, atrial fibrillation may go undetected for a very long time in horses that do light work, such as pleasure horses, trail horses, and show hunters. In a race horse, atrial fibrillation would be picked up very soon.