NEWS
Lung disease in the racehorse
Posted: Tuesday, November 28, 2006
Respiratory disease is second only to lameness in negative impact on racing career
by Kenneth L. Marcella, D.V.M.
EQUINE ATHLETES are faster, stronger, and tougher than most of their pasture-grazing relatives, but are they any healthier? The answer to that question may surprise you.
Racehorses represent a section of the equine population that is well cared for, fit, and receiving regular strenuous exercise. They are some of the most elite athletes of the equine world. Yet racehorses are much more likely than slower, weaker, less fit horses to develop lung infections such as pneumonia, infectious bronchitis, and lung abscesses.
The types of viruses and bacteria that tend to cause these problems in racehorses are not exotic or particularly aggressive strains, either. They are the types of infectious agents that all horses are exposed to and that are generally fought off by the average immune system.
Racehorses seem unable to successfully defend themselves against causes of lung disease, and Dorothy Ainsworth, D.V.M., Ph.D., a veterinary researcher, clinician, and professor of large animal medicine at Cornell University, is looking into the reasons why.
Career threat
"One of the greatest risk factors for developing pneumonia, pleuritis [an infection of the lining of the lungs and chest], or lung abscesses in athletic horses is strenuous exercise or racing itself," Ainsworth said.
"Equine athletes get lung infections such as pneumonia, infectious bronchitis, and lung abscesses significantly more often than other horses do," Ainsworth said. "Because of immunosuppression [caused by a racehorse's environmental conditions and exercise demands], various bacteria and other agents can take advantage of the depressed respiratory-tract defense mechanisms. We need to improve the pulmonary defense mechanisms so that viruses and bacteria can be effectively cleared before they are able to proliferate and cause illness."
Aside from the time lost to actual infections, respiratory diseases often require lengthy convalescent periods. Because the time schedule for a racehorse to compete, win, and establish itself often is so short, any time lost effectively can close the door on a career.
"Respiratory disease is so important that it is second only to lameness as the most common cause of decreased performance, lost training days, and premature retirement among racehorses," said Ainsworth. Yet it is becoming apparent that racing and strenuous exercise, the actual substance of being a racehorse, is exactly what is putting these horses at risk.
Exercise physiologists, veterinary researchers, and observant horse trainers have long thought that the stresses of training and overexertion might make horses more prone to developing respiratory diseases. The idea that high-intensity exercise and other factors in the racehorse environment may be able to weaken the immune system lead to the concept of a "window of susceptibility" for these animals.
"Because of immunosuppression, aspirated [inhaled] bacteria, which normally reside in the pharyngeal cavity [the back of the mouth], can take advantage of the depressed respiratory-tract defense mechanisms, rapidly proliferate, and develop into infections," Ainsworth said. This immune suppression opens the window of susceptibility and also can allow viruses to establish themselves more easily in the respiratory tract, which, in turn, can predispose the horse to secondary bacterial infections.
Ainsworth has been awarded a grant from the Harry M. Zweig Memorial Fund for Equine Research to investigate the mechanisms of how high-intensity exercise weakens the immune system. In her work, she has found that exercise is only one component of the stress of being a racehorse.
Immune stressors
Horses in training at the track typically receive little turn-out time. Constant stall confinement increases the amount of dust and particulate matter they inhale. Poor ventilation, dusty bedding, mold spores from even good-quality hay, transport stress, poor air quality due to some urban track locations, and similar environmental factors have all been correlated to increased immune suppression and increased risk of respiratory disease.
Researchers at Oklahoma State University tested human and equine athletes to look for those factors that would affect competitors in training and produce respiratory disease. These scientists identified specific Th2-cell cytokines (specific inflammatory cells in the body that can cause tissue damage and disrupt normal cell function) that were increased significantly by strenuous exercise. These exercise-released cytokines may sufficiently damage the tissue of the respiratory system so that an increased sensitivity to other environmental allergens (foreign proteins that irritate the immune system) is produced.
The research team from Oklahoma State felt they had identified a mechanism for racehorse lung disease. Lead researcher Michael Davis, D.V.M., Ph.D., assistant professor of physiological sciences, said: "We believe our data are the first to provide a specific mechanism for the exercise-induced, open-window effect as a local pulmonary phenomenon. The potential deleterious effect of increased IL-10 [cytokine] expression is increased susceptibility to pathogens, particularly those that are normally cleared by cell-mediated immunity. ... The most prominent antigens identified in this syndrome are mold spores found in hay and straw bedding."
These findings are in agreement with Ainsworth's research on the stress of the racehorse environment, and they lead to some suggestions for reducing lung disease in the equine athlete.
"Most of the things that are best for reducing stress to the respiratory system are difficult to achieve in a racing environment," Ainsworth said. But ways do exist to decrease irritants at the racetrack and to strengthen the horse's respiratory system, including:
•Increased turn-out time;
•Better ventilation (shedrow stalls as opposed to totally enclosed barns);
•Reduced dust;
•Cleaner bedding;
•Quality hay that is low in mold spores (best fed after soaking in water if there are any concerns over quality); and
•Decreased transport stress.
While some of these items may seem difficult to implement at present, it only takes one individual who alters how racehorses are trained and campaigned and who then wins to change the management of these athletes.
Recurrent airway obstruction
Perhaps the most common lung disease seen, especially among racehorses, is recurrent airway obstruction (RAO). The condition has been previously termed chronic obstructive pulmonary disease (COPD) and is known by most horsemen as heaves.
Obstruction of airways in this condition occurs as a result of chronic inflammation and spasm of the bronchioles with accumulation of mucus and dead cells (exudate).There is mounting evidence that most cases of RAO are the result of hypersensitivity of the horse's immune system to allergens most likely found in dust or molds associated with hay. The equine athlete's world usually creates conditions where overexposure to these allergens is common. Lack of pasture turn-out and exposure to green grass means more stall dust and hay nets, which increase the chances of developing RAO.
Affected horses exhibit a cough; thick, mucopurulent nasal discharge; wheezing and other abnormal lung sounds; weight loss; and the eventual development of increased chest muscles used to help move air through constricted, inflamed airways. Another similar condition seen primarily in young horses (less than two years of age) has been called inflammatory airway disease. It is thought that this variation of airway spasm and mucus buildup is associated with repeated bacterial infections in these growing horses and not the allergy reaction seen with RAO.
Diagnosis and treatment
Diagnosing the type of respiratory ailment a horse may have relies on ultrasound, radiology, endoscopy (putting a thin fiber-optic scope into the horse's nostril and through its nasal passages into the trachea and larger bronchi to visualize these areas), transtracheal wash (a small tube is placed into the windpipe and fluid is flushed into the trachea and then withdrawn for examination for cell types and bacterial content), and bronchoalveolar lavage (this technique places a tube deeply in the lung airways to collect a more representative sample of the cells involved with various conditions).
Nasal polyps or masses are generally easily diagnosed because the affected horse breathes with a wheeze or noise caused by air moving in a narrow passage. Endoscopic examination usually reveals the location and cause of the problem. Other obstructive problems in the horse's larynx are more numerous and perhaps more serious.
Conditions such as dorsal displacement of the soft palate, epiglottic entrapment, laryngeal paralysis, and lymphoid hypertrophy of the pharynx can cause swelling through the back of the throat. Infectious or viral tracheitis can be seen, as well. Strangles, a serious infection caused by the bacterium Streptococcus equi, can lead to massive pus-filled abscesses that occasionally constrict the throat or larynx.
Treatment options are then determined based on results of the above tests and usually involve antibiotics, anti-inflammatory medications, bronchodilators, and, perhaps most importantly, environmental management to reduce stress and allergen exposure.
Ainsworth and other researchers believe that they are approaching a means of helping these horses stay competitive. "We think that allergen desensitization through immunotherapy just may be a viable and safe treatment option for [RAO] in all kinds of horses, including racehorses," Ainsworth said.
Other aspects of ongoing research are looking at various medications that boost or improve immune system function. Recently, one such new medication, Zylexis, has become available commercially in this country. Ainsworth and other researchers in this field feel that further work with immune system medications may produce even more positive results.
Kenneth L. Marcella, D.V.M., is a practicing veterinarian in Canton, Georgia.
