CHAPTER 62 Inflammatory Airway Disease in Racehorses
Inflammatory airway disease (IAD) is a syndrome characterized by airway inflammation in the absence of overt signs of systemic disease such as fever and respiratory distress in the resting animal. Investigations in the United Kingdom (UK) suggest that IAD is most prevalent in horses newly introduced to the training environment, whether they are 2-year-old flat racing thoroughbreds or older horses racing over fences, and that bacterial infections, especially Streptococcus zooepidemicus,Streptococcus pneumoniae, and Actinobacillus spp., are associated with many but not all cases of IAD. Environmental factors such as particulates and endotoxin in the stable may be a primary or a synergistic component of IAD.
Every day the equine practitioner is faced with the dilemma of diagnosing IAD. One issue that complicates matters is that most racehorses do not outwardly exhibit clinical signs of respiratory disease. Some of the clinical signs are subtle, with the only detectable signs coming from the highly skillful attentiveness of the trainer, groom, or exercise person. As with all sports medicine problems in horses, the equine practitioner must sort through many possible causes of a performance problem. It is important to enter the diagnostic process with an open mind and not to focus immediately on one problem. In racehorses, lameness is always an uppermost concern, and it is easy to focus on, for example, mild effusion in the carpal joint, and fail to examine the respiratory system, which may be the primary cause of the problem.
In racetrack practice, the most common manifestation of lower airway inflammation is failure to perform to expectations during racing or training. Veterinarians are generally called to examine such horses immediately after training is over. The time for the workout may be below the trainer’s expectation given the horse’s level of fitness. Jockeys or exercise riders may report that the horse fails to sustain effort at the quarter or eighth pole, or they may hear respiratory noises such as gurgling; alternatively the horse may begin to lift its head while racing. In this group of horses, there is rarely any recent history to suggest a respiratory problem. Horses are typically eating well, have no fever, and are alert, responsive, and playful.
A second presentation is the appearance of nasal discharge during the cool-out period, which may or may not be associated with failure to perform. The nasal discharge is usually white and forms frothy rings around each nostril. Occasionally this discharge is combined with blood. Clear, watery discharge also may occasionally accompany airway inflammation and therefore warrants endoscopic examination. Thick yellowish green nasal discharge is most common in horses that have recently arrived at the racetrack and have mingled with other horses and been exposed to pathogens to which they are immunologically naïve. Occasionally this type of discharge is unilateral and originates from the upper airway. In these latter horses it is important to conduct a complete examination to rule out the lungs as a source of the discharge.
Although coughing is a specific sign of airway inflammation, many horses with airway inflammation do not cough. Coughing tends to be rare if the horse has small accumulations of airway mucus and no other clinical signs of respiratory disease. Coughing after racing also can accompany exercise-induced pulmonary hemorrhage (EIPH) and dorsal displacement of the soft palate (DDSP), as well as lower airway inflammation. Coughing generally is more common when systemic signs of disease accompany the respiratory problem.
When airway inflammation is severe, the trainer may notice initial signs of inappetence, lethargy, and depression accompanied by fever. Within a few hours, coughing and nasal discharge may develop. In younger horses, these signs may spread throughout the peer group. Older horses often are unaffected.
Depending on geographic location, bouts of airway inflammation may be seasonal and associated with introduction of new animals or with climate changes. In the Great Lakes region of the United States, the prevalence of airway inflammation increases in spring and late summer. Dry summers can cause an increase in airborne particulate concentrations and subsequent airway irritation.
Prevalence of IAD varies among trainers. There are several possible reasons for this. Differences in housekeeping, barn design, and location with respect to prevailing winds can all result in differences in airborne particulate concentrations. In some stables, horses are shipped in and out frequently, effecting the introduction of new microorganisms to which the other horses may not be immune.
It is very important to examine the horse as a whole, before focusing on the respiratory system. A horse may have two or more problems affecting its performance, for example, lameness and airway disease. However, a chronic orthopedic problem may be insignificant at the time of examination, while airway inflammation is the dominant cause. Physical examination must include measurement of rectal temperature and auscultation of the heart and lungs. Unless the horse has complications such as pleuropneumonia, it is rare to hear abnormal lung sounds in a horse with IAD.
The most important diagnostic tool is the endoscopic examination. The optimal time for examination is from 40 minutes to 2 hours after exercise. At this time, any abnormalities such as blood or excessive mucus from deeper within the lung tissues have had time to be transported up the tracheobronchial tree. The large volumes of fluid that are transferred between the pulmonary circulation and the lung tissue during exercise complicate results of endoscopic examination immediately after racing. Endoscopic examination beyond 2 hours after exercise is still valuable because excess mucus production associated with inflammation is continual and clinically significant mucus accumulations are still evident.
Even when the tracheobronchial tree is suspected of being the primary cause of a performance problem, it is important to examine the whole respiratory system. For this reason, the upper airway is completely examined as the endoscope is advanced into the trachea. Routinely, one nasal passage is examined unless there is a unilateral discharge or there is a bilateral discharge unaccompanied by tracheal accumulations of mucoid secretions. Examination of the upper airway can rule out lesions such as ethmoid hematoma and discharges from the paranasal sinuses and guttural pouch openings. Particular attention is paid to the severity of follicular lymphoid hyperplasia. Pharyngeal hyperemia or edema accompanied by excessive mucus within the tracheobronchial tree suggests respiratory infection. If the lymphoid follicles in the pharynx are white and inactive without large polyps and the lower airway mucus is watery or frothy, there is less suspicion of infection and more suggestion of an environmental or allergic cause of lower airway inflammation.