“What is interesting to understand about the horse is that at rest, it breathes about 86,000 liters of air a day, about 25,000 gallons, which may carry quite a bit of dust particles that can really injure the respiratory tract,” said Laurent Couëtil, DVM, Dipl. ACVIM, of Purdue University in the In Depth: Inflammatory Airway Disease (IAD) session at the 2002 American Association of Equine Practitioners Convention. The result is a chronic cough and mild exercise intolerance, seen in 11-65% of racehorses today, depending on the diagnostic criteria used (endoscopy or cytology) and whether the measurements were taken before or after exercise.

IAD typically occurs in younger horses, and its characteristics include:

  • Clinical signs of a chronic cough, mucopurulent exudates (small amounts of pus, in this case deposited in the airways), mild exercise intolerance, and chronic respiratory tract inflammation
  • Multifactorial etiology (caused by many different factors);
  • Mild airway obstruction;
  • Non-recurrent;
  • Bronchitis/bronchiolitis.

Couëtil explained that up until a few years ago, many terms were used mistakenly to describe different airway disorders. IAD is most often confused with recurrent airway obstruction (RAO) or heaves, which tends to occur in older horses and includes all of the clinical signs of IAD, but with the addition of abnormal lung sounds, increased respiratory efforts at rest, and marked exercise intolerance. Also, researchers believe RAO has an allergic etiology, causes moderate to severe airway obstruction, and is recurrent. Typically IAD lasts seven weeks, but historically, it can last anywhere from four to 22 weeks.

IAD can affect any breed of horse, but, “It’s certainly more common in Standardbred and Thoroughbred racehorses,” said Couëtil. “It just happens in racehorses that they use 100% of their cardio-respiratory potential (pulmonary gas exchanges are a major limiting factor and a mild degree of IAD might have a profound effect), and are overly represented as a result,” explained Couëtil. Also, training, racing, and frequent travel often are stressful to the immune system and might result in lower airway disease, along with extended confinement in stables with suboptimal ventilation. In racing, “The last place horse is six times more likely to have mucus present in the airways than horses finishing first or second. In a nutshell, horses that finish last are more likely to have IAD,” said Couëtil.

Mucopurulent exudates found in the tracheobronchial tree (the junction of the trachea and the bronchial tubes) and an increased percentage of neutrophils (a type of white blood cell) in bronchoalveolar lavage (BAL) have been associated with exercise intolerance and poor performance, said Couëtil. Clinicians can determine what type of IAD is affecting a horse through examining the cells in a bronchoalveolar lavage, andthey can differentiate IAD from recurrent airway obstruction.

Suspected etiologies for IAD include bacteria, viruses, and inhaled environmental pollutants. While bacterias such as Streptococcus spp., Pasteurella/Actinobacillus spp., and Bordetella spp. are frequently isolated from tracheal wash samples, the role of bacteria in the pathogenesis of the disease is unclear. According to Couëtil, no more than one-third of IAD-affected horses show bacteria upon culture testing, and if they do, it might result from decreased clearance of the airways and not necessarily from infection. Also, the trachea is not a sterile environment, and bacteria can be isolated from healthy horses. Scientists also think that the bacteria might be opportunistic invaders in horses with IAD rather than causative agents, however they might play a role in young racehorses entering training.

“Contrary to a common belief, respiratory viruses do not appear to play an important role in IAD,” wrote Couëtil in his review paper. There does not appear to be an association between “viral infections in horses with IAD based on serology or virus isolation aimed at detecting equine herpesviruses, influenza, adenovirus, and rhinoviruses,” he noted.

However,he continued, “exposure to environmental dust does play a role in the pathogenesis of IAD. There are several types of data that can demonstrate that.” He mentioned one study in which healthy yearlings were fed hay in a barn, and others at pasture. After three months the groups were switched between pasture and barn for another three months. Yearlings had evidence of IAD when housed in the barn but not when they were kept at pasture. Another study showed that horses bedded on straw took a longer time to recover from IAD than horses kept in stalls with shredded paper (which has a lower dust content).

Travel and head elevation can have an influence on IAD as well, and can cause it to elevate into a more serious condition. “If you keep a horse’s head elevated more than six hours, you start seeing increasing numbers of bacteria in the tracheal fluid and start seeing accumulation of mucus,” he said. He explained that some horses have developed full-blown pneumonia after having their heads elevated for 24 hours. “It’s very important for a horse to be able to act like a horse” and have the ability to put its head down, he said. “A lot of horses are transported over long distances with their heads kept up—so that is food for thought,” he added

No genetic data is available currently for IAD, so it is unclear what role genetics might play in IAD susceptibility, but genetic predisposition remains a possibility like it is with heaves in horses and asthma in people.

Horses with clinical signs of IAD are more likely to perform poorly, but scientists still aren’t sure of the mechanism that causes exercise intolerance in these horses. It could be a result of impaired gas exchanges due to airway obstruction. Airway hyperresponsiveness is thought to play an important role with triggering the cough.

A retrospective study performed at Purdue University on 163 horses presented for poor performance evaluation showed that 20% of the cases had a combination of respiratory disease and at least one problem in another body system. This emphasizes the importance of performing a comprehensive evaluation of poor performers.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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