AAEP Convention 2005: Inflammatory Airway Disease

A racehorse running at top speed breathes about 120 times per minute, moving about 12-15 liters of air per breath or 1,400-1,800 liters per minute. With this amount of airflow, it's not hard to imagine that any amount of airway inflammation can significantly affect performance. Unfortunately, the problem is common in horses, said Susan J. Holcombe, VMD, PhD, Dipl. ACVS, ACVECC, at the 2005 American Association of Equine Practitioners Convention, held Dec. 3-7 in Seattle, Wash.

"Airway inflammation is a key component of respiratory disease in all mammals," she explained. "It is responsible for bronchoconstriction (narrowing of airways), mucus/edema production (which can clog airways and decrease ventilation), and cause tissue damage (possibly leading to exercise-induced pulmonary hemorrhage). It can also cause tissue remodeling and fibrosis (this can impair gas exchange) and perhaps chronic changes in the lung that can decrease a horse's ability to perform long-term."

Holcombe discussed two types of airway inflammation: Upper airway inflammation (pharyngitis) and inflammatory airway disease (IAD), which affects the lower airways.

Pharyngeal Lymphoid Hyperplasia

"Pharyngeal lymphoid hyperplasia (PLH) is an inflammatory condition of the nasopharynx or throat of the horse," said Holcombe. "It is very common in young horses, although its prevalence decreases as horses age and develop immunity. However, the problem can have infectious causes as well, and it has been associated with stabling in one study." She cited another study that found pharyngitis of grades 3-4 (on a four-point scale) in 37% of 2-year-old Thoroughbred racehorses. However, the disease nearly disappeared in horses six years old or older.

The problem might not really be a disease, just an adaptation of a young horse to its environment, she said. However, the problem can have infectious causes as well, and it has been associated with hay (compared to pasture forage) and stabling in some studies.

"Because pharyngitis is frequently self-limiting and has not been definitively associated with poor performance, this disease is usually not treated," she reported. But if treatment is desired, anti-inflammatory therapy and/or systemic and inhaled corticosteroids might be used.

Although pharyngitis might seem a mild problem, Holcombe warned: "Accumulating evidence suggests that regional inflammation of the upper airway may predispose individuals to obstructive upper airway disease, such as nasopharyngeal collapse, dorsal displacement of the soft palate, and aryepiglottic fold collapse."

Inflammatory Airway Disease--A Bigger Problem

"Horses with an array of clinical signs that include poor racing performance, coughing, and mucus accumulation in the airways are frequently diagnosed with IAD," said Holcombe. "These horses may fade at the three-quarter pole, have reduced stamina during training, and have protracted recovery after hard workouts," evidenced by breathing too hard for too long compared to unaffected horses.

Bouts of IAD usually last about eight weeks and recur in some horses. The disease is less common as horses get older; it affects 12 - 33% of racehorses but nearly 80% of 2-year-olds.
Horses entering training are also at increased risk.

Development of IAD--Some inciting factor directly causes an inflammatory response, or something first might incite neutrophil proliferation (a type of immune cell). This causes large amounts of mucus accumulation in the lower airways. If a horse has both significant mucus and neutrophilic inflammation, Holcombe said this is highly suggestive of IAD.

Not quite IAD--However, high neutrophil numbers alone might not be a problem. Both horse and human athletes have exhibited significantly higher neutrophil numbers than sedentary controls, suggesting that higher exercise intensities might be associated with low-grade inflammatory lung disease. "These changes were interpreted as secondary to repeated exercise-induced hyperventilation and/or increased airway exposure to inhaled allergens or pollutants," Holcombe explained.

IAD causes--Researchers don't yet know what causes this disease, but they suspect several factors, including the bacteria Streptococcus zooepidemicus, S. pneumoniae (only in younger horses), Actinobacillus, Pasteurella spp., and Mycoplasma equirhisnis. Other causes could be equine herpesvirus, genetic susceptibility, and environmental problems including poor ventilation, respirable particulates (dust), endotoxin, and the quality of bedding and feed. Also, IAD bouts last longer when horses are kept in a dusty environment.

Diagnosis--The disease is diagnosed based on a history of poor performance with signs of respiratory compromise (such as coughing or nasal discharge), complete physical examination, and attempt to make the horse cough by palpating the trachea and larynx. Veterinarians also look for neutrophilic inflammation of the airways and mucus accumulation (detected with visual endoscopic examination and scored from 0 to 4). Bronchoalveolar lavage to look for infectious agents and inflammatory cells might also be performed.

Performance effects--A tracheal mucus score of 2 or higher has been associated with decreased racing performance in Thoroughbred horses; horses with lower scores of 0 or 1 were nearly twice as likely to finish well in a race in one study, Holcombe reported. Another study of Standardbreds found that horses with tracheal mucus were more likely to finish last or next to last than first or second. In horses with moderate to severe mucus, the higher the mucus score, the farther back the race placing.

Treatment--The goal is to decrease airway inflammation and eliminate any infectious agents (most commonly Streptococcus bacteria) that are present. Non-steroidal anti-inflammatory drugs might also be used, as well as systemic or aerosolized corticosteroid after the infection is resolved. Clenbuterol is also helpful, she reported, as is environmental management such as improving ventilation, and decreasing dust in bedding and hay.

Unfortunately, "No one has looked at duration of clinical signs with and without treatment, or at efficacy," said Holcombe. When they do, any results regarding treated racehorses should be quite revealing, as she notes: "Racing might be the optimum lung function test."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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