PULMONARY EOSINOPHILIA ASSOCIATED WITH INCREASED AIRWAY RESPONSIVENESS IN YOUNG RACING HORSES

Horses are known to acquire small airway disease (SAD), an allergen-in duced naturally occurring syndrome of reversible obstructive lung dise ase accompanied by airway hyperresponsiveness and increased inflammato ry cell numbers on bronchoalveolar lavage (BAL). This disorder has rec eived scant attention in young racehorses. The purpose of the present report was to examine the effect of BAL eosinophilia in young racehors es on clinical examination, BAL, hematology, airway responsiveness, an d on pulmonary function at rest and after a standardized exercise chal lenge. Five (3 males, 2 females; age 2.6 +/- 0.9 years) with a history of respiratory compromise and BAL eosinophil differential count > 5% and 6 controls (4 males, 2 females; age 3.5 +/- 1.0 years) training an d performing to expectation with normal BAL cell differential (eosinop hils < 1%) were studied. Respiratory system clinical examination was p erformed and expressed as a clinical score. Arterial blood gas measure ments, CBC, and pulmonary function testing were performed at rest. Pul monary mechanics measurements were repeated 1 hour and 20 hours after a standardized treadmill exercise challenge. Incremental histamine inh alation challenge was performed and the concentration of histamine eff ecting a 35% decrease in dynamic compliance (PC35C(Dyn)) was determine d. Significant differences were noted between and controls with regard to clinical score (P = .01), blood eosinophils (P = .04), BAL cell co unt (P = .04), BAL macrophage differential (P = .04), PC35C(Dyn) (P = .008), and tidal volume and respiratory rate at 20 hours following exe rcise challenge (P = .05). We conclude that pulmonary eosinophilia and airway hyperresponsiveness are manifest in some young horses without overt airway obstruction at rest. We speculate that these may be early events in the natural progression of heaves.