Effect of treatment with erythromycin on bronchoalveolar lavage fluid cell populations in foals

Objective. To determine whether oral administration of erythromycin alters the inflammatory response to bronchoalveolar lavage (BAL) in young horses. Animals. 12 healthy, unweaned, mixed-breed foals of either sex, between 2 and 4 months old. Procedure. BAL was performed; 250 ml of phosphate-buffered saline solution (300 mOsm, pH 7.4) was administered in 50-ml aliquots. Foals were carefully monitored for 4 days, then erythromycin base (25 mg/kg of body weight, PO, q 12 h) was given to foals of the treated group. After 4 days, foals were reanesthetized, and the same lung was relavaged. Cytologic examination was performed on BAL fluid (BALF) samples from both groups of foals. At 12 hours after administration of the final dose, erythromycin A and anhydroerythromycin A concentrations were determined in plasma of treated foals. Results. In the second BALF sample from the same lung of control foals, percentage of neutrophils was significantly increased (3 ± 38.0%), compared with that from erythromycin-treated foals (4.88 ± 3.66%, P < 0.05), and was associated with apparent decrease in the ability of BALF cells from erythromycin-treated foals to migrate toward a chemoattractant source. Significantly fewer BALF cells adhered to a cell culture substratum after erythromycin treatment of foals. Erythromycin A was not detected in plasma of any treated foal at the time of the second BAL; anhydroerythromycin A, a degradation product of erythromycin, was detected in plasma of 5 of 6 foals (mean concentration, 0.2 ± 0.06 μg/ml). Conclusion and Clinical Relevance. BAL induces neutrophilic inflammation, which persists for at least 4 days in the lungs of young horses. Erythromycin (25 mg/kg, PO, q 12 h) diminishes this inflammatory response through a mechanism that may involve alteration of BALF cell function. Degradation of erythromycin to biologically active products or presence of parent drug in pulmonary secretions may be responsible for alterations in pulmonary lavage cell chemotaxis and adherence. Erythromycin administered orally to foals at clinically relevant doses appears to have nonantimicrobial effects that may interfere with host cell metabolism and decrease inflammatory reponses in airways.