EFFECT OF TREATMENT WITH ERYTHROMYCIN ON BRONCHOALVEOLAR LAVAGE FLUIDCELL-POPULATIONS IN FOALS

Objective-To determine whether oral administration of erythromycin alt ers 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 p hosphate buffered saline solution (300 mOsm, pH 7.4) was administered in 50-ml aliquots. Foals were carefully monitored for 4 days, then ery thromycin base (25 mg/kg of body weight, PO, q 12 h) was given to feat s of the treated group. After 4 days, foals were reanesthetized, and t he same lung was relavaged. Cytologic examination was performed on BAL fluid (BALF) samples from both groups of foals. At 12 hours after adm inistration of the final dose, erythromycin A and anhydroerythromycin A concentrations were determined in plasma of treated foals. Results-I n the second BALF sample from the same lung of control foals, percenta ge 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 s ource. Significantly fewer BALF cells adhered to a cell culture substr atum after erythromycin treatment of foals. Erythromycin A was not det ected in plasma of any treated foal at the time of the second BAL; anh ydroerythromycin A, a degradation product of erythromycin, was detecte d in plasma of 5 of 6 foals (mean concentration, 0.2 +/- 0.06 mu g/ml) . Conclusion and Clinical Relevance-BAL induces neutrophilic inflammat ion, which persists for at least 4 days in the lungs of young horses. Erythromycni (25 mg/kg, PO, q 12 h) diminishes this inflammatory respo nse through a mechanism that may involve alteration of BALF cell funct ion. Degradation of erythromycin to biologically active products or pr esence of parent drug in pulmonary secretions may be responsible for a lterations in pulmonary ravage cell chemotaxis and adherence. Erythrom ycin administered orally to foals at clinically relevant doses appears to have nonantimicrobial effects that may interfere with host cell me tabolism and decrease inflammatory reponses in airways.