ANTIBIOTIC-PROPHYLAXIS OF LOWER RESPIRATORY-TRACT CONTAMINATION IN HORSES CONFINED WITH HEAD ELEVATION FOR 24 OR 48 HOURS

Objective To evaluate the administration of procaine peni- cillin prio r to or during confinement with head elevation as a means of reducing the associated accumulation of inflammatory lower respiratory tract se cretions and increased numbers of bacteria within the lower respirator y tract of confined horses. Design and Procedure Two experiments were conducted to evaluate the efficacy of different dose rates and dosing frequencies. In experiment A a single low dose (15,000 IU/kg) of proca ine penicillin was administered to four horses immediately prior to co nfinement with head elevation for 48 hours. The systemic leucocyte res ponse, gross and cytologic characteristics of transtracheal aspirate a nd bacterial numbers in lower respiratory tract samples were compared with corresponding samples from two horses confined with heads elevate d but not given penicillin. The efficacy of higher dose rates (20,000 IU/kg and 40,000 IU/kg) given before and during confinement with heads elevated for 24 hours was evaluated in experiment B. Results Treatmen t with procaine penicillin had no effect on the systemic leucocyte res ponse or on the accumulation of inflammatory lower respiratory tract s ecretions at any of the dosing schedules evaluated. The number of bact eria isolated from trans-tracheal samples was reduced at 12 hours for treated horses in experiment A and at 24 hours for experiment B. beta- haemolytic Streptococcus sop were not isolated from treated horses in either experiment. Bacterial species isolated from treated horses were predominantly Pasteurella and/or Actinobacillus spp, however, members of the family Enterobacteriaceae and a Staphylococcus sp were isolate d from treated horses. One treated horse in experiment A developed cli nically apparent pulmonary disease. Conclusions The prophylactic admin istration of penicillin before or during confinement did not reliably reduce bacterial numbers or prevent the accumulation of purulent lower respiratory tract secretions in horses confined with their heads elev ated. Numbers of beta-haemolytic Streptococcus spp were reduced follow ing treatment, suggesting that the repeated administration of procaine penicillin may have some merit as part of a strategy to prevent trans port-associated respiratory disease. However. methods directed at mini mising the duration of confinement with head elevation, augmentation o f the clearance of accumulated secretions and prompt identification of animals in which airway inflammation has extended to the pulmonary pa renchyma remain the best ways of minimising transport-associated respi ratory disease.