Plasma and tear concentrations of antibiotics administered parenterally to cattle.

Chloramphenicol, erythromycin, gentamicin, oxytetracycline, penethamate and procaine benzyl penicillin were administered parenterally to cattle and the concentrations of these antibiotics in plasma and tears were assayed microbiologically. Concentrations in plasma and tears were significantly correlated for all antibiotics tested but the concentration of antibiotic in tears and the tear flow rate were not correlated. Lipophilic drugs diffused into the tears in higher concentrations than did drugs which were not lipophilic. Concentrations of lipophilic but not hydrophilic antibiotics in tears could be predicted from the Henderson-Hasselbach equation. In cattle, it is possible through parenteral administration of chloramphenicol, erythromycin, gentamicin or oxytetracycline to achieve antibiotic concentrations in the tears which are bacteriostatic to Moraxella bovis, a primary aetiological agent of infectious bovine keratoconjunctivitis.