Absence of therapeutic blood concentrations of tetracycline in rats after administration in drinking water.

Because of ease of administration and broad antibacterial spectrum, tetracycline often is administered in drinking water to control infectious diseases of rats. Assay of serum after a gavage bolus of tetracycline (300 mg/kg body weight) revealed little absorption of tetracycline by this route. Rats were given water containing tetracycline at several concentrations (400 mg/liter, 4g/liter, and 4 g tetracycline plus 50 g sucrose/liter) ad libitum and serum concentrations of tetracycline were monitored. Bioassay of serum samples from these animals, taken during 72 hours of water medication, revealed no detectable tetracycline concentrations (greater than 0.2 mcg/ml) in the 400 mg and 4 g/liter groups. Two of eighteen serum samples from the group given 4 g tetracycline with 50 g sucrose/liter had minimal therapeutic tetracycline concentrations (0.3 mcg/ml) effective for Mycoplasma pulmonis. Some of the animals given tetracycline ad libitum in drinking water drank very little and lost weight compared to control animals. These findings indicate that the practice of adding tetracycline to drinking water of rats may be ineffective in controlling systemic diseases, and also be detrimental to the treated animals.