Studies on the Biovailability of Tetracycline Chloride after Oral Administration to Calves and Pigs

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Abstract
The serum concentrations of tetracycline (TC) were studied in calves after administration in the milk replacer and in pigs after administration in feed and drinking water. Serum concentrations of potential therapeutic value (> 1 .mu.g/ml) were obtained in the calves when TC was fed in the milk replacer at a dose of 25 mg/kg body weight twice daily. Significantly higher serum concentrations were achieved after a single oral dose (50 mg/kg) given in the milk replacer than after the same dose given as a water drench four hours after the first daily milk feeding. The difference was explained by the fact that the calves probably consumed their daily amount of concentrate and an unknown amount of hay previous to the drench. Earlier studies showed that concentrates reduce the bioavailability of tetracyclines more than a conventional milk replacer. Serum concentrations of potential therapeutic value could not be acheived in the pigs in spite of recommended doses. The TC concentrations in the water were 400 mg/l and 800 mg/l. Tetracycline did not appear to influence the water consumption. Serum concentrations were highest in the evening and fell during the night. The highest levels afte 400 mg/l water were 0.30-0.47 .mu.g/ml. The corresponding levels after 800 mg/l were 0.65-1.15.mu.g/ml. It was concluded that feed serously reduced the bioavailability of TC. The influence of feeding was demonstrated in a single dose study. Fasted animals showed much higher serum levels than non-fasted ones. Very low and constant serum levels (about 0.2 .mu.g/ml) occurred when TC was mixed in the feed at a concentration of 1,000 mg/kg. It was suggested that the therapeutic use of TC in feed and drinking water in pigs may be questioned because of a very low bioavailability.