Circadian serum concentrations of tylosin in broilers after feed or water medication

1. Because tylosin is a time-dependent antibacterial agent, and because feeding and drinking of broilers decreases in late afternoon and ceases in the dark, it was hypothesised that serum concentrations of this drug are greatly reduced during the dark period. 2. The trial was carried out in a commercial poultry house, under standard broiler husbandry conditions, with food and water withdrawn from 22:00 until 07:00 h next morning and exposed to a natural light cycle of 13L:11D. 3. Broilers were given tylosin tartrate, in either feed or water, for 5 d as follows: 100, 200 and 300 ppm in feed, equivalent to 12.6, 25.2 and 37.8 mg/kg/d, respectively; and 200 and 400 mg/l in drinking water, equivalent to 51 to 102 mg/kg/d, respectively. 4. At 07:00 h on d 4, and for the next 40 h, hourly serum samples were obtained and analysed for tylosin by means of a microbiological assay. 5. Day vs night concentrations of tylosin expressed as area under the curve (AUC) in all groups revealed greater values during the day. The highest AUC and AUC(24)/minimal inhibitory concentration (MIC) ratio were obtained in the group medicated with 400 mg/l and the corresponding lowest values were found in the group medicated with 100 ppm in feed. 6. In conclusion, tylosin did not reach therapeutic serum concentrations during the dark period, at all dose rates tested when administered in feed or water. A sustained release form of this drug is needed to solve this inadequacy of tylosin medication in broilers.