Preliminary clinical pharmacological investigations of tylosin and tiamulin in chickens.

The minimal inhibitory concentrations (MIC) of tiamulin and tylosin for mycoplasma, Gram-positive, and Gram-negative micro-organisms isolated from chickens were determinated by the agar dilution method. Median MIC values for tiamulin against Mycoplasma gallisepticum (0.05 microgram/ml) and Mycoplasma synoviae (0.10 microgram/ml) were 2 to 4 times lower than the corresponding values for tylosin. Tiamulin was also slightly more effective in vitro in inhibiting Escherichia coli, Pasteurella multocida, and beta-haemolytic streptococci than was tylosin. Groups of chicken were offered tiamulin medicated drinking water at rates of 125 and 250 mg/litre for 48 hours. Average serum tiamulin concentrations were 0.38 and 0.78 microgram/ml, respectively. When tylosin tartrate was added to the drinking water at 500 and 700 mg/litre, average serum drug levels were 0.12 and 0.17 microgram/ml, respectively. Tiamulin was 45% bound in chicken serum, as against 30% serum protein binding for tylosin. Correlations were made between free (non protein bound) serum drug levels and the MIC values of the two drugs. Such comparisons suggest that when tiamulin is given in the drinking water at rates of 125 to 250 mg/litre, better antimycoplasmal activity is to be expected in vivo than by giving tylosin tartrate in the drinking water at 500 to 700 mg/litre. Based on these data, no clinical efficacy of these dose rates can be expected in flocks infected by gram-negative micro-organisms such as E. coli or P. multocida. The tylosin tartrate rate of 500 to 700 mg/litre, may be clinical ineffective the treatment of Staphylococcus aureus infections.