The minimum inhibitory concentration of kitasamycin, tylosin and tiamulin for Mycoplasma gallisepticum and their protective effect on infected chicks.

The minimum inhibitory concentration (m.i.c.) of kitasamycin, tylosin and tiamulin for Mycoplasma gallisepticum (Mg) were compared with 10(6), 10(4), and 10(2) CFU/ml of the organisms with the drug incorporated in mycoplasma agar. The lowest m.i.c. was obtained with tiamulin and the highest with kitasamycin and, in general, the m.i.c.'s were directly influenced by the concentration of mycoplasmas. Chick embryos at 19 days of incubation were infected with Mg and the hatched infected chicks were used for comparing the protective effect of the three drugs. They were given in the drinking water when the chicks were 2, 3 and 4 days of age. Weight gains for the infected treated birds were similar for all three drugs; they were significantly lower than for those of uninfected chicks and significantly higher than for those of untreated infected chicks. However Mg could be isolated from a high proportion of chicks in the infected treated and untreated groups at 32 days of age.