Efficacy of tilmicosin in the control of experimental Mycoplasma gallisepticum infection in chickens.

This study was conducted to evaluate the efficacy of 5-day, "in water" tilmicosin medication for the prevention of experimental Mycoplasma gallisepticum (MG) disease in 10-day-old specific-pathogen-free (SPF) chickens. Birds were inoculated intratracheally and into the sinus with the MG R-P10 strain. A limited dose titration of the antibiotic over the expected effective range was included, using six groups of 60 birds each: UI: uninfected untreated group; IUT: infected untreated group; IT1 to IT4: four infected treated groups, which were administered 50, 100, 200, or 300 mg/liter of tilmicosin. The birds were given tilmicosin from 8 to 13 days of age and were inoculated at 10 days of age. The birds were observed for 11 days postchallenge before terminal postmortem examination was completed including, assessment of lesions and sampling for mycoplasma culture and serology. Body-weight gains of the different groups were compared. The results showed that tilmicosin medication at dose levels of 50-300 mg/liter significantly decreased growth losses and respiratory signs due to MG infection (P < 0.05). Significant reduction in air sac and peritonitis lesions were obtained by treatment with 100, 200 or 300 mg/liter for 5 days (P < 0.05). A significant reduction in the proportion of MG-culture-positive birds was obtained at a dose level of 50 mg/liter (P < 0.05). Increasing the dose resulted in a further decrease in the number of MG shedding chickens to the extent that with the two highest doses of tilmicosin, no bird was serologically positive on day 21, compared to 46/58 positively infected untreated birds (day 21).