Objective: To evaluate the effect of in-feed tilmicosin on the morbidity, mortality, and production parameters of pigs experimentally challenged with a virulent field strain of Haemophilus parasuis. Methods: The study consisted of three replicates of a trial in which 19- to 22-day-old pigs were randomly assigned to one of four treatment groups (nine pigs per group) on Day -4: unchallenged-unmedicated, challenged-unmedicated, challenged-tilmicosin 200 mg per kg, and challenged-tilmicosin 400 mg per kg. Challenged groups were randomly assigned to one of three rooms. The unchallenged group was assigned to the same room for the three replicates to minimize the potential for H parasuis cross-contamination. Medicated feed was provided ad libitum from Day 0 to trial termination (Day 21). Aerosol challenge occurred on Day 7 with a virulent H parasuis serotype 5 isolate. Outcomes assessed included mortality, gross lesions and culture of H parasuis at necropsy, clinical scores, and growth parameters. Results: Among challenged pigs, compared to unmedicated pigs, treatment with tilmicosin at 200 mg per kg of feed reduced the frequency of gross lesions consistent with H parasuis infection, culture of H parasuis, and H parasuis-specific mortality (P<.05), and improved clinical scores and growth parameters (P<.05), except for feed:gain. In addition, a significant trend (P<.05) towards further improvements in all parameters with increasing tilmicosin dose (400 mg per kg) was demonstrated by linear regression. Implications: Under the conditions of this study, tilmicosin was effective in controlling H parasuis infection in weaned pigs challenged by aerosol inoculation with a virulent field strain.
