Effect of a new macrolide antibiotic (tilmicosin) on pneumonia experimentally induced in calves by Mycoplasma bovis and Pasteurella haemolytica.

Two gnotobiotic calves were treated once with tilmicosin (20 mg kg-1) six hours before they were infected by the intratracheal route with Mycoplasma bovis and Pasteurella haemolytica serotype 1. This treatment prevented colonisation of the lungs by P haemolytica and considerably reduced colonisation by M bovis, and the clinical scores and the extent of pneumonic consolidation, compared with two untreated gnotobiotic calves, both of which had to be killed in extremis for humanitarian reasons within 24 hours of infection. In a second experiment, 10 conventionally reared calves were similarly exposed to infection and, at the onset of clinical disease, five were treated once with tilmicosin (20 mg kg-1). Colonisation by P haemolytica and M bovis, the clinical scores and extent of pneumonic consolidation were suppressed or greatly reduced in the treated compared with the untreated calves, one of which had to be killed in extremis two days after infection. It was concluded that tilmicosin had a beneficial effect.