Treatment of experimental enzootic pneumonia of the pig by norfloxacin or its 6-chloro analogue

The 6-chloro analogue of norfloxacin (compound A) administered continuously in the feed at 400 ppm for 21 days markedly reduced the extent and activity of pneumonic lesions in pigs with pneumonia induced experimentally with an homogenate of pneumonic lung and broth cultures of Mycoplasma hyopneumoniae. Norfloxacin at 100 ppm or compound A at 200 ppm in the feed did not reduce the extent of lung lesions, although half the pigs treated with norfloxacin had lesions which appeared histologically to be healing. M hyopneumoniae was detected either by culture or immunofluorescence in the lungs of 60 per cent of the pigs treated with compound A at 400 ppm compared with all the pigs in the other groups. These results were related to the amount of drug in the lungs and body fluids during therapy. Only compound A at 400 ppm produced concentrations in the lungs and bronchial secretions exceeding the minimum inhibitory concentration against M hyopneumoniae. Mycoplasmacidal concentrations were not reached either in the lungs or bronchial secretions which might account partly for the frequent detection of M hyopneumoniae in the lungs after treatment. Drug resistance did not appear to be responsible for the persistence of M hyopneumoniae in vivo since the M hyopneumoniae isolates from the pigs after therapy were sensitive in vitro to both quinolones. As daily weight gain and feed-conversion efficiency improved in all groups of treated pigs compared with the controls, these effects were probably unrelated to the antimycoplasmal activities of the two quinolones.