A method for assessing induced resistance to enzootic pneumonia of pigs

The resistance induced in pigs by the intravenous, intraperitoneal and subcutaneous inoculation of live Mycoplasma hyopneumoniae was tested by placing inoculated pigs in contact with pigs artificially infected with enzootic pneumonia. Control pigs inoculated by the same routes with sterile culture medium were exposed simultaneously to the infected pigs and the incidence and extent of the pneumonia in both groups were compared. A score derived from the cube root of the calculated or estimated volume of the lung lesion, was used to evaluate severity. Ninety-six per cent of the control pigs developed lesions characteristic of enzootic pneumonia but only 29 per cent of the pigs that had previously been inoculated with M hyopneumoniae developed lesions. Induced lesions differed from those seen in natural cases in extent rather than in character. The method of challenge was satisfactory in that a high proportion of control pigs became infected without the resistance of a majority of inoculated pigs being overwhelmed.