Characterization of Pleuropneumonia-Like Organisms of Avian Origin I. Antigenic Analysis of Seven Strains and Their Comparative Pathogenicity for Birds

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Abstract
On the basis of signs and lesions produced in chickens and turkeys, the virulence of 7 strains of pleuropneumonia-like organisms (PPLO) of avian origin was graded from highly virulent to completely avirulent. Nervous signs, joint lesions, and air-sac lesions were among the signs and lesions produced by the pathogenic strains. The pathogenicity studies with two-day-old chickens gave results comparable to the turkey-sinus inoculation method. Four distinct PPLO types ( designated as Groups I, II, III, and IV) were detected by the cross agglutination test with sera of rabbits hyperimmunized with the various strains. The organisms of Group I comprised strains which were pathogenic for birds while those of Groups III and IV were not pathogenic. An organism designated strain C which was serologically classified into Group II was mildly pathogenic for birds. However, evidence was presented which suggested that the C strain inoculum for the pathogenicity studies was mixed with an organism antigenically related to the pathogenic Group I. The presence of the latter component was evident only after bird inoculation and selection for that type. These findings should aid in the clarification of the role of PPLO in chronic respiratory disease of chickens and infectious sinusitis of turkeys.