Pasteurellosis: an outbreak amongst sheep

Abstract

From the three cases of pasteurellosis in sheep studied, five different strains of pasteurellae were obtained, 182 from ram 1, 181 from ram 2, and 247, 247a, and 247c from ram 3. Of these strains 182 and 247 were highly pathogenic for both sheep and guinea-pigs, and almost non-pathogenic for rabbits and pigeons. Both showed the same biochemical reactions and both produced similar pathological changes in experimental animals inoculated with live cultures. They resembled each other also in morphology, staining and cultural characteristics. Pasteurellae 182 and 247 can therefore be regarded either as identical or so closely related that they cannot be differentiated by the methods employed. Both these organisms have a predilection for pulmonary tissue and serous membranes and both produced lesions in experimental animals that could not be differentiated from those found in natural cases studied. These two organisms are considered to have been the cause of the mortality among the Ryeland sheep at the experimental farm of the University of Pretoria. An identical disease in experimental sheep was produced by the injection of organ emulsions and cultures made from the original cases from the University farm. Berkeveld filtrates of organ emulsions from natural cases did not produce the disease. So far no success has yet been attained with immunisation tests in laboratory animals and no properly controlled immunisation experiments have been carried out with sheep. The pathogenesis of the disease under natural conditions is still obscure. Pasteurellae 181 and 247a cannot be distinguished from each other by the tests employed; both are very slightly pathogenic for guinea-pigs and both have the same biochemical reactions, and they agree in morphology, cultural and staining characteristics. Both have originated from small colonies picked from primary cultures of pulmonary material. Pasteurella 247c does not resemble either of the two groups of organisms mentioned above. It is entirely non-pathogenic for laboratory animals. These results indicate: (1) that the small colonies picked from the primary growths on media seeded with material from affected lungs yielded cultures which were either non-pathogenic (247c) or only very slightly pathogenic (247a, 181); (2) that the large colonies obtained from similar growths gave rise to highly pathogenic cultures (182 and 247); (3) that when several colonies were picked from the same primary growth, highly pathogenic, slightly pathogenic and non-pathogenic cultures may be obtained e.g. cultures 247, 247a, and 247c); (4) that if only one colony is picked from the primary growth either a highly pathogenic culture (182) or one which is barely pathogenic (181) may result. It is possible that the non-pathogenic and slightly pathogenic pasteurellae occur as saprophytes in the respiratory passages of sheep in certain areas and that they invade the lungs only when the way has been paved for them by the entrance of pathogenic pasteurellae of the type 182 and 247. These latter enter the tissues and set up disease under conditions which have not yet been determined. In making a bacteriological study of a case of pasteurellosis, therefore, it is recommended that several colonies of different sizes be picked from the primary growth, and that the pathogenicity of each one be studied separately. Only in this way may the presence of pathogenic pasteurellae be determined.