Berry, Donald Michael Enzootic pneumonia in cattle M. V. Sc., May, 1960 73 P* tables and photomicrographs. Professor H. C. Rowsell, Supervisor of Research Pneumonia and enteritis in calves are a constant problem for the veterinarian and a continual source of loss to the livestock owner. The two syndromes have been associated since the publication of Nocard in 1901, and have been the subject of extensive research. Until the last decade bacteria notably Pasteurellae, coliforms, staphylococci, and streptococci are considered as primary aetiological agents. From Nagel (1937), Baker (1943), Gallo and Galderon (1948), Kiuchi et al (1952), Moll (1956), and Matumoto et al (1955), increasing evidence supported viruses a^ primary agents and members of the psittacosis lymphogranuloma (P. L. V. ) group of viruses were implicated in particular. This study attempted to evaluate the condition and to determine possible agents associated with the disease in Ontario. An abattoir study of 867 sets of lungs from apparently healthy calves aged three to six months showed 21% to areas of pulmonary consolidation: 7^ of lungs examined had severe consolidation involving one whole lobe or more, and 14% had consolidation involving one quarter to three quarters of a lobe. Bacteriological examination of 178 sets of lungs enabled Pasteurellae to be recovered from 34%,, streptococci, staphylococci and pneumococci from 15%, no growth was recorded in 51%. Pathological examination revealed peribronchiolar and perivascular lymphocytic infiltrations with lymphocytic aggregations in areas adjacent to the bronchioles. Areas of alveolar collapse and local emphysema with alveolar epithelia metaplasia was noted. A superimposed bacterial pneumonia was frequently observed. A serological study was then undertaken of herds with histories of respiratory disease and of herds currently affected with pneumonia. From this an incidence of 42% of cattle with pneumonia were seen to be seropositive to the psittacosis antigen, 20% to be seropositive to an unidentified virus obtained from an experimentally reproduced case of pneumonia in a colostrum free calf, 20* to be seropositive to adenovirus antigen an? less than 1% to be seropositive to Para influenza Material from areas of consolidation in lungs obtained from the battoir, from field oases and from allantoic fluid infected with Miyagawanella bovis reproduced a mild to moderate consolidation in colostrum free calves exposed to them. Egg adapted Psittacosis virus produced a moderate pneumonia in colostrum free calves. Psittacosis virus recently isolated from a parakeet produced a severe pneumonia, which on calf to calf passage developed into a fulminating syndrome with death in 24 hours. Human strains of Adenovirus type 3 and type and of Para influenza 3 and 433 and resulted in inapparent infections with a serum titre rise. Pigeons and unconfined poultry on premises with histories of respiratory disease showed high seropositive titres to the psittacosis antigen, while confined poultry were seronegative. It is possible that pigeons, 35% of which are carriers of P. L. V. group viruses in southern Ontario, and unconfined poultry roosting in cattle barns during the winter months act as reservoirs an are able to transmit P. L. V. group viruses to susceptible cattle. It would appear possible that the Psittacosis virus by itself was capable of producing a severe pneumonia with increasing mortality on calf to calf passage in inapparent infections. It is probable that many viruses may cause pneumonia in cattle.