Three naturally-occurring epizootics of Paeteurella pseudotuberculosis in canaries were studied. Cultural details and gross histopathological lesions were described for birds from two of the aviaries. Epizootiological observations were made in all three cases following visits to the premises and recovery of data on management, first clinical signs, and mortalities. A reasonably complete study was made of early and current literature concerning pseudotuberculosis infections and incidence in birds and mammals, both feral and domesticated. Although this disease has been commonly reported in canaries in Europe from 1884 onwards, the epizootics herein reported are, as far as the author is aware, the first bacteriologically confirmed canary Infections to be reported on the North American Continent. Because of the high mortalities encountered, and because of the known potential of the causative organism to produce human disease, the epizootiological considerations were extended to Include experimental studies on the faecal excretion rate of viable Pasteurella pseudotuberculosis by naturally and artificially Infected canaries. With the evolution of suitable culturing techniques, the recovery of viable organisms of the inoculation strain from experimentally infected canaries was performed with ease. This allowed counts to be carried out on total daily faecal samples from twenty inoculated and four control birds for a three-week period following oral inoculation. It was found that in the group of twenty inoculated birds which suffered five (20%) mortalities, some sixteen (80%), including those birds that died, excreted organisms for periods ranging from three up to nineteen days. Peak amplitudes for the estimated faecal counts of Pasteurella pseudotuberculosis per day varied from Log 4.3 up to Log 8.1. The four dead birds showed gross lesions typical of naturally-occurring pseudotuberculosis, and yielded recovery cultures from various organs. The remaining twelve shedder birds all ceased shedding viable P. pseudotuberculosis by the twentieth day following -the conclusion of oral Inoculations (24th experimental day), and although none yielded a positive culture from various organs, there were slight-to-marked splenic lesions in all but two when they were autopsied on the twenty-fifth experimental day. Four birds which never shed detectable numbers of viable P. pseudotuberculosis were found to have no visible gross lesions when sacrificed and autopsied. Four non-inoculated control birds also failed to shed detectable numbers of viable P. pseudotuberculosis. Attempts were made in the laboratory to allow naturally and experimentally infected canaries to transmit the infection to healthy contact birds. These attempts were unsuccessful, and it was concluded from this and from direct observations on the natural epizootics that predisposing factors other than the presence of the organism (such as climatic or poor-management stress, or gastro-intestlnal irritation) are required at times for the disease to become epizootic in canaries. From the faecal excretion rates of viable pseudotuberouloeis measured in experimentally infected birds, and from epizootiological observations, it was concluded that canaries infect each other (rather than the infection coming from a common source) and that they are potential spreaders of infection (or of the infecting organism) to other species in contact, including man. Gross and histopathological observations of experimentally Infected birds correlated with their faecal counts, and gross pathological observations on naturally-Infected birds, indicate that lesions in the bowel wall, in particular caecal abscesses, are the lesions which when present predispose to a high potential of infectivity in the shed faeces. / Science, Faculty of / Zoology, Department of / Graduate