By inoculating chickens intranasally with a collection of strains of infectious bronchitis virus (IBV) of the Massachusetts serotype and of of different serotypes, a pool of viral and bacterial strains was selected which, on inoculation, consistently produced a highly lethal disease closely resembling the natural disease produced by these two organisms. The conditions for reproducing the experimental disease were not rigorous in that, within broad limits, the size of the viral and bacterial inocula were not important; neither were the times at which both organisms were administered in relation to each other. The breed or strain of chicken used was important and the resistance of chickens to fatal infection increased with age. When the strains of the pool were inoculated intranasally without the IBV component, the chickens remained well; bacteriological examination of chickens inoculated with one of the strains, O18, revealed little evidence of invasion of the tissues or even of persistence of the inoculated strain in the upper respiratory tract. A minority of the IBV strains examined were lethal for chickens when inoculated without but many of them only produced a substantial mortality when the were included in the inoculum; IBV strains in this latter category included the vaccine strains H52 and H120. High concentrations of IBV strain M41 and O18 persisted in the upper respiratory tract for a number of days after they had been inoculated together. Much lower concentrations of IBV M41 were found in the internal organs, such as the spleen; O18 was only found in these sites in some of the inoculated chickens. Coliform organisms proliferated in the upper respiratory tract of chickens inoculated with IBV alone; they were rarely found in their internal organs.


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