Comparative morphological, histological, and biological studies suggest a close relationship between the meningopneumonitis virus of Francis and Magill and a virus recovered from thiamin-deficient pigeons. Both of these viruses are morphologically identical with typical psittacosis, and it seems probable that they are biologically modified strains of psittacosis. They both differ from typical psittacosis in that they are regularly more pathogenic for the pigeon after intracranial injection, and fail to produce hepatic necrosis after intraperitoneal injection in mice. A virus recently isolated from human cases of atypical pneumonia by Eaton, Beck, and Pearson may also be closely related to these two viruses.

A number of psittacosis viruses of pigeon origin showed a similarly increased pathogenicity for pigeons by the intracerebral route, as compared with psittacosis viruses of parrot origin. The viruses of parrot origin, however, commonly produced latent infection in pigeons even when clinical illness was not evidenced.

For the isolation of psittacosis of pigeon origin from human sputum the intracranial injection of mice or pigeons may be essential, although it is probable that the intranasal injection of mice would be successful. The intraperitoneal injection of mice may give negative results.

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