Papillomas occur frequently on the oral mucosa of domestic rabbits procured in the metropolitan area of New York. They are small and benign, and are situated mostly on the under side of the tongue. A filtrable virus can be extracted from them with which growths can be reproduced in the oral mucosa of several species of rabbits and hares but which fails to cause lesions when inoculated into other rabbit tissues and into the oral mucosa of other species. The virus differs notably from the Shope virus, which causes cutaneous papillomas in rabbits but proves innocuous to oral mucosa: rabbits solidly immune to the oral papilloma virus are fully susceptible to the Shope virus and vice versa.
The oral papillomas are not highly contagious, for susceptible animals kept in individual cages in the same rooms with others carrying the growths, fed the same kind of food, and cared for by the same attendants, do not "catch" them. They are found much more frequently in the offspring of dams that carry the growths than in those of mothers free from them, and the causative virus can be recovered from the mouth washings of rabbits having no growths. The observations indicate that the virus may be spread by transfer from the mother to the young during the period of suckling, and that it may lie latent in the mouth, doing no harm unless the mucous membrane is injured. The slight trauma occurring now and then when coarse foods are chewed may furnish the required tissue nidus under natural conditions, for papillomas occasionally appear after virus has been dropped into the mouths of uninoculated rabbits; but the more extensive injury and healing resulting from experimental tattoo inoculations proves regularly effective in this respect. Tar can also act as an efficient adjuvant to the virus, the incidence of "spontaneous" oral papillomas being much higher in domestic rabbits that had had the opportunity to lick tar from their ears and paws during long periods than in normal control animals. The virus is recoverable in quantity from the oral papillomas of tarred domestic rabbits, and the findings indicate that it is their essential cause, the tar acting merely to prepare the tissue for the virus' action. For the same tar does not elicit oral papillomas in wild cottontail rabbits, which do not carry the causative virus though fully susceptible to it.
The implications of the findings are discussed.