MASKING 81 



the papilloma has all of the earmarks of a virus-induced tumor and is 

 analogous in this respect to the chicken tumors of the Rous type. No 

 investigator would seriously doubt causal relationship of the papilloma 

 virus to the epithehal tumors which follow its inoculation into cottontail 

 rabbits. 



The case of the domestic rabbit, however, presents quite another set 

 of circumstances. As mentioned previously, if papilloma virus of 

 cottontail rabbit origin is applied to the scarified skin of domestic rab- 

 bits, papillomas result just as they do in the wild rabbit and their general 

 appearance is the same. However, if one attempts to transmit the virus 

 serially beyond the first domestic rabbit passage, one makes the rather 

 startling discovery that the domestic rabbit papillomas contain no virus 

 demonstrable by direct means. Rabbits, either cottontail or domestic, 

 inoculated in the usual fashion with suspensions of domestic rabbit 

 papillomas fail to develop papillomas or exhibit any evidence of infec- 

 tion. The situation in the domestic rabbit is therefore a very puzzling 

 one. The evidence that the domestic rabbit papilloma has a virus as its 

 cause is about as direct as could be hoped for — the wild rabbit virus 

 was placed on the scarified domestic rabbit skin and the tumors devel- 

 oped at the sites where the virus was deposited. However, if an investi- 

 gator unfamiliar with the method used in inducing the warts were 

 presented with a domestic rabbit suffering full blown papillomatosis 

 and were told to determine the etiology of the condition, he would be 

 completely unable to demonstrate that it had a viral cause. He would 

 almost certainly characterize the tumor as a mammalian neoplasm of 

 nonviral etiology. As such, the condition can be transmitted in series 

 to other domestic rabbits by tumor grafting in a manner similar to that 

 employed in working with nonviral mammahan tumors. 



Thus wdth a single clinical entity (papillomatosis), in two species 

 of animals, we have examples of a condition which in one species, the 

 cottontail, is a typical readily transmissible virus tumor and in the 

 other species, the domestic rabbit, is apparently a nonviral tmnor typical 

 of this class of mammalian neoplasms. Fortunately, for the sake of com- 

 pleteness of the scientific record and with wild rabbit virus available 

 as a test reagent, there are indirect means of demonstrating that pap- 

 illoma virus is present in the domestic rabbit tumors even when these 

 become malignant and progress to cancer. These methods are immuno- 

 logical and, briefly, make use of the virus neutralization test or the 

 demonstration of active immunity. Thus a domestic rabbit afflicted 

 with papillomatosis will have antibodies in its blood serum capable of 

 completely neutrahzing papilloma vims of cottontail origin. Such a 



