8o SHOPE 



plications mentioned previously may nullify any useful comparison 

 between the work with animal viruses that I propose to discuss and 

 observations in the bacterial or plant virus field, I am going to present 

 brief statements on the general subjects of "masking," transformation, 

 and interepidemic survival of animal viruses and hope that they may 

 initiate some mutually interesting discussion. 



1. Virus ''Masking" 



For the sake of the present discussion, the term "masked" virus will 

 be used to indicate a living virus which, for reasons that we do not fully 

 understand, has been rendered noninfective and therefore not directly 

 detectable by any of the tests for infectivity ordinarily used in dem- 

 onstrating its presence. A "masked" virus is one which is known by 

 circumstantial evidence or by a series of indirect tests to be present but 

 which is not of itself directly demonstrable. If, in the case of the bac- 

 teriophage, the time gap between the disappearance of the initial virus 

 and the appearance of the mature virus in an infected cell were a mat- 

 ter of days instead of minutes, the phage during this period might 

 possibly be thought of as "masked" in the sense in which that term 

 can be used in the animal virus field. 



Two very good examples of "masking" of virus in the animal field 

 and the two with which I have had the most personal experience are 

 the cases of the papilloma virus as it exists in the tumors it causes in 

 domestic rabbits and the swine influenza virus as it exists in its inter- 

 mediate host, the swine lungworm. Since these two examples are fairly 

 clear-cut and since each represents a phenomenon which may be of 

 wide general importance, in the tumor field in the case of papilloma 

 virus and in the broad field of epidemiology in the case of the swine 

 influenza virus, they should furnish fit subject matter for this discussion. 



a. "Masked" papilloma virus 



I shall outline the case of the papilloma virus first. Briefly, the 

 situation is as follows: Cottontail rabbits in our Middle Western States 

 have a disease characterized by the occurrence of papillomas (warts) 

 over various parts of their bodies. These warty growths are rich in a 

 virus which when applied to the scarified skin of other cottontail rabbits 

 or of domestic rabbits induces the appearance of papillomas apparently 

 identical to those seen on the naturally infected wild cottontails (i). 

 The papilloma virus is usually readily transmissible in series through 

 cottontail rabbits, and ordinarily one has no difficulty in passing the 

 agent indefinitely from cottontail to cottontail. In this species therefore 



