MASKING 85 



been successful in demonstrating the presence of "masked" papilloma 

 virus in domestic rabbit timiors have failed to shed light in the case of 

 the influenza virus in its lungworm intermediate host. Thus far "un- 

 masking" of the virus has been effected only by the rather complicated 

 procedure of transmitting the infected lungworms to susceptible swine 

 and there provoking the virus to activity in vivo. Fully infective virus is 

 detectable only at either end of the procedure. Stanley, a long time ago, 

 compared the phenomenon very aptly with that of a train passing 

 through a tunnel: One can see the train as it enters and as it leaves, 

 but it is no more apparent while in the tunnel than is swine influenza 

 virus while in its intermediate host. Incidentally this same tunnel 

 simile could be just as well applied in the case of bacteriophage infec- 

 tions of susceptible bacteria in which the initial virus disappears only 

 to reappear later as the mature virus. 



2 . Inter epidemic Survival of Animal Viruses 



a. Swine influenza 



The preceding account of the "masking" of swine influenza virus 

 in its lungworm intermediate host fairly well explains the mechanism 

 by which swine influenza virus survives in nature from one outbreak 

 to the next. Field experiments have shown that the phenomenon is not 

 an isolated laboratory trick, but actually does take place on a large scale 

 in nature. Lungworm larvae from earthworms dug in the pig pastures 

 on numerous Midwestern farms where influenza is of annual occurrence 

 have been shown experimentally to be carriers of "masked" influenza 

 virus (7). 



b. Salmon poisoning 



Salmon poisoning in dogs is another infectious disease of probable 

 viral origin in which the causative virus hides out in a reservoir worm 

 host between its appearances as the cause of sporadic disease in dogs 

 (8). In this case, however, the worm is a trematode having a fish and a 

 snail as intermediates in its life cycle. Here again the virus appears to 

 be "masked" during at least a part of the cycle in its intermediate host 

 since it has never been demonstrated by direct means during the part 

 of its life history that it spends in the snail, 



c. Bovine pseudorabies 



Bovine pseudorabies, a virus disease of cattle, tides over from one 

 appearance to the next in a rather interesting fashion. The disease 

 caused in cattle by this virus is not contagious, and so far as anyone 

 knows, never transmits directly from one sick cow to a normal one. The 



