MASKING 87 



would constitute the analogue of the lysogenic carrier strain in being 

 the healthy carrier of Snotsiekte virus which it transmits by contact as 

 a serious disease-producing agent to domestic cattle, the analogue of the 

 indicator organism. 



There are lots of other viruses which survive from one periodic out- 

 break to the next, but unfortunately our knowledge of how they survive 

 and where they hide out during the time that they are not causing such 

 diseases as hog cholera, poliomyelitis, measles, cattle plague, foot-and- 

 mouth disease, etc., is not as complete as in the instances that I have 

 just outlined. 



The examples that were cited above embody only two mechanisms 

 for interepidemic survival, namely, persistence of the causative virus in 

 a worm intermediate host and persistence of the virus in a species in 

 which it causes either no or only mild symptoms. Both of these mech- 

 anisms are admirably suited to insure the prolonged survival of viruses 

 and to perpetuate the diseases they cause from the period of mass 

 immunization at the end of one epidemic to the period of mass suscep- 

 tibility signaling the onset of the next epidemic. It would be strange if 

 nature abandoned these mechanisms for perpetuating virus infections 

 after only a few applications, and it would seem quite reasonable that 

 some, if not many, of the viruses with whose whereabouts between epi- 

 demics we are not as yet familiar might fall within one or the other of 

 the two mechanisms outlined. 



3. Transformation of Viruses 



Though frequently suspected of taking place under natural condi- 

 tions, there appears to be but one experimentally reproducible example 

 of transformation in the animal virus field. This is the changing of 

 rabbit fibroma virus to that of infectious myxoma — the Berry-Dedrick 

 phenomenon. This phenomenon appears superficially to be something 

 of a mixture of "recombination" and "multiplicity reactivation" as these 

 terms are applied in the bacteriophage field and should therefore be a 

 good subject to present for discussion at this symposium. Since some of 

 you may not be completely familiar with the background of the Berry- 

 Dedrick transformation, I shall review it briefly. 



The two viruses involved in the reaction are the myxoma and the 

 fibroma viruses, both of rabbit origin. The myxoma virus appeared for 

 the first time in Brazil and has not occurred as a cause of natural dis- 

 ease in domestic rabbits outside of that country except for its introduc- 

 tion from there into California rabbitries. The disease that this virus 

 causes in domestic rabbits is highly fatal and readily contagious. 



