PROBLEMS CONCERNING THE TUMOR VIRUSES 311 



indicate that this tumor virus has solved the problem of transmission from 

 generation to generation and this gives it better assurance for survival. Of the 

 known tumor viruses, none has yielded easily to the epidemiological approach 

 aimed at prevention of the disease. 



Efforts to elucidate the contagious nature of this virus have contributed 

 another generalization which is of importance to those interested in the 

 control of cancer. As the work progressed it became clear that the exposure 

 of a virus as the causative agent of this tumor was no assurance that the 

 disease would yield readily to preventive measures. Discovery that the virus 

 was egg-borne showed why the usual procedures for the prevention of contact 

 infection were almost hopeless, and suggested as control measures the 

 acquisition of stocks of chickens which were highly resistant to the virus, 

 or the almost impractical procedure of maintaining chickens in virus-free 

 quarters. Indeed, it now appears that the more practical approach to control 

 resides in the discoveries of antibodies in eggs and susceptibility of newly 

 hatched chicks. These leads can be exploited by immunizing the hen before 

 the eggs are laid, in hopes that the chicks will be protected from infection 

 during the brief and critical period after they are hatched. 



Work with the virus of visceral lymphomatosis has made another contribu- 

 tion to the basic knowledge of tumor viruses. This stems from its established 

 relationship to other viruses of the fowl leukosis complex. All who review this 

 interesting problem emphasize the similarities and dissimilarities of various 

 diseases of the complex and of the viral agents implicated in their causations. 

 It is the consensus that while each disease and virus possess distinct properties, 

 all the diseases, as well as all the viruses, possess undeniable relationships. 

 Another review of the investigation leading to this generalization is not 

 essential to the present discussion, for this was done in an excellent summa- 

 tion by Oberling and Guerin (1954). Instead, a brief account of the recent 

 publications dealing with the subject will serve to illustrate several avenues 

 of approach and the attending results. 



Beard's (1957) review of the contributions from his laboratory deals for 

 the most part with two diseases of the complex: erythroblastosis, character- 

 ized by the presence of large numbers of erythroblasts, and myeloblastosis, by 

 large numbers of myeloblasts, in the circulation. The viruses of both diseases 

 were found in blood plasma and, with this as the point of departure, a series 

 of investigations contributed significantly to knowledge of the biological, 

 physical, and chemical properties of the viruses. The blood plasma from 

 individual chickens showed a wide range in the number of virus particles and 

 members of the most susceptible stock of chickens varied widely in their 

 degree of susceptibility to the two viruses. Fewer erythroblastosis than 

 myeloblastosis particles were needed to induce disease and the erythro- 

 blastosis virus elicited symptoms and killed its hosts earlier. Three-day-old 



