PROBLEMS CONCERNING THE TUMOR VIRUSES 309 



the size of the animals; the incidence of resultant tumors was 95 % in 2-day- 

 old hosts but only 31 % in 114-day-old animals. It was also established that 

 chicks were far more susceptible than adults when exposed to natural 

 infection through contact with infected birds. Of paramount importance to 

 the problems of tumor viruses was the finding that infected birds did not 

 develop the disease until months after they had acquired the virus and many 

 lived their life spans without revealing signs of the disease. The ability to 

 parasitize without producing overt disease is also a characteristic of other 

 tumor viruses. 



Genetic factors were found to be of prime importance in determining the 

 host's degree of susceptibility to the virus of visceral lymphomatosis. Waters 

 (1945, 1951) has developed inbred lines of chickens showing pronounced 

 variations in their susceptibilities to the virus when it is transmitted under 

 natural or artificial conditions. The importance of the host's genetic con- 

 stitution in problems of the tumor viruses will be emphasized throughout 

 this chapter. 



Efforts to acquire knowledge of the epidemiological factors concerned with 

 the spread of the disease have supplied further information of host-virus 

 relationships as well as a fascinating story in virus transmission (Burmester, 

 1957). Exposure of normal chicks to infected chicks resulted in the former 

 showing a high incidence of the disease, and this was explained by detecting 

 the virus in the debris of incubators. Succeeding experiments revealed the 

 virus in feces and oral washings and showed that the saliva of chicks became 

 highly infectious soon after exposure to the virus. These findings established 

 the routes of virus transmission as contaminated food and water when the 

 young birds were kept in close contact in crowded pens. 



Meanwhile, some workers had observed the occurrence of the disease in 

 chicks hatched from eggs in areas where chickens had never been kept and in 

 others hatched in germ-free incubators. Such observations strongly suggested 

 transmission of the virus through the egg and direct evidence to this end was 

 obtained when the virus was found in livers from chick embryos. Egg 

 transmission has been studied intensively and, among other findings, it was 

 shown that the virus is more prevalent in eggs laid by hens from an inbred 

 stock susceptible to the disease than by hens belonging to a resistant stock. 

 Further, more virus was in the eggs laid by hens one year of age than in those 

 laid by the same hens one year later. 



Egg transmission was used to further knowledge of host-virus relationships. 

 Progeny were hatched from eggs of birds known to shed virus in their eggs 

 and these progeny also laid eggs which contained the virus. Throughout the 

 experiments the hens remained in good health during the egg-laying period 

 and chicks from infected eggs developed normally. Thus, the virus persisted 

 in hen, egg, embryo, and chick without exerting any untoward effect upon 



