PROBLEMS CONCERNING THE TUMOR VIRUSES 335 



D. Fibroma of Rabbits 



A fibroma of rabbits, also discovered by Shope (1932a), was shown to have 

 a virus as the causative agent and this virus became an important tumor 

 virus when Shope (1932b) found it was related to the virus of rabbit myxo- 

 matosis. The fibroma virus will be discussed briefly because reviews by Smith 

 (1952) and by Oberling and Guerin (1954) contain all essential information, 

 with the exception of recent findings with tissue culture techniques. 



The disease is characterized by the occurrence of subcutaneous masses 

 consisting of cells resembling fibroblasts, interspersed with collagenous 

 fibrils and inflammatory cells. Natural transmission through contact is 

 suspected but not proved; Kilham and Dalmat (1955) were able to demon- 

 strate insect transmission of the disease under laboratory conditions. 



The virus can be transmitted to domestic rabbits from which, in contrast 

 to the virus of rabbit papillomatosis, it can be recovered from the tumor, 

 blood, spleen, and other organs. Tbe resultant tumor is benign and regresses 

 within 4 to 5 weeks in domestic rabbits, but persists longer in wild hosts. The 

 virus is very labile and distinct strains have been procured during the course 

 of experiments designed to ascertain its properties (Smith, 1952). Local 

 irritants accelerate the occurrence of tumors and, when the virus is injected 

 intravenously, it may induce a generalized fibromatosis. Duran-Reynals 

 (1940b) produced a generalized disease resembling myxomatosis in very 

 young rabbits by the administration of a large amount of virus. 



Kilham (1955) discovered a similar disease in squirrels which, when trans- 

 mitted to suckling squirrels, produced generalized lesions but, when trans- 

 ferred to adult squirrels, produced local temporary lesions. Neutralization 

 tests suggested a relationship between the squirrel and rabbit viruses. 



The most interesting property of the virus is its relationship to the virus of 

 rabbit myxomatosis. Animals immune to the fibroma virus are resistant to 

 contact infection with the highly fatal myxoma virus and serological studies 

 with the viruses reveal similar and dissimilar antigens. This suggests that 

 both viruses could have a common ancestor which, it will be recalled, has 

 also been postulated for a variety of chicken tumor viruses. The rabbit and 

 squirrel viruses extend this concept to the inclusion of other species. Evidence 

 at hand, however, permits only the speculation that some tumor viruses are 

 able to produce the natural disease in different species. The occurrence of 

 fibromas in different species and their suggested relationships should 

 encourage further investigation. 



An important and stimulating discovery with the fibroma virus was reported 

 by Berry and Dedrick (1936a,b) who found that the inoculation of domestic 

 rabbits with a mixture of fibroma virus and heat-inactivated myxoma virus 

 produced myxomatosis in the test animals. This indicated that the inactivated 

 myxoma virus contained a substance capable of transforming the fibroma 



