88 SHOPE 



The fibroma virus was first isolated from a timior in a cottontail 

 rabbit shot near Princeton Junction, New Jersey. Sporadic cases of 

 fibroma virus infection have since been observed in various scattered 

 regions of New Jersey, in northern New York State, and in some areas 

 of the Middle West. It is not a commonly recognized condition in wild 

 rabbits. The virus is readily transmissible by inoculation to both cotton- 

 tail and domestic rabbits but is not contagious in either species. The 

 condition is never fatal and causes only benign fibromatous tumors at 

 sites of inoculation in either species. 



From the above facts it appeared that the two viruses and the dis- 

 eases they caused were quite different and distinct; myxoma, a disease 

 apparently native to domestic rabbits was contagious and regularly 

 fatal, while fibroma, a disease seemingly native to cottontail rabbits, 

 was not contagious and never fatal. The two conditions did have one 

 point of similarity, however, and this was the frequent superficial re- 

 semblance of the initial lesion, developing at the site of inoculation with 

 myxoma virus, to the early fibromas developing at the sites of inocula- 

 tion with fibroma virus. It was this similarity between the initial local 

 lesions of the two conditions that early led to an immunological com- 

 parison of the two viruses and the discovery that infection of rabbits 

 with fibroma virus rendered them immune to fibroma and unusually 

 refractory to myxoma. Rabbits that had recovered from fibromatosis 

 and that were subsequently inoculated with myxoma virus came down 

 with a myxomatosis showing less manifestations of generalization than 

 usual and furthermore such animals almost never died (12). It ap- 

 peared from this that a previous fibroma virus infection, though not 

 immunizing rabbits against myxoma virus, did give sufficient protection 

 to prevent a fatal outcome. This state of increased resistance to myx- 

 oma virus induced by fibroma infection began very early and at some 

 time between the second and fourth day was frequently demonstrable. 

 At no time, however, did antibodies capable of neutralizing myxoma 

 virus appear in the blood serum of fibroma recovered animals, though 

 fibroma virus-neutralizing antibodies were readily demonstrable. The 

 immune response in the reverse direction was more complete and the 

 very rare rabbit that survived a primary myxoma virus infection was 

 not only resistant to infection both with the myxoma and the fibroma 

 viruses, but also possessed antibodies in its blood serum that were capa- 

 ble of neutraUzing both viruses. 



The immunological and serological data obtained in the cross-pro- 

 tection studies outlined briefly above indicated that the myxoma virus 

 contained antigenic components essential to the production of complete 

 fibroma virus immunity. On this basis, the incomplete protection of 



