270 AJSTNTJAL REPORT SMITHSONIAN INSTITUTIOrN', 1942 



temper, influenza, and others. It is now being widely employed in 

 the case of equine encephalomyelitis virus. In the second method, 

 the production of a strain which will cause an innocuous disease 

 but which Avill immunize against virulent strains is most important. 

 In the third method, the production of an inactivated virus which 

 will, upon injection, immunize against active virus is most important. 

 It seems likely that many of the failures to achieve the latter result 

 have been due to the use of methods of inactivation which cause too 

 great a change in the structure of the virus. In our studies of the 

 viruses which we have purified, we have found that the same pro- 

 cedure which will inactivate a given virus without causing wide- 

 spread loss of structure and loss of characteristic antigenic proper- 

 ties will cause the complete disintegration and loss of characteristic 

 antigenic properties in another virus. It is obvious, therefore, that 

 it is of extreme importance that viruses be obtained in purified form 

 and studied so that for each virus a method may be evolved for in- 

 activating the virus without destroying its immunizing potency. The 

 change in structure that results in inactivation need not necessarily 

 be very great, and it may even be reversible. For example, we have 

 found that the inactivation resulting from the addition of formalde- 

 hyde to tobacco mosaic virus may be reversed and active virus once 

 again prepared from the inactive material. The fact that this 

 reaction may be reversed is some indication that a major change in 

 structure is not involved. 



Closely related to the general problem of inactivating viruses with 

 the least possible change in structure are the studies on ways and means 

 of producing the less virulent strains of a virus which are so important 

 in connection with the second general method of immunization. So 

 far, the production of less virulent strains which have proved of great 

 practical importance has been achieved by the simple expedient of 

 passing the virus through another host. Thus, a strain of virus which 

 will protect against smallpox may be obtained by infecting a calf with 

 smallpox virus and reisolating the virus produced after several pas- 

 sages in calves. A useful strain of yellow fever virus was secured in 

 a similar manner by Theiler by passage through mouse brains. The 

 change in enviroimient during the production of virus in the second 

 host apparently results in the production or selection of a strain of 

 virus which is much less virulent in the first host. Practically nothing 

 is known as to why a less virulent strain is prepared or of the change 

 in structure which must be involved. However, it does not appear un- 

 reasonable to expect that definite chemical reactions which result in a 

 change in the structure of a given virus without causing inactivation 

 will achieve the same result and yield strains of the virus, some of 

 which may cause a less virulent disease and be useful for immuniza- 

 tion against virulent strains. Leaves diseased with different strains 



