CHAPTER IV 

 THE VARIABILITY OF VIRUSES 



1. The Inheritance of Altered Structure 



Viruses are liable to alter in their properties, this being recog- 

 nized as one of their characteristics. The altered property is occasion- 

 ally transmitted to newly produced viruses, a phenomenon analogous 

 to the mutation of organisms, though appearing to occur more fre- 

 quently than the mutation of higher organisms. 



Higher organisms may have acquired the prominent characters 

 through their extremely long history of the struggle for existence. 

 They seem to have elaborated wise mechanisms to maintain the good 

 quality of these prominent characters, whereas, as this is not the case 

 with viruses, viruses appear to change their structure abruptly and 

 extensively with the change of environmental conditions. 



For example, the virus of smallpox, normally infecting only man, 

 undergoes variation to the virus of vaccinia when repeatedly trans- 

 ferred through the skin of rabbits. The virus has lost the capacity 

 for causing smallpox, although antigenically it is still related to the 

 variola virus, immunizing against it. Another variant of interest is 

 a strain of yellow fever developed by passage through tissue cultures. 

 This strain gives rise to excellent immunity to yellow fever and» 

 since it is devoid of all tendencies to cause yellow fever in man, it 

 can be employed as a live virus vaccine of high immunizing potency 

 with safety. In a similar way, rabies virus changes its character 

 when inoculated subcutaneously into a rabbit and passed in succession 

 through a series of rabbits. The virus thus reduced in its virulence 

 for man is likewise used as a vaccine. 



For a virus, such a host change must be a great environmental 

 change, so that viruses may be altered in their properties on continued 

 passages in new host. A phage newly isolated from chicken feces is 

 usually non-specific, being capable of affecting simultaneously several 

 strains of bacteria, but when repeatedly transferred through a certain 

 strain of bacteria, the phage will obtain the property to affect the 

 strain specifically with the vigorous phage production. We (13) were 

 able to reduce the virulence of phage by removing Ca and Mg from 

 culture media of the host bacteria. "Virulence" designates here the 



