VIII. B. 1 Ashburn, Vedder, Gentry: Variola and Vaccinia 21 



The heifer being more resistant to vaccine virus, it foUov^^s 

 that virus growing on the heifer must be able to withstand the 

 greater resistance there offered, only the more vigorous and 

 counter-resistant virus will survive, and by that process of selec- 

 tion the virulence of the strain becomes exalted, a conclusion 

 supported by the following observations: 



Brinckerhoff and Tyzzer(i8) found that vaccinia more 

 thoroughly protected monkeys from subsequent vaccination than 

 did a previous attack of variola inoculata, although both pro- 

 tected against later variola inoculata. They quote Roger and 

 Wiel as having made similar observations. 



Dupont(l9) found 22 per cent of 2,601 Sudanese, who had 

 had variola vera or variola inoculata, still susceptible to vac- 

 cinia, although he says that the immunity of these people to 

 smallpox is usually complete and permanent. 



Schamberg(20) quotes Martin as having obtained 35 per cent 

 of successful vaccinations with old, long humanized virus, and 

 80 per cent with animal virus and early human removes. 



It is stated (2) that strains of vaccine passed from man to 

 man, in China, for a hundred years, are almost inert. How- 

 ever, Immermann says that bovine lymph also deteriorates 

 when passed exclusively from bovine to bovine host. Still, that 

 does not affect the fact that the pock-forming element of small- 

 pox virus first becomes exalted by passage through cattle. 



Under our hypothesis there is another element in that virus, 

 apparently one without which it cannot produce a contagious, 

 generalized, mortal disease with a distinct preeruptive phase and 

 initial rashes. This element dies or is otherwise eliminated by 

 the animal passages, but such elimination does not always occur 

 on the first passage. 



Copeman and Immermann both discuss this matter rather 

 fully, and agree that smallpox occasionally may be transmitted 

 from cattle to man after two or three passages. 



Brinckerhoff and Magrath(2i) state that they carried a strain 

 of variola virus from man to monkey, thence through 4 gen- 

 erations on rabbits' corneas, and then back to a monkey, causing 

 smallpox, with the formation of a propustule and a secondary 

 eruption. We know of no instance of smallpox production at 

 a later remove than this from the human case. A single bovine 

 passage may suffice to change the virus to vaccine. 



It may be inferred, we think, that although the whole disease, 

 smallpox, does not occur in cattle, the whole virus may live in 

 them for a time, the separation of elements not always occur- 

 ring suddenly. 



