758 MODE OF SPREAD OF INFECTIVE DISEASES. 



Value of the lation differs in different species of animals : for example, 



protective in- . _ . L 



oculation for guinea-pigs and rabbits cannot be made immune by the 

 attenuated anthrax bacilli ; rats often resist inoculation 

 with the virulent material, but do not thereby acquire 

 immunity. In like manner the effect is different in 

 sheep and cattle, the two species of animals chiefly in- 

 oculated in practice. In the case of sheep the protec- 

 tive inoculation appears to be particularly uncertain in 

 its results ; too weak vaccines do not give the necessary 

 guarantee for immunity ; too strong materials, on the 

 other hand, readily cause the death of the animal ; 

 further, the duration of the protection is very short, 

 probably not lasting more than a year; and it has also 

 been shown by Koch that sheep inoculated with strong 

 vaccines are not able to resist natural infection produced 

 by feeding them with anthrax spores. In the case of 

 cattle the result is probably more favourable, because 

 stronger vaccines can be borne, the losses by the inocula- 

 tions themselves are less, and the chances of immunity 

 are better. Nevertheless even here we cannot make any 

 definite statements as to the duration of the protection, 

 nor as to its potency against the natural modes of in- 

 fection. In any case the strong vaccines present a 

 certain amount of danger for the people who have to 

 attend on the animals, and without doubt these protec- 

 tive inoculations can only be looked on as a temporary 

 means to be employed till rational means of prophy- 

 laxis, proper modes of disinfection, change in the pas- 

 ture, care in burying the dead bodies, &c., have been 

 generally introduced. 



Protective in- The protective inoculation which has most recently 

 oculation attracted attention is that employed by Pasteur against 

 Method/' rabies. When the method of attenuating the rabic 

 virus described on page 660 had been ascertained, 

 Pasteur attempted to rentier dogs immune by first in- 

 oculating them with virus which had been dried for the 

 longest time, and which was the weakest ; then two days 

 later less virulent material was used, and so on till 

 finally the virulent virus was employed. When these ex- 

 periments had led to uniform results. Pasteur made pro- 



