156 Veterinary Medicine. 



Youth has its influence, even if it means only that the system 

 that has never before been exposed to the poison, retains all its 

 native susceptibility, and has none of that acquired immunity 

 which comes from a previous exposure to the virus and successful 

 resistance. 



Acquired immunity must of course be reckoned with. After a 

 non- fatal attack this is usually to be relied on for several years or 

 even for the rest of the lifetime, yet it varies with the individual 

 animals, and , under the baleful combination of a specially potent 

 germ and strongly conducive accessory causes, it may be- 

 come worn out in a year. Yet the older horses can always be 

 trusted to show a large measure of this immunity, so that in the 

 absence of extraordinary epizootics it is mainly the young that 

 suffer, and it is only when a country has had no general invasion 

 for a length of time, or when the germ has acquired an unusual 

 pathogenic potency, or when these two conditions conjoin, that the 

 invasion of the equine population becomes universal, as it virtually 

 was in the United States and Canada in 18^2-3. Under other 

 circumstances the germ, temporarily shorn of its power, lingers 

 in city and dealers stables, biding its time until circumstances 

 become more favorable for a new general outbreak. 



Immunity largely explains the comparative mildness of the 

 last cases in any particular locality. The more susceptible ani- 

 mals are attacked first and most severely, while the partially 

 immune ones, which for a time resist, throw off the disease with 

 greater readiness. The explanation has been sought in a lessen- 

 ing potency of the germ, but though this may hold true of some 

 cases, it manifestly does not apply when slight lingering cases 

 only are left in one locality, and'the disease is advancing over the 

 neighboring state with all its original force and vigor. 



Incubation. This appears to vary within certain limits. When 

 during an epizootic a sick horse is brought into a new locality and 

 stable, other cases usually develop in from one to three days. 

 Trasbot gives examples of one day, Salle, Cadeac and others of 

 two, others claim four, seven and even, exceptionally, fifteen 

 days. One reason for an apparently prolonged incubation may 

 be found in the seclusion of the germs in the alimentary canal, so 

 that they escape only when passed with the faeces. The patho- 

 genic potency of individual germs, and the varying susceptibility 

 of the animals exposed must also be taken into account. 



