258 EPIZOOTIC DISEASES. 



to depend on damp, exposure, malaria, improper feeding, and 

 similar local causes. 



The line of demarcation between these two classes of dis- 

 ease is very essential in the present study, and is perhaps 

 more easily established in veterinary than in human medicine. 

 It is to the first class only that the word epizootic is applied. 

 The diseases included in it having originated in some definite 

 region of the globe, spread in all directions without regard 

 either to the breeds of animals, or to soil, climate, and other 

 local influences. Such are the following : The typhoid or 

 enteric fever of cattle, which always spreads from the 

 Eussian steppes; the pleuro-pneumonia of cattle, ever 

 extending from central Europe, though probably traceable 

 to Asia and Africa, in some parts of which it is a very 

 common disease ; the epizootic aphtha, or vesicular murrain, 

 which seems indigenous to Hungary and south-eastern 

 Europe, to which it may first have passed from Asia ; the sheep- 

 pox, or variola ovina, also imported from Asia and Africa, 

 but constant in eastern Europe, and spreading westwards 

 through central Europe. 



All these maladies are contagious. They are mildest, as a 

 rule, where they develope spontaneously, or where they 

 have been, as it were, acclimatized, and are very destruc- 

 tive in their progress beyond such regions. They are 

 most severe when they first break out in a fresh district, 

 and many causes combine to diminish their fatality as they 

 continue. Fresh accessions of virus by renewed importa- 

 tions, and the constant renovation of stock (in consequence 

 of the short lives of our animals, and the rapidity with 

 which they are bred), lead to local exacerbations, the fre- 

 quency and severity of which are in proportion to the extent 

 of imported disease, and the number of susceptible animals in 

 the locality. 



