536 DISEASES OF THE HORSE. 
body, but chiefly, and often exclusively, upon the pasterns and fet- 
locks. The eruption may commence upon the lips, or about the nos- 
trils or eyes. 
This disease was described by the early Roman agricultural writers 
and by the veterinarians of the eighteenth century. It received its 
first important notice from the great Jenner, who confounded it with 
grease in horses, since animals with this disease are very liable to have 
the eruption of variola appear on the fetlocks. He saw these cases 
transmit the disease to cattle in the byres and to the stablemen and 
milkmaids who attended them, and furnish the latter with immunity 
from smallpox, which led to the discovery of vaccination. Horsepox 
is also frequently mistaken fcr the exanthemata attending some 
forms of venereal disease in horses. 
Varicla in the horse, while it is identical in principle, general 
course, complications, and lesions with variola in other animals, is a 
disease of the horse itself, and is not transmissible in the form of 
variola to any other animal; nor is the variola of any other animal 
transmissible to the horse. Cattle and men, if inoculated from a case 
of horsepox, develop vaccinia, but vaccinia from the latter animals 
is not so readily reinoculated into the horse with success. If it does 
develop, it produces the original disease. 
Causes.—The direct cause of horsepox is infection. A large num- 
ber of predisposing causes favor the development of the disease, as 
in the case of strangles, and this trouble, like almost all contagious 
diseases, renders the animal which has had one attack immune. The 
chief predisposing cause is youthfulness. Old horses which have not 
been affected are less liable to become infected when exposed than 
younger ones. The exposure incident to shipment, through public. 
stables, cars, etc., acts as a predisposing cause, as in the other infec- 
tious diseases. The period of final dentition is a time which renders 
it peculiarly susceptible. 
Dupaul states that the infection is transmissible through the at- 
mosphere for several hundred yards. The more common means of 
contagion is by direct contact or by means of fomites. Feed boxes 
and bridles previously used by horses affected with variola are prob- 
ably the most frequent carriers of the virus, and we find the lesions in 
the majority of cases developed in the neighborhood of the lips and 
nostrils. Coition is a frequent cause. A stallion suffering from this 
disease may be the cause of a considerable epizootic, as he transmits 
it to a number of brood mares and they in turn return to the farms 
where they are surrounded by young animals to which they convey 
the contagion. The saddle and croup straps are frequent agents of 
infection. The presence of a wound greatly favors the inoculation of 
the disease, which is also sometimes carried by surgical instruments 
or sponges. Trasbot recites a case in which a set of hobbles, which 
