missible 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 number 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 infectious diseases.
The period of final dentition is a time which renders it peculiarly
susceptible.

Dupaul states that the infection is transmissible through the atmosphere
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 probably 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 had been used on an animal suffering from
variola, were used on a horse for a quittor operation and transmitted
the disease, which developed on the edges of the wound.

_Symptoms._--There is a period of incubation, after an animal has been
exposed, of from five to eight days, during which there is no
appreciable alteration in the health. This period is shorter in summer
than in winter. At the end of this time small nodes develop at the point
of inoculation and the animal becomes feverish. The horse is dull and
dejected, loses i