VARIOLA—POX IN ANIMALS 475 
Some authors believe that bovine and equine variola are a form of 
human variola modified by the transmission through animals. 
Others entertain the opinion that the diseases are separate and inde- 
pendent. Variola affects cattle, sheep, horses, goats, pigs, dogs, 
buffaloes, camels, monkeys, and man. With the possible exception 
of sheep pox, the infection of one species can be conveyed to another 
by accidental infection or by inoculation. Generally speaking, the 
variola of man and sheep is a serious disease followed by rather high 
mortality while the pox of other species is usually mild. 
Etiology. The cause of variola has not been definitely determined 
other than that it is known to exist in the contents of the nodules and 
vesicles and that under sufficient pressure it passes through certain of 
the porcelain filters, which places it among the ultra-microscopic or 
filterable viruses. The infectiousness of the blood and of the exhaled 
air of affected animals during the fever period is doubtful. The 
secretions and excretions do not seem to contain the virus except where 
they become mixed with the contents of the vesicles. When inocu- 
lated into the skin characteristic pox develops at the point of inocula- 
tion in susceptible individuals and in certain instances results in a 
general eruption. The epithelial layers of the skin and mucous 
membranes are the most favorable places for the propagation of the 
pox virus. It seems to be able to enter direct between the epithelial 
cells and if the virus is injected intravenously or into the trachea it 
finds its way apparently through the blood stream of the epithelial 
tissue where it becomes localized. 
A large number of microérganisms have been described as having 
some relation to the cause of variola. Guarnieri believed the cause 
to be certain enclosures within the epithelial cells. These were called 
Cytorhyctes variolae. “There is a large literature on this subject and 
the conclusions are quite contradictory. Councilman found bodies 
in the lesions of cow pox and termed them C. vaccinae. 
While variola in the different species presents like manifestations 
and while it is believed that their etiological factor is either identical 
or very closely related the disease in each of the different species 
should be briefly mentioned. 
Immunity. Individuals that recover from attacks of variola are 
immune to subsequent attacks for several years. This is true whether 
the infection was naturally or artificially produced. Reciprocal 
immunization in the different forms of pox exists to acertain extent. 
