158 VARIOLA EQUINA — HORSE POX. 



diffusible infecting agent, its transmission, mode of access to 

 the system, and its extension amongst a number of animals, or 

 over a considerable district of country, can only be accounted 

 for by the contact, mediate or immediate, of the actually 

 diseased with the healthy. No doubt many of the outbreaks 

 of the disease which have been chronicled, being exceedingly 

 difficult, or it may be at the time of their investigation im- 

 possible to trace to their source, have been regarded as of 

 spontaneous origin. This, however, judging from analogy, is 

 improbable, and would, to be received, require very convinc- 

 ing evidence. 



In the transmission of the virus of this disease, where the 

 specific lesions are situated on the head and extremities — parts 

 of the animal most constantly in contact with fodder or litter 

 — we may comprehend how readily a healthy animal, when 

 placed where the diseased have j^reviously been located, will 

 become contaminated. Placed in direct contact, either during 

 work, in the stable, or at pasture, there is little difficulty in 

 the transmission of the disease. The infecting material, most 

 probably the saliva, having mingled with it, the lymph from the 

 vesicles or the secretion from the sores on the limbs may be 

 brought in contact with and placed upon the mucous mem- 

 branes, or on some abraded cutaneous surface frequently exist- 

 ing on the parts most susceptible of receiving the inoculation, 

 the heels and fetlocks. 



That equine variola may be transmitted to the horse and 

 other animals by artificial inoculation, experimental investiga- 

 tion abundantly proves. It also proves that the susceptibility 

 to the action of the virus of variola when inoculated varies in the 

 different species, being greatest in the horse and less in cattle 

 and sheep ; it also indicates that the individual susceptibility 

 of different animals of the same species varies considerably. 

 Further, we seem to be taught from experimentation that the 

 virus loses its power after passing through the systems of 

 several animals. Whether the passing through one attack of 

 the disease exempts the horse from a successful reinoculation, 

 does not from experimentation or observation amongst the 

 diseased appear to be determined. 



Diagnosis. — When considering the subject of glanders and 

 farcy, attention was directed to this disease as one likely to be 



