336 Veterinary Medimie. 



the habitual source of cowpox. Sacco recognized it at the begin- 

 ning of the 19th century, Hertwig in Berlin in 1830, Roll in 

 Vienna in 1855, and Bouley and others later in different parts of 

 Europe. 



Animals Susceptible. Variola in some form affects man, sheep, 

 cattle, horses, goats, pigs, dogs, buffaloes, camels and monkeys. 



Microbiology and Infection. It has long been well established 

 that variola is due to contagion alone. The habitual dread of 

 contact with a smallpox patient, shows the general appreciation 

 of the danger of contagion, and the many epidemics, started 

 by the introduction of a smallpox patient and thereafter 

 spreading from that as a centre, together with the long continued 

 immunity of certain insular or trans-oceanic countries illustrate 

 this. One of the most striking examples is the immemorial im- 

 munity of the New World until the landing of the variolous slave 

 in Mexico in 1520, and the immediate, rapid and destructive 

 spread of the disease among the native tribes. Sheeppox offers 

 a no less .striking example. Prevailing for centuries in Asia and 

 Europe, its extension to a new district was always the manifest 

 result of the movement of infected sheep ; England remained 

 immune until her first invasion in 1847, and the second in 1862, 

 in both cases the source was easily traced , and the disease com- 

 pletely extinguished by the destruction of the infectioti in its 

 circumscribed area ; the more distant sheep raising countries, 

 America, North and South, Australia, Tasmania, New Zealand, 

 South Africa, in the absence of importation of infected sheep re- 

 main free to the present time. For horsepox and cowpox the 

 demonstration is more difficult as limited outbreaks, have occurred 

 at intervals in different localities, traceable more or less clearly to 

 infection from vaccinated persons, yet often mistakenly attributed 

 to spontaneous developments of the disease. Before the days of 

 Jenner however it prevailed habitually in certain dairying dis- 

 tricts (Gloucestershire), and I can point to localities in New 

 York, in which the infection is manifestly laid up in the stables, 

 and the disease develops yearly in the heifers coming into milk 

 for the first time and in newly purchased cows, that have not 

 been previously exposed. 



The contagion varies greatly in force in the different forms of 

 variola, the milder horsepox or cowpox, requiring actual contact 



