The name cow-pox, or vaccinia, is employed to describe a special disease which in animals of the bovine species is characterised by the development of pustules at points where the skin is fine, and more particularly the mammary region.
It can be conveyed both to man and the domestic animals.
This disease has been known from time immemorial, and it would appear that first of all in the East and later in England it was a general belief that its attacks rendered human beings proof against small-pox. Medical men, it must be admitted, long regarded this belief as a popular delusion, as is proved by their continuing to practise inoculation with true small-pox material.
Jenner in 1770 was the first to declare the truth of this popular opinion, and by his wise foresight to confer on humanity one of the most beneficent discoveries ever made, although the weight of modern opinion is in favour of the identity of cow-pox and human variola. Having observed that milkmaids who happened to have small cuts or sores about the hands sometimes contracted the disease in a mild form, and that they did not afterwards suffer from small-pox, he was struck with the advantages consequent on such a discovery, and having proved the possibility of inoculating human beings artificially, he immediately formulated the principles of vaccination. A child eight years of age was vaccinated with cow-pox, and afterwards inoculated with pus from a small-pox patient. It contracted vaccinia in consequence of the first inoculation, but entirely resisted the attempt to inoculate it with small-pox. Vaccination had been discovered.
Jenner furthermore proved that cow-pox was transmissible from cow to cow and from man to man, but it seemed to him that the original disease was to be sought elsewhere, and that the pustular affection originated primarily with the horse. The horse is sometimes the subject of a pustular disease called horse-pox; this disease when inoculated in man confers immunity against small-pox, just as does cow-pox, and Jenner believed that the disease did not attack cows unless they had been accidentally inoculated through the medium of the people about the farm. Unfortunately, he named the pustular disease of the horse which he had studied “sore heels,” and for a long time all those who busied themselves with the question of vaccine confounded “sore heels” with a number of different diseases, although as early as 1802 Loy had experimentally proved that so-called “grease” (in reality horse-pox) was transmissible by inoculation to the cow, in which it produced cow-pox.
Loy’s “grease” and Jenner’s “sore heels” only represent forms of horse-pox, but for more than fifty years the origin of vaccine was sought in grease, lymphangitis, and other diseases which attack the extremities of horses’ limbs. Pételard (1845–1868) rediscovered and redescribed horse-pox and proved its transmissibility to man; Lafosse and U. Leblanc discovered it in an epizooty which broke out at Rieumes; and Bouley in 1862 furnished a synthetical description of it under the designation of horse-pox. He shows that horse-pox is always a pustular disease, but that it may sometimes appear in the form of a discrete eruption around the lips and nostrils, sometimes of an eruption limited to the pasterns or extremities of the limbs when inoculation has been effected in this region, sometimes of lymphangitis, and sometimes of a more or less confluent and generalised eruption.
Symptoms. The disease as discovered and described by Jenner was soon rediscovered and redescribed on all sides—by Sacco in Italy, Hering in Germany, etc.
The pustular eruption usually appears on the udder in the case of cows, and on the muzzle, nose, and lips in that of calves. In exceptional cases the eruption may become generalised.
The pustules are round or slightly elliptical, and are preceded by the appearance of red congested patches, followed by infiltration and thickening of the skin.
The pustule is moderately prominent, and after some days there is exudation at its centre, transforming it into a vesico-pustule. The exuded liquid collects under the thickened layer of epidermis, which it raises, and on examination it appears as a white or transparent little central patch, with a thin grey periphery surrounded by a reddish inflammatory zone. This liquid becomes thicker and the pustule is flattened at its centre, then, towards the eighth or ninth day, the pustule is ruptured, owing to tearing of the epidermic patch. The vaccine thus escapes.