Thus the disease makes its progress from the Horse (as I conceive) to the nipples of the Cow, and from the Cow to the Human Subject. (Pp. 2 and 6.)
This conception of the origin and progress of the disease was not Jenner’s specially: he shared it with the farmers to whom it was a novelty—
The rise of Cowpox in this country may not have been of very remote date, as the practice of milking Cows might formerly have been in the hands of women only; which I believe is the case now in some other dairy countries; and consequently that the Cows might not in former times have been exposed to the contagious matter brought by the men servants from the heels of Horses. Indeed a knowledge of the source of infection is new in the minds of most of the farmers in this neighbourhood, but has at length produced good consequences; and it seems probable from the precautions they are disposed to adopt, that the appearance of the Cowpox here may either be entirely extinguished or become extremely rare. (P. 56.)
Thus Cowpox was to be extinguished by forbidding milkers to handle Horses’ greasy heels. Jenner himself tried to produce Cowpox in the manner described, but without success—
It is very easy [he wrote] to procure pus from old sores on the heels of Horses. This I have often inserted into scratches made with a lancet on the sound nipples of Cows, and have seen no other effects from it than simple inflammation. (P. 45.)
What was requisite for success, he concluded, was the limpid fluid from the Horse’s heel at an early stage of the disease, and that it should be applied to the Cow’s nipples at a certain season—
The virus from the Horses’ heels is most active at the commencement of the disease, even before it has acquired a pus-like appearance; indeed I am not confident whether this property in the matter does not entirely cease as soon as it is secreted in the form of pus. I am induced to think it does cease, and that it is the thin darkish-looking fluid only, oozing from the newly formed cracks in the heels, similar to what sometimes appears from erysipelatous blisters, which gives the disease. Nor am I certain that the nipples of the Cows are at all times in a state to receive the infection. The appearance of the disease in the spring and the early part of the summer, when they are disposed to be affected with spontaneous eruptions so much more frequently than at other seasons, induces me to think, that the virus from the Horse must be received upon them when they are in this state in order to produce effects. Experiments, however, must determine these points. (P. 45.)
Whilst thus explicit as to what was requisite for the infection of the Cow by the Horse, Jenner did not succeed in producing Cowpox from Horsegrease. He had to write—
The spring of the year 1797, which I intended particularly to have devoted to the completion of this investigation, proved from its dryness remarkably adverse to my wishes. No Cowpox appeared in the neighbourhood; for it most frequently happens that while the farmers’ Horses are exposed to the cold rains of spring their heels become diseased. (P. 44.)
Yet without proof, he argued as if he had proof, saying—