Wherefore, he argued, many thus affected may fancy themselves secure from Smallpox, supposing they have suffered Cowpox, when they have undergone nothing but Horsegrease; and in the event of incurring Smallpox, would bring discredit on the virtue of true Cowpox. How easy it was to confound the two diseases, he illustrated from the case of William Morris, a servant, aged 32, who applied to him on 2nd April, 1798--
His symptoms and the sores on his hands were so exactly like the Cowpox, that I pronounced he had taken the distemper from milking Cows. He assured me he had not milked a Cow for more than half a year, and that his master’s Cows had nothing the matter with them. I then asked him if his master had a Greasy Horse, which he answered in the affirmative; and further said, that he had constantly dressed him twice a day for the last three weeks or more, and remarked that the smell of his hands was much like that of the Horse’s heels. (P. 97.)
Thus Horsegrease sores so simulated those of Cowpox, or of Horsegrease Cowpox, as to be indistinguishable from them. At this time it was Jenner’s opinion that Horsegrease, _per se_, afforded no protection from Smallpox: it had to pass through the Cow to acquire its sovereign efficacy. The opinion is noteworthy in view of its absolute surrender at a later period, when the virus from the Horse’s heel came to be described by him as “the true and genuine life preserving fluid,” and was used by him for inoculation without any reference to the Cow.
One of the aims of _Further Observations_ was “to enforce the precaution of subduing the inoculated pustule as soon as it had sufficiently produced its effect on the constitution.” True Cowpox, according to Jenner, was a serious affection. “The sores ate into the flesh.” (P. 77.) They were capable of producing violent effects. They were attended with erysipelas. “They closely resembled Smallpox of the confluent sort.” (P. 111.) To prolong such suffering he considered useless, for the virus conferred its protective influence on the constitution as soon as received--
The symptoms which (as in the accidental Cowpox) affect the patient with severity, are entirely secondary, excited by the irritating processes of inflammation and ulceration; and it appears to me this singular virus possesses an irritating quality of a peculiar kind; but as a single Cowpox pustule is all that is necessary to render the variolous virus ineffectual, and as we possess the means of allaying the irritation, should any arise, it becomes of little or no consequence. (P. 110.)