Such experiments, made before the days of careful antiseptic, or aseptic, laboratory methods, by men who were daily engaged in making vaccinations, cannot be very implicitly relied on, yet the success of Thiele in Central Asia, the early home of variola, may indicate the possibility of a transition, under given eastern conditions, which, to say the least, is exceedingly rare in Western Europe or America.
The experiments of Klein, conducted under modern methods, are more conclusive, and seem to imply the possibility of smallpox passing into cowpox, in the bovine system, under some not yet clearly defined conditions. Until such conditions are sufficiently well known, so that they can be controlled at will, no one can be justified in attempting to produce lymph for vaccination by simply passing smallpox virus through the system of the cow.
It seems important to note one or two instances of the evident transmission of smallpox from man to man through the bovine system.
In 1860, Martin inoculated variolous matter, from a man who had just died of smallpox, on a cow’s udder, and subsequently inoculated about fifty persons from the eruption caused in the cow. Most of those so inoculated had unmistakable smallpox and three died.
Reiter had a very similar experience.
Chauveau (French Commission) inoculated twelve susceptible cattle with smallpox virus and produced, in all but one, small conical (smallpox) papules and vesicles, and in ten of these, on subsequent inoculation with cowpox, six proved immune, three had rudimentary pustules, and one had a distinct cowpox eruption.
A milch cow and two heifers were inoculated with smallpox and cowpox on two sides of the vulva, with the result that each disease appeared in the seat of its inoculation, with its characteristic vesicles, and the two developed side by side. The smallpox vesicles were by inoculation conveyed from ox to ox with steadily decreasing activity. Inoculated from the cow on a child, it caused great hyperthermia, vomiting, one large vesicle like vaccinia and a general eruption like varioloid. Inoculation from this child upon another produced a mixed eruption of cowpox and varioloid. Inoculation from the second child on a bull and heifer produced papular eruption only.
Smallpox virus, inoculated on a horse produced a papular eruption, but failed to affect another horse that had been previously vaccinated. Cowpox virus inoculated on the first horse which had had the papular eruption, caused a second papular eruption (not cowpox). The virus from a vesicle in the first horse caused a similar eruption in another horse, on which it was inoculated. The lymph from the papular eruption led to a similar eruption in cattle, on which it was inoculated, but did not protect against cowpox, subsequently inoculated.
The lymph from the papular eruption in the horse, inoculated on two children, produced fever, vomiting, a general papular (smallpox) eruption, in which a few of the pustules only showed a tendency to umbilication. A child and its mother in the same ward contracted varioloid. A child inoculated from one of the first named children, had six large umbilicated vesicles like cowpox and a general papular (smallpox) eruption. Another child inoculated from the last had six large umbilicated vesicles, and a general papular (smallpox) eruption. From the papular eruption of one of these children a horse and seven cattle were inoculated and in all a varioloid eruption resulted.
The rational conclusion is, that while there is every indication of a primal identity of the two diseases, and indeed of all forms of variola, as shown by a disposition of the virus from one genus, when inoculated upon a totally different genus, to show some indication of the characteristic eruption of the latter, yet the generic type, which comes from the long continued growth in the one class of animal, becomes so fixed, that it cannot be overcome at once, and sometimes apparently not at all, by transferring it to an animal of another class.