'I have lent,' he said, 'to the expression vaccination an extension that I hope science will consecrate, as a homage to the merit and immense services rendered to humanity by one of the greatest men of England—Jenner.'
Still, while rendering homage to the sentiment which induced Pasteur to efface himself in favour of Jenner, we may be permitted to say that there is no likeness between the two discoveries. Great as was the discovery of Jenner, it was but a chance observation, which had no ulterior development; and for a whole century, medicine has not been able to derive anything from it beyond its actual application, which is the one result achieved. 'Vaccination is vaccination,' an opponent of Pasteur's, who was driven hard, was obliged to say. The opponent found no other answer, and he could not have found any other. The cow-pox is a malady belonging exclusively to a race of animals. Man can only observe it; he cannot produce it. Suppress cow-pox and there will be no more vaccination. In the French discovery, on the contrary, it is the deadly virus itself which serves as a starting point for the vaccine. It is the hand of man which makes the vaccine, and this vaccine may be artificially prepared in the laboratory, in sufficient quantity to supply all needs. What a future is presented to the mind in the thought that the virus and its vaccines are a living species, and that in this species there are all sorts of varieties susceptible of being fixed by artificial cultivation! The genius of Jenner made a discovery, but Pasteur discovered a method of genius.
'This is but a beginning,' said M. Bouley on the day when Pasteur announced these facts to the Academy of Sciences. 'A new doctrine opens itself in medicine, and this doctrine appears to me powerful and luminous. A great future is preparing; I wait for it with the confidence of a believer and with the zeal of an enthusiast.'
THE VACCINE OF SPLENIC FEVER.
We have seen how the facts have been established with regard to the microbe of fowl cholera. Immunity against a virulent disease may be obtained by the influence of a benign malady which is induced by the same microbe, only weakened in virulence. What a future there would be for medicine if this method could be applied to the prevention of all virulent diseases! As splenic fever was at that time being studied in the laboratory of the École Normale, it was upon this fever that the research was first attempted. But the success of this research, said Pasteur, can only be hoped for if the disease is non-recurrent. It is only in this case that inoculation with the weakened microbe can protect from the deadly splenic fever. Unfortunately, human medicine is dumb as regards this question of non-recurrence. The man who is smitten with malignant pustule rarely recovers. If there are any cases of recovery—and there are some authentic ones—he who has so narrowly escaped death could not confidently count upon his chance of protection from the disease in future. In order to acquire such a sense of security he would have to expose himself to experiments of direct inoculation, which he would hardly care to do. Animals alone offer the possibility of solving this problem. Yet it is not to all species of animals that we can have recourse. Every sheep inoculated with splenic fever infection is a sheep lost; but the ox and the cow have more power of resistance. Among them there are frequent cases of cure. An incident occurred which enabled Pasteur to push very far this experimental study.
In 1879 the Minister of Agriculture appointed him to give judgment upon the value of a proposed mode of cure for cows smitten with splenic fever, which had been devised by M. Louvrier, a veterinary surgeon of the Jura. Choosing M. Chamberland as his assistant to watch the application of M. Louvrier's remedy, Pasteur instituted a series of comparative experiments. Some cows were inoculated, two and sometimes four at a time, with the virulent splenic fever virus. Half of these cows were treated by M. Louvrier's method; the other half were left without treatment. A certain number of the cows under M. Louvrier's care resisted the disease, but an equal number of those not under treatment recovered also. The inefficacy of the remedy was demonstrated as well as the cause of the inventor's illusions. But one precious result remained from the trial of this remedy. Pasteur and Chamberland had thus at their disposition several cows which had recovered from splenic fever, and which had experienced in their attack all the worst symptoms. At the places of inoculation enormous swellings were formed, which extended to the limbs, or under the abdomen, and which contained several quarts of watery fluid. The fever had been intense, and at one time death had appeared imminent. When these cows recovered they were reinoculated with great quantities of virulent virus. Not the least trace of disease showed itself, even in cases where the inoculation was performed after an interval of more than a year.
The question was solved; splenic fever, like most of the virulent diseases which it has been possible to study, was non-recurrent. The immunity obtained has a long duration. With that valiant ardour which always urges him on, Pasteur next proposed to examine the vaccine of splenic fever. In view of these new investigations, which would require long and careful labours, and which necessitated a certain amount of medical knowledge, Pasteur associated with himself, in addition to M. Chamberland, a young savant, now a doctor of medicine, M. Roux.
Following the rigorous course of his deductions, Pasteur naturally turned to the oxygen of the air in his attempts to modify the virulence of the splenic microbe. But a difficulty presented itself at the outset. Between this microbe and the microbe of fowl cholera there exists an essential difference. The microbe of fowl cholera, as is the case with a great number of microscopic organisms, reproduces itself only by fission. The parasite of splenic fever, on the contrary, has another mode of generation; it forms spores, nothing analogous to which is found in the microbe of cholera.