THE ATTENUATED VIRUS, OR VACCINATION
OF VIRULENT DISEASES.
THE VACCINE OF FOWL CHOLERA.
Among the scourges which afflict humanity there are none greater than virulent diseases. Measles, scarlatina, diphtheria, small-pox, syphilis, splenic fever, yellow fever, camp typhus, the plague of the East—what a terrible enumeration! I pass over some, such as glanders, leprosy, and hydrophobia. The history of these diseases presents extraordinary circumstances. The most strange, assuredly, is that which has been from all time established with a great number of these diseases, that they are non-recurrent. As a general rule, notwithstanding some rare exceptions, man can only have measles, scarlet fever, plague, yellow fever once. What explanation, even hypothetically, can be given of such a fact? Still more difficult is it to explain how vaccination, which is itself a virulent though benign disease, preserves from a more serious malady, the small-pox? Has there ever been a discovery more mysterious in its causes and origin, standing, as it does, alone in the history of medicine, and for more than a century defying all comparison?
After dwelling long on Jenner's discovery this question arose in Pasteur's mind: If contagious maladies do not repeat themselves, why should there not be found for each of them a disease different from them, but having some likeness to them, which, acting upon them as cow-pox does upon small-pox, would have the virtue of a prophylactic? A chance occurrence, one of those chances which not unfrequently occur to those who are steadfastly looking out for them, opened out to Pasteur the way to a discovery which may well be called one of the greatest discoveries of the age.
In causing the microbe of fowl cholera to pass from culture to culture, in an artificial medium, a sufficient number of times to render it impossible that the least trace of the virulent matter from which it originally started should still exist in the last cultivation, Pasteur gave in an absolute manner the proof that infectious microbes are the sole authors of the diseases which correspond to them. This culture may be repeated ten, twenty, a hundred, even a thousand times: in the latest culture the virulence is not extinguished, or even sensibly weakened. But it is a fact worthy of attention that the preservation of the virulence in successive cultures is assured only when no great interval has been allowed to elapse between the cultures. For example, the second culture must be sown twenty-four hours after the first, the third twenty-four hours after the second, the hundredth twenty-four hours after the ninety-ninth, and so on. If a culture is not passed on to the following one until after an interval of several days or several weeks, and particularly if several months have elapsed, a great change may then be observed in the virulence. This change, which generally varies with the duration of the interval, shows itself by the weakening of the power of the contagium.
If the successive cultures of fowl cholera, made at short intervals, have such virulence that ten or twenty inoculated birds perish in the space of twenty-four or forty-eight hours, a culture which has remained, say, for three months in its flask, the mouth of which has been protected from the introduction of all foreign germs by a stopper of cotton wool, which allows nothing but pure air to pass through it—this culture, if used to inoculate twenty fowls, though it may render them more or less ill, does not cause death in any of them. After some days of fever they recover both their appetite and spirits. But if this phenomenon is extraordinary, here is one which is surely in a different sense singular. If after the cure of these twenty birds they are reinoculated with a very virulent virus—that, for instance, which was just now mentioned as capable of killing its hundred per cent. of those inoculated with it, in twenty-four or forty-eight hours—these fowls would perhaps become rather ill, but they would not die. The conclusion is simple; the disease can protect from itself. It has evidently that characteristic of all virulent diseases, that it cannot attack a second time.
However curious it may be, this characteristic is not a thing unknown in pathology. Formerly it was the custom to inoculate with small-pox to preserve from small-pox. Sheep are still inoculated to preserve them from the rot; to protect horned cattle from peripneumonia they are inoculated with the virus of the disease. Fowl cholera offers the same immunity; it is an additional scientific acquisition, but not a novelty in principle.
The great novelty which is the outcome of the preceding facts, and which gives them a distinct place in our knowledge of virulent diseases, is that we have here to do with a disease of which the virulent agent is a microscopic parasite, a living organism cultivated outside of the animal body, and that the attenuation of the virulence is in the power of the experimenter. He creates it, he diminishes it, he does what he wishes with it; and all these variable virulences he obtains from the maximum virulence by manipulation in the laboratory. Looked at in juxtaposition with the great fact of vaccination for small-pox, this weakened microbe, which does not cause death, behaves like a real vaccine relatively to the microbe which kills, producing a malady which may be called benign, since it does not cause death, but is a protection from the same malady in its more deadly form.
But for this enfeebled microbe to be a real vaccine, comparable to that of cow-pox, must it not be fixed, so to speak, in its own variety, so that there should be no necessity for having recourse again to the preparation from which it was originally derived? Jenner, when he had demonstrated that cow-pox vaccination preserved from small-pox, feared for some time that it would be always necessary to have recourse to the cow to procure fresh vaccinating matter. His true discovery consisted in establishing that the cow-pox from the cow could be dispensed with, and that inoculation could be performed from arm to arm. Pasteur made his enfeebled microbe pass from one cultivation to another. What would it become? Would it resume its very active virulence, or would it preserve its moderate virulence?
The virulence remained enfeebled and, we may say, unchanged. This showed it to be a real vaccine. Some veterinary surgeons and farmers, on the announcement of this discovery, applied to Pasteur for a vaccine against the disease which was so disastrous among their poultry. Some trials were made, and all succeeded beyond expectation. To preserve this vaccine it must be secured from contact with the air, the cultures being enclosed in tubes, the extremities of which are sealed by the flame of a blowpipe.
What takes place during that interval of time intentionally placed between two successive cultivations of the cholera microbe—that interval which is employed in effecting the attenuation and producing the vaccine? What is the secret of this result? The agent which intervenes is no other than the oxygen of the air. Here is the proof. If the cultivation of this microbe is carried on in a tube containing very little air, and if the tube is then closed by the flame of a lamp, the microbe, by its development and life, quickly appropriates all the free oxygen contained in the tube, as well as the oxygen dissolved in the liquid. Thus, completely protected from contact with oxygen, the microbe does not become sensibly weakened for months, sometimes even for years.
The oxygen of the air, then, appears to be the cause of modification in the virulence of the microbe.
But how, then, is the absence of influence on the part of the atmospheric oxygen, in the successive cultures which are practised every twenty-four hours, to be explained? There is, in Pasteur's opinion, but one possible explanation; it is that the oxygen of the air in this latter case is solely employed in the life of the microbe. A culture has a duration of some days; in twenty-four hours it is not terminated. The air which comes in contact with it is then entirely employed in nourishing and largely reproducing the microbe. During the longer intervals of culture, the air acts only as a modifier, and at last there arrives a moment when the virulence is so much weakened as to become nil.
This very extraordinary fact is, then, established that the virulence may be entirely gone while yet the microbe lives. The cultures offer the spectacle of a microbe indefinitely cultivable, yet, on the other hand, incapable of living in the bodies of fowls, and in consequence deprived of virulence. May not this domesticated microbe, as M. Bouley calls it, be compared to those inoffensive microbes of which there are so many in nature? May not our common microbes be those organisms which have lost their former virulence? But may not these harmless microbes, become infectious in some particular circumstances? And if there are fewer virulent maladies now than there were in times past, might not the number of these maladies again increase?
Questions multiply as the facts relating to the attenuation of a virus suggest inductions, awaken ideas, and throw new lights upon a problem which, until within these last few years, has remained so obscure. Formerly it was believed that these viruses were morbid entities. A virus was a unity. This opinion has still its declared upholders. According to Pasteur a virus has different degrees of virulence; it can pass from the weakest virulence to the maximum. Modifying, at will, the virus of fowl cholera, Pasteur inoculates some hens, for instance, with a virus too attenuated to protect from death, but which nevertheless is effectual in securing them against a virus stronger than itself. The second virus will preserve them from the attacks of a third virus, and thus passing from virus to virus they end by being guaranteed against the most deadly virulences. The whole question of vaccination resolves itself into knowing at what moment a certain degree of virus attenuation is a guarantee of protection against the mortal virus.
It seems that between small-pox and cow-pox facts of a similar kind take place. It is probable that vaccination rarely gives perfect security against the infection of a very malignant small-pox; moreover, during epidemics of small-pox many persons who have been previously vaccinated are attacked, and some even die of the disease.
As regards the practice of vaccinating fowls against the cholera peculiar to them—which, though it certainly is not of the same importance as human vaccination, is a scientifically capital fact—we may hope that whatever the differences of receptivity in different races, or in different individuals of the same race, there will be found vaccines to suit them all, special care being taken to resort to the employment of two successive vaccines of unequal power, employed after an interval of ten or fifteen days. The first vaccine may always be chosen of a degree of weakness which will not in any case cause death, and yet of sufficient strength to prevent dangerous consequences from the second vaccine, which would in some cases be fatal if employed at once, and to enable it to act as a vaccine against the most virulent virus.
With regard to the preparation of vaccines, and the ascertaining of their proper strength, it is necessary to make trials upon a certain number of fowls, even at the risk of sacrificing a few in these preliminary experiments. Beyond such questions of manipulation there remains still a scientific question. How are the effects of vaccination to be conceived? What explanation can be given of the fact that a benign disease can preserve from a more serious and deadly one? Pasteur long sought for the solution of this problem. Without flattering himself that he has unravelled the difficulty, he has nevertheless amassed facts which, amid these physiological mysteries, permit us to frame a hypothesis which can satisfy the mind. Pasteur believes, for example, that the vaccine, when cultivated in the body of the animal, robs the globules of the blood, for example, of certain material principles which the vital actions take a long time to restore to the system, and which to the most deadly contagium is a condition of life. The impossibility of action of the progressive virus and of the deadly virus is thus accounted for.
When Pasteur communicated to the Academy of Sciences these important and unforeseen facts, they were at first received with hesitation. It was not without some surprise that the word vaccination, hitherto exclusively reserved for Jenner's discovery, was heard applied to fowl cholera. At the International Medical Congress held in London in August 1881, Pasteur, in the presence of 3,000 doctors of medicine from all parts of the world, who received him with an enthusiasm which reflected glory on France, justified the name that he had given to his prophylactic experiments.
'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.'