Part 24

But, notwithstanding these happy results, the question of the application of vaccines to different breeds requires still further investigation, before the vaccination of pigs can become general.

Soon afterwards a method, different from that of the atmospheric oxygen, for weakening the virulence of the fever virus, was tried in the laboratory.

Pasteur had proved that viruses are not morbid entities, that they can assume numerous forms, and especially physiological properties, dependent on the medium in which they live and multiply. The virulence belongs to living microscopic species, but is at the same time essentially modifiable. It may be weakened or intensified, and each of these states is capable of being made permanent by culture. A microbe is virulent in an animal, when it has the power of swarming in the body of that animal, after the manner of a parasite, and of producing, by the renewal of its own life, disturbances which cause disease and death. If this microbe has lived in any species of animal--that is to say, if several times over it has passed from the body of one individual into that of another of the same kind, without having been subjected to any sensible exterior influence during its passage--we may consider that the virulence of this parasite has reached a fixed and maximum state for the individuals of that race. The splenic fever parasite pertaining to sheep, for instance, varies little from one subject to another or from one year to another in the same country; this must be attributed, doubtless, to the fact that, in its successive passages through the sheep, the habit of the parasite to live in sheep has, so to speak, attained a definite state. It is thus with the virus of the Jennerian vaccination. But the virulence of a virus which is not at its maximum may be essentially modified by its passage into a succession of individuals of the same race. It will be remembered how, when Pasteur and his assistants wished to increase progressively the virulence of the virus of chicken cholera and splenic fever, so as to bring them at last to their maximum intensity, these viruses were first inoculated into young subjects, and from them successively into older ones.

'The Academy remembers, without doubt,' said Pasteur in a recent communication, 'that, some time ago, we discovered a microbe virus in the saliva of hydrophobia. This microbe, though very virulent for rabbits, is shown to be harmless for adult guinea-pigs, but it kills rapidly guinea-pigs only some hours or days old. In following out this inoculation from young guinea-pigs, we have seen the virulence increase, and easily arrive at the point of causing death to older guinea-pigs. There was even at last a marked difference in the lesions. The increase of virulence, by successive passages through individuals of one race, was clearly shown.

'But the new and unexpected result that I wish to point out to the Academy consists in this: that the microbe, after having increased its virulence by successive passages through the bodies of guinea-pigs, shows itself to be less virulent in relation to rabbits than it was before.

'In these new conditions, it gives to the rabbit a disease which is spontaneously curable; and, moreover, having once gone through the malady, the animal becomes refractory in regard to the microbe which is deadly to rabbits. From this arises the all-important consequence, that the habit of living in one species (the guinea-pig) at a definite corresponding degree of virulence, can change this virulence in relation to another species (the rabbit), so much diminishing its effects as to cause it to become a vaccine for this latter species.

'The importance of this result cannot fail to be perceived by everyone, for it contains the secret of a new method of attenuation, which can be applied to some of the most virulent viruses. We will give an example and an application of it.

'If a pigeon be inoculated in the pectoral muscle with the microbe of swine fever, the pigeon dies in an interval of six or eight days, after having shown the apparent exterior symptoms of fowl cholera.

'When a second pigeon is inoculated with the blood of the first, a third with that of the second, and so on in succession, the microbe acclimatises itself to the pigeon. The symptoms of forming itself into a ball, and of somnolence, which are the habitual characteristics of the disease, appear in a much shorter time than with the first pigeons of the series. Death likewise comes on more rapidly. Finally, the blood of the last pigeons exhibits much more virulence in the pig than even the most infectious products of a pig that has died of what is called spontaneous fever.

'The passage of the swine fever microbe through rabbits leads to quite a different result. Rabbits inoculated with the infectious products of a pig that has died of the fever, or with the cultivations of them, are always made ill and most frequently die.

'When the virus is inoculated from rabbit to rabbit, the microbe acclimatises itself to the rabbit. All the animals die, and death comes in a very few days. The cultures of the blood of these rabbits in sterilised media become progressively easy and more abundant. The microbe itself changes its aspect somewhat, grows rather larger than in the pig, and appears in the form of an 8, without the filiform lengthening out characteristic of certain other cultures.'

'When pigs are inoculated with the blood of the last rabbits, and the results compared with those obtained from the first of the series, it is found that the virulence has been progressively diminishing from the first rabbit to the following ones. Very soon the blood of the rabbits ceases to cause death in the pigs, though it renders them ill. On recovery they are proof against the deadly swine fever.'

III.

But in the midst of these investigations undertaken by Pasteur, there is one which is paramount over all the others, one on which for three years all his efforts, as well as those of his pupils, have been concentrated, and this is Hydrophobia. Mysterious in its incubation, alarming in its symptoms, Pasteur's attention had for a long time been drawn to it, when in 1880 he finally attacked it. Besides the attraction which an obscure problem had for him, he felt that if he succeeded in discovering the probably microbean etiology of such a disease, he would carry all minds with him into the current of these new ideas. He had been very often struck, if not with the opposition, at least with the prudent and circumspect reserve, shown in the examination of his doctrine, by a considerable number of physicians who, possessed by the idea that the moral element could cause modifications in the symptoms and development of a malady in man, are not disposed to recognise the least assimilation between human diseases and those of the animal species. No doubt the emotional qualities, grave family cares, the terror of approaching death, the dread of the great unknown, may modify the course of the evil in man, may aggravate it, even hasten it; but, whilst recognising--for never was there a man more a creature of sentiment than he--what there is of deep truth in this opinion, Pasteur could not help thinking that the first origin, the cause of every contagious malady, is physiologically the same in the two groups, and that our bodies, notwithstanding our superior moral qualities, are exposed to the same dangers, to the same disorders, as the bodies of animals.

To overcome these resistances, it was necessary, as in the great experiments on splenic fever, to attack a disease common both to men and animals--one in which experimentation, the only, but great, strength of Pasteur, was supreme. Hydrophobia offered these conditions.

Again, it was Dr. Lannelongue who introduced Pasteur to his first case of hydrophobia. On December 10, 1880, a child of five years old, who had been bitten in the face a month previously, was dying in the Trousseau Hospital. Devoured at the time by a raging thirst, and seized with a horror for all liquids, he approached with his lips the spout of a closed coffee pot, then suddenly started back--the throat contracted--a prey to such fury that he insulted the nursing sister who was attending on him. He was at the same time attacked by aerophobia to a prodigious degree. At a certain moment, the heel of one of his feet protruded from the bed. An assistant blew on it. The child had not seen the assistant, and the breath of air was so light as to be almost imperceptible. The poor child flew into a rage, and a violent spasm seized him in the throat. The next day delirium began, a frightful delirium. The frothy matters which filled his throat suffocated him.

Four hours after his death, the mucus from the palate of the child was collected. It was diluted with a little water, and two rabbits were inoculated under the skin of the abdomen. The rabbits perished in less than thirty-six hours. The saliva of these dead rabbits also transmitted the disease to fresh rabbits. Did it not seem as if one had got hold of an inoculation of hydrophobia? Such was in fact the conclusion of Dr. Maurice Raynaud, who, having been informed, at the same time as Pasteur, of the illness of the child, had made, on his own account, some experiments on rabbits. His rabbits were dead. Already a year previously M. Maurice Raynaud had announced the transmission, by the saliva, of rabies from man to rabbits. 'We are, then, in the presence of a new fact of this kind,' he said, 'and we really believe, until a proof to the contrary is given us, that these latter rabbits died of hydrophobia.'

With his habitual prudence, and trusting more to the results of experiment than to medical observation alone, Pasteur was not in a hurry to form such positive conclusions. He began by doing what Dr. Maurice Raynaud had neglected to do. He examined with the microscope the tissues and the blood of the rabbits inoculated in the laboratory; he discovered, both in those that were dead, and in those which were on the point of death, the presence of a special microbe, easily cultivable in a pure state and of which the successive cultures caused the death of other rabbits. Invariably, the same microbe appeared in the blood. As one or two days sufficed to cause death, hydrophobia could not have had time to make its appearance. Pasteur, moreover, found this same microbe in the saliva of children who had died of common maladies, and even in the normal saliva of healthy adults. It was a new microbe, causing a disease unknown up to that time. To Pasteur it seemed, in the case of the experiments made with the mucus from the child's palate, to be simply an accompaniment of the rabic virus.

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This microbe of the saliva is very easily cultivated in sterile infusions--that of veal, for example--and successive cultures can be made in the usual way. The virulence continues. Could the virulence be attenuated, asked Pasteur, by the action of the oxygen of the air? This would, by a new example, go to establish the generalisation of the method of attenuation by oxygen. The attempt succeeded. When care is taken, as with the attenuation of the fowl cholera contagium, not to allow more than some hours' interval to elapse between one cultivation and the succeeding one, the virulence of the successive cultivations of the microbe of the saliva is preserved in some sort indefinitely. In other words, if it be arranged that the cultures succeed each other every twelve hours, the rabbits inoculated from the last cultures die as quickly as those inoculated from the first. Thuillier had had the patience to make, in this manner, eighty cultures in contact with air, and eighty cultures in a vacuum; the microbe of the saliva being both aerobic and anaerobic. The eightieth culture killed as quickly as the first. But by allowing the successive cultures to remain for some time in contact with the air, before passing from one culture to the following one, the virulence of the cultures becomes enfeebled. Thus, then, as in fowl cholera, attenuated cultures of the microbe can be obtained. Unlike what happens with cholera, however, the cultures of the microbe of the saliva, exposed to the contact of air, perish very quickly. Two or three days of keeping suffice for the parent cultivation to lose all virulence. The seed, taken in any quantity from it, does not fertilise a new cultivation. But, before perishing, this culture passes through very different degrees of progressively weakened virulence, and it is easy with these cultivations to render rabbits ill without causing their death. Once cured, they resist all inoculation which would be mortal for others. The oxygen of the air is manifestly the transformer of this virulent virus into vaccine virus; for, if the virulent blood or cultivations remain inclosed in their tubes, sealed from all entrance of the air, they retain not only for some hours, but for months, their life and their original virulence.

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But though these results were as new as they were unexpected, and though one cause of confusion in the study of this terrible problem was removed, yet these first researches were not marked by any progress in the etiology of hydrophobia. The question remained wholly unsolved.

Impatient with the length of time required for the incubation of the disease, and with the obligation of waiting whole months for the result of an experiment, when the subject demanded such numerous ones, Pasteur began to seek some means of producing hydrophobia with certainty and of making it appear more rapidly. Notwithstanding the assertion of a professor of the veterinary school at Lyons that the saliva of the rabid dog alone contained the virus of the disease, and that he had failed in every attempt to inoculate, whether with the substance of the brain or with the spinal marrow of rabid dogs; Pasteur, with due care as to purity, introduced under the skin of some rabbits and some dogs, divers parts of the brain of a dog which had died in a rabid state. Hydrophobia declared itself in both dogs and rabbits, with a duration of incubation about equal to that which followed the ordinary bite of a dog. Although it was necessary still to submit to this long uncertainty with regard to the incubation, one great result was obtained: hydrophobia could be inoculated with other matter than saliva. Not only is the saliva always impure, containing a saliva microbe, which is endowed with a special virulence of its own, but it presents other inconveniences. It is necessary, in these researches, to have a supply of material constantly at hand. Now, the saliva loses its rabic virulence in twenty-four hours. The existence of the rabic virulence in the brain substance placed, on the contrary, at the disposal of the experimenter, an abundance of the virus, in a state of great purity and capable of long preservation.

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The idea then occurred to Pasteur and his assistants, to inoculate the virulent rabic matter in its pure state under the dura mater on the surface of the brain of a dog. Why not carry the virus, said Pasteur, directly to the place of its activity and development? After having trepanned the skull of a chloroformed dog, a little bit of the medulla of an animal which had died of hydrophobia was deposited on the surface of the brain. As soon as the influence of the chloroform was dissipated, the dog recovered its healthy appearance. It ate its food that same evening. But after some days the symptoms of hydrophobia appeared. The animal became dejected and restless; it tossed its litter about, refused all nourishment. A doleful, sharp howling was the first indication of the rabic voice, which is but one long cry of suffering and appeal, mingled with barkings from hallucinations. The stomach became depraved; the dog swallowed hay and straw. It soon grew furious, agitated with violent convulsions; finally, after a last fit, it died. During all this time there was great rejoicing in the laboratory. They were at last in possession of a method for singularly shortening the period of incubation, and for communicating the disease with certainty. The experiments were multiplied; all the dogs which were trepanned, and which received on the surface of the brain a little of the medulla of the rabid animal, succumbed to the disease, with very rare exceptions, within a period of twenty days. Did not the method pursued demonstrate, among other things, that hydrophobia is a disease of the brain; that the seat of the rabic virus, far from being exclusively in the saliva, belongs, above all, to the cerebral matter?

Other results, in addition to this one, were not slow in revealing themselves. It was established that not only the brain, but the spinal marrow, along its whole length, may be rabic, and that the nerves themselves throughout their whole system, from the centre to the periphery, may contain the virus of hydrophobia. If there exists a microbe of hydrophobia, its medium of cultivation in the body is, par excellence, the brain, the spinal marrow, and the nerves. It was also established that there were localisations of virus in certain parts of the mucous system, and that the very considerable differences of rabic symptoms which exist in different cases of hydrophobia, must be sought for in this fact. At the moment of death the medulla oblongata is always rabic. Finally, it was established that hydrophobia could be given (and almost as rapidly as by trepanning) by inoculating rabic nervous matter into the circulation of the blood by a vein.

In presence of such facts it is easy to account for what takes place in the case of a bite from a mad dog. The circulation of the blood carries the virus to the surface of the brain, or to the surface of the spinal marrow; there it houses itself in particular spots, and, little by little, invades the nervous matter. This last would be progressively attacked throughout, if death from the medulla oblongata did not almost always supervene before the propagation of the virus can become general.

The saliva glands are often rabic, doubtless because the virus oozes into them, little by little, from the nerves which enter these glands. Thus may the presence of this virus be explained in the saliva of mad dogs, where, at all times since the disease was first known, it has been found to exist. When the first point attacked by the virus is the spinal marrow, or certain portions of it, a general paralysis often precedes death. In this case the howling and biting symptoms are for the most part absent, and the dog continues to be caressing until it dies.