II.
In 1882, a new malady occupied the attention of the laboratory of the École Normale, a malady the name of which was not even known in Paris, but which made great ravages in the country—namely, swine fever (rouget). Here, again, it is a microbe which causes the disorder. This microbe was first perceived by Thuillier, in a little commune of the Département de Vienne, when examining the blood and humours of pigs which had died of the fever. Experiments were at once set on foot in the laboratory, with the view of proving that the microbe was really the cause of the disease. The microbe was cultivated in a sterilised infusion of veal. This cultivation was passed on to a succeeding one, a small drop of the preceding cultivation being always taken for seed. Inoculations from these last cultivations produced the fever in certain breeds of pigs. The proof was thus given that the microbe was really the origin of the disease.
Pasteur then, accompanied by Thuillier and a young assistant, M. Loir, went, in his turn, to study the disease in the Department of Vaucluse. He remained more than a month in the canton of Bollène, in the house of a veterinary surgeon, M. Maucuer, who took him to all the pigsties in the arrondissement. After having had recourse to the oxygen of the air to attenuate the virulence of the microbe, Pasteur made some experiments in vaccination. Some pigs which had been vaccinated remained in the canton of Bollène, under the supervision of M. Maucuer, the owners having pledged themselves to keep their vaccinated pigs for at least a year. In the ensuing September, when swine fever raged everywhere in the canton of Bollène and in the arrondissement of Orange, not a single vaccinated pig was attacked. 'They are all flourishing,' wrote M. Maucuer. An address of thanks was sent to Pasteur by the municipal council of Bollène.
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.'