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.'
_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.
In the blood of animals, as in the cultures at the beginning, the
splenic fever microbe appears at first in transparent filaments, more
or less divided into segments. Up to that point, the resemblance
between the microbe of splenic fever and the microbe of cholera is
complete. But this blood, or the cultures exposed to the free contact
of the air, instead of continuing this first mode of generation,
frequently exhibit, even in the course of twenty-four hours, spores
distributed more or less regularly along the length of the filaments.
All around these corpuscles the matter of the filaments is absorbed,
in the manner formerly illustrated by Pasteur in the diagrams of
his work on the diseases of silkworms, when treating of the bacilli
of putrefaction. Little by little, all cohesion between the spores
disappears, and the whole collection soon forms nothing more than
a dust of germs. But--and here lies the great difficulty which
experimenters encountered in applying to splenic fever the method
of gradual attenuation which was practised with the microbe of
cholera--these germs of splenic fever may be exposed for years to
the air without losing their virulence, always ready to reproduce
themselves without any appreciable change, and to manifest their
effects in the bodies of animals. How can it be hoped to discover a
vaccine of splenic fever by the method used with the contagium of fowl
cholera, since the splenic fever virulence, at the end of twenty-four
hours, is concentrated in a spore? Before the oxygen of the air has had
time to attenuate the contagium, the virulence of the parasite would be
encased in these spores. Yet this objection did not appear insuperable
to Pasteur. Since (said he to himself), under its filamentous form,
the microbe of splenic fever is quite analogous to the microbe of fowl
cholera, may not the problem of exposing the splenic microbe to the
air be reduced to the following one: to determine the conditions which
would prevent the production of spores? The difficulty would thus
be surmounted; for, once we have got rid of the spores, the splenic
filaments might be maintained in contact with air for any length of
time, and we might then no doubt fall back upon the conditions which
had produced the attenuation of the cholera microbe.
Pasteur and his two assistants gave themselves up to this research.
Days passed and experiments were multiplied. Pasteur became more and
more engrossed: he had, what his daughter called, 'the face of an
approaching discovery.'
'Ah! what a grand thing it would be,' he was heard from time to time
to murmur to himself with a suppressed voice, 'if one could arrive at
that--if the fact that the attenuation of the microbe of fowl cholera
proved not to be an isolated one!' But if anyone ventured to ask him a
timid question as to the phase his experiments were going through, he
would reply, 'No, I can tell you nothing. I dare not express aloud what
I hope.'
At last one day he came up from his laboratory with a triumphant face.
His joy was such that tears stood in his eyes. I have never seen a
more radiant expression of the highest and most generous emotions than
emanated from his countenance.
'I should never console myself,' he said while embracing us, 'if a
discovery such as my assistants and I have just made were not a French
discovery.'
* * * * *
And with the clearness which is the charm of this powerful mind, he
related to us the most recent discoveries of his laboratory.
In neutralised chicken infusion the splenic microbe can no longer be
cultivated at a temperature of 44 or 45 degrees. Its cultivation, on
the contrary, is easy at 42 or 43 degrees; and in these conditions the
microbe produces no spores. At this latter temperature, therefore,
and in contact with pure air, we can maintain a culture of filamentous
parasites of splenic fever, deprived of all germs. In some weeks the
crop dies--that is to say, when this culture is sown in fresh broth
the sterility of the broth remains complete. But during the preceding
days life exists in the cultivating liquid. If after two, four, six, or
eight days of exposure to the air and to heat, the contagium is tried
upon animals, its virulence is found to be continually changing with
the time of its exposure to the air, and, consequently, it represents
a series of attenuated contagia. From the moment when the formation
of the spores of the splenic fever bacillus is prevented, all becomes
substantially the same as in the case of the microbe of fowl cholera.
Moreover, as in the cholera microbe, each of these states of attenuated
virulence can be reproduced by cultivation. Finally, splenic fever not
being recurrent, each of these splenic fever microbes constitutes a
vaccine for the more virulent microbe.
In order to apportion the virulence of the vaccine to the species
it is desired to vaccinate, it must be tried on a certain number
of individuals of the same species. If some vaccinated animals are
inoculated with the virulent virus, and none of them perish, the
vaccine is good. Among individuals of the same species, however,
the difference of receptivity is in general great enough to make it
prudent, and even necessary, to have recourse to two vaccines, one
weak and the other stronger, with an interval of from 12 to 15 days
between the two inoculations.
* * * * *
It was on February 28, 1881, that Pasteur communicated to the Academy
of Sciences, in his own name, and in those of his two fellow workers,
the exposition of this great discovery. Loud applause burst forth with
patriotic joy and pride. And yet so marvellous were these results that
some colleagues could not help saying, 'There is a little romance in
all this.' All this reminds one, in fact, of what the alchemist of
Lesage did to the demons which annoyed him. He shut them up in little
bottles, well corked, and so kept them imprisoned and inoffensive.
Pasteur shut up in glass bulbs a whole world of microbes, with all
sorts of varieties which he cultivated at will. Virulences attenuated
or terrible, diseases benign or deadly, he could offer all. Hardly had
the journals published the _compte-rendu_ of this communication when
the President of the Society of Agriculture in Melun, M. le Baron de la
Rochette, came, in the name of the Society, to invite Pasteur to make a
public experiment of splenic fever vaccination.
Pasteur accepted. On April 28 a sort of convention was entered
into between him and the Society. The Society agreed to place at
the disposal of Pasteur and his two young assistants, Chamberland
and Roux, sixty sheep. Ten of these sheep were not to receive
any treatment; twenty-five were to be subjected to two vaccinal
inoculations at intervals of from twelve to fifteen days, by two
vaccines of unequal strength. Some days later these twenty-five sheep,
as well as the twenty-five remaining ones, were to be inoculated with
the virus of virulent splenic fever. A similar experiment was to be
made upon ten cows. Six were to be vaccinated, four not vaccinated; and
the ten cows were afterwards, on the same day as the fifty sheep, to
receive inoculation from a very virulent virus.
Pasteur affirmed that the twenty-five sheep which had not been
vaccinated would perish, while the twenty-five vaccinated ones would
resist the very virulent virus; that the six vaccinated cows would not
take the disease, while the four which had not been vaccinated, even if
they did not die, would at least be extremely ill.
As soon as the agricultural and scientific press had published this
programme, and recorded Pasteur's prophecies, several of his colleagues
at the Academy of Sciences, startled by such boldness in reference to a
subject which had hitherto been enveloped in such profound obscurity,
and fearing to see the illustrious company somewhat compromised by
these affirmations in relation to problems of physiology and pathology,
addressed some observations to M. Pasteur on what they called 'a
scientific imprudence.'
'Take care,' they said to him, 'you are committing yourself without
possibility of retreat. Your experiments in the laboratory hardly
authorise you to attempt experiments like those at Melun.'
'No doubt,' Pasteur answered, 'we have never had in our experimental
studies so many animals at our disposition to inoculate; but I have
full confidence. What has been already done in my laboratory is to me a
guarantee of what can be done.'
And M. Bouley, confident also in the assurances of his illustrious
friend, and arranging to meet him, to witness these audacious
experiments, said to his anxious colleagues, 'Fear nothing; he will
come back triumphant.'
The experiments began on May 5, 1881, at four kilometers' distance
from Melun, in a farm of the commune of Pouilly-le-Fort, belonging to
a veterinary doctor, M. Rossignol, secretary-general of the Society
of Melun. At the desire of the Society of Agriculture, a goat had
been substituted for one of the twenty-five sheep of the first lot.
On the 5th of May they inoculated, by means of the little syringe of
Pravaz--that which is used in all hypodermic injections--twenty-four
sheep, the goat, and six cows with five drops of an attenuated splenic
virus. Twelve days after, on May 17, they reinoculated these thirty-one
animals with an attenuated virus, which was, however, stronger than the
preceding one.
On May 31 very virulent inoculation was effected. Veterinary doctors,
inquisitive people, and agriculturists formed a crowd round this
little flock. The thirty-one vaccinated subjects awaiting the terrible
trial stood side by side with the twenty-five sheep and the four cows,
which awaited also their first turn of virulent inoculation. Upon the
proposal of a veterinary doctor, who disguised his scepticism under the
expressed desire to render the trials more comparative, they inoculated
alternately a vaccinated and a non-vaccinated animal. A meeting was
then arranged by Pasteur and all other persons present for Thursday,
June 2, thus allowing an interval of forty-eight hours after the
virulent inoculation.
More than two hundred persons met that day at Melun. The Prefect of
Seine-et-Marne, M. Patinot, senators, general counsellors, journalists,
a great number of doctors, of veterinary surgeons, and farmers; those
who believed, and those who doubted, came, impatient for the result. On
their arrival at the farm of Pouilly-le-Fort, they could not repress
a shout of admiration. Out of the twenty-five sheep which had not
been vaccinated, twenty-one were dead; the goat was also dead; two
other sheep were dying, and the last, already smitten, was certain
to die that very evening. The non-vaccinated cows had all voluminous
swellings at the point of inoculation, behind the shoulder. The fever
was intense, and they had no longer strength to eat. The vaccinated
sheep were in full health and gaiety. The vaccinated cows showed no
tumour; they had not even suffered an elevation of temperature, and
they continued to eat quietly.
There was a burst of enthusiasm at these truly marvellous results.
The veterinary surgeons especially, who had received with entire
incredulity the anticipations recorded in the programme of the
experiments, who in their conversations and in their journals
had declared very loudly that it was difficult to believe in the
possibility of preparing a vaccine capable of triumphing over such
deadly diseases as fowl cholera and splenic fever, could not recover
from their surprise. They examined the dead, they felt the living.
'Well,' said M. Bouley to one of them, 'are you convinced? There
remains nothing for you to do but to bow before the master,' he added,
pointing to Pasteur, 'and to exclaim--
"I see, I know, I believe, I am undeceived."'
Having suddenly become fervent apostles of the new doctrine, the
veterinary surgeons went about proclaiming everywhere what they
had seen. One of those who had been the most sceptical carried his
proselytising zeal to such a point that he wished to inoculate himself.
He did so with the two first vaccines, without other accident than a
slight fever. It required all the efforts of his family to prevent him
from inoculating himself with the most virulent virus.
An extraordinary movement was everywhere produced in favour of
vaccination. A great number of agricultural societies wished to repeat
the celebrated experiment of Pouilly-le-Fort. The breeders of cattle
overwhelmed Pasteur with applications for vaccine. Pasteur was obliged
to start a small manufactory for the preparation of these vaccines in
the Rue Vauquelin, a few paces from his laboratory. At the end of the
year 1881, he had already vaccinated 33,946 animals. This number was
composed of 32,550 sheep, 1,254 oxen, 142 horses. In 1882, the number
of animals vaccinated amounted to 399,102, which included 47,000 oxen
and 2,000 horses. In 1883, 100,000 animals were added to the total of
1882.
From the commencement of the practical application of this new system,
the results were topical. Among flocks where half had been vaccinated
and the other half not vaccinated--all the animals continuing to live
together--the mortality from splenic fever in 1881 was ten times less
in the vaccinated sheep than in the non-vaccinated, being 1 in 740 as
against 1 in 78; and in cows and oxen fourteen times less, being 1
in 1,254 against 1 in 88. In 1882 also, the mortality was ten times
greater among the non-vaccinated than among the vaccinated animals.
In 1883 it was proved that the duration of the immunity generally
lasted longer than a year. It is, however, prudent to vaccinate
every year, and to select for performing the operation a period when
splenic fever has not yet become developed--in March and April. If
the vaccinating is postponed until the fever is in the sheepfolds,
there is the risk of attributing to vaccination the losses which in
reality belong to the natural disease. Just as human vaccination cannot
preserve from small-pox a patient who is already under the influence of
small-pox, so the splenic vaccinations are powerless against a fever
already in process of incubation.
It must not be assumed that the duration of immunity to animals
after splenic vaccination cannot be compared with the duration of
immunity from small-pox after Jennerian vaccination. Jenner and his
contemporaries believed that vaccination was able to preserve from
small-pox during the whole life. That illusion disappeared long ago,
and now ten years has been fixed as the average duration of that
immunity and of the interval which ought to separate successive
vaccinations. This interval, moreover, is too long for a certain
number of individuals. Besides, in order to judge of the immunity of
antisplenic vaccination, we must not lose sight of the formidable trial
which vaccinated animals have to undergo when inoculated with the most
virulent virus. What doctor would dare to subject a vaccinated child
to inoculation from virulent small-pox a year after its vaccination?
Finally, taking into consideration the commercial and economic view of
the life of a sheep--if such an expression may be used--the average
scarcely exceeds three years. The duration, then, of the immunity that
vaccination confers is about a third of the duration of the animal's
life.
_THE RETURN TO VIRULENCE._
After having reduced the microbes of fowl cholera and splenic fever
to all degrees of virulence, and brought them to a point where they
could no longer multiply in the bodies of animals inoculated with
them, and fixed them in media appropriate to their life, Pasteur asked
himself whether it would not be possible to restore to these attenuated
microbes--weakened to such a degree as to have lost all virulence--a
deadly virulence, and to render them again capable of living and
multiplying in the bodies of animals.
Experiment soon confirmed this mental prevision. An attenuated
splenic fever virus which could cause no danger of disease or death
to guinea-pigs of a year, or a month, or even a week old, could kill
a little guinea-pig just born, or one or two days old. The attenuated
microbe could multiply itself in the blood of so young an animal. We
can well imagine that in an animal, scarcely formed, the power of
oxygenation of the blood globules is not as yet capable of preventing
the aerobic microbe from turning to its own account the oxygen of the
blood. The disease does its work and death supervenes.