FOWL CHOLERA._
If fowls are naturally impervious to
the infection of splenic fever, there is a disastrous malady to which they
are subject, and which is commonly called 'fowl cholera.' Pasteur thus
describes the disorder:--'The bird which is attacked by this disease is
without strength, staggering, the wings drooping. The ruffled feathers of
the body give it the shape of a ball. An overpowering somnolence
takes possession of it. If forced to open its eyes, it appears as if it
were awakened out of a deep sleep. Very soon the eyelids close again,
and generally death comes without the animal changing its place, or
without any struggle, except at times a slight movement of the wings for a
few seconds.' The examination after death reveals considerable
internal disorders.
Here, again, the disease is produced by a
microscopic organism. A veterinary surgeon of Alsace, M. Moritz by name, was
the first who suspected the presence of microbes in this disease; a
veterinary surgeon of Turin, M. Peroncito, depicted it in 1878; a professor
of the veterinary school of Toulouse, M. Toussaint, recognised it, in
his turn, in 1879, and sent to Pasteur the head of a cock which had
died of the cholera. But, however skilful they were, these observers
had not succeeded in deciding the question of parasitism. None of them
had hit upon a suitable cultivating medium for the parasite, nor had
they reared it in successive crops. This, however, is the only method
of proving that the virulence belongs exclusively to a parasite.
It is
absolutely necessary, in the study of maladies caused by microscopic
organisms, to procure a liquid where the infectious parasite can grow and
multiply without possible mixture of other organisms of different kinds. An
infusion of the muscles of the fowl, neutralised by potash, and rendered
sterile by a temperature of 110 to 115 degrees, has proved to be wonderfully
appropriate to the culture of the microbe of fowl cholera. The facility of
its multiplication in this medium is almost miraculous. In some hours the
clearest infusion begins to grow turbid, and is found to be filled with a
multitude of little organisms of an extreme tenuity slightly strangulated at
their centres. These organisms have no movement of their own. In some days
they change into a multitude of isolated specks, so diminished in volume
that the liquid, which had been turbid to the extent of resembling
milk, becomes again almost as clear as at first. The microbe here
described belongs to a totally different group from that of the vibrios. It
is ranged under the genus called 'micrococci.' 'It is in this group,'
said Pasteur on one occasion, 'that the microbes of the viruses which
are yet unknown will probably be one day found.'
In the cultivation of
the microbe of fowl cholera, Pasteur tried one of the cultivating liquids
which he had previously made use of with most success--the water of
yeast--that is to say, a decoction of yeast in water rendered clear by
filtration and then sterilised by a temperature of over 100 degrees. The most
diverse microscopic organisms find in this liquid suitable nourishment,
particularly if it has been neutralised. When, for example, the bacterium of
splenic fever is sown in the liquid, it assumes in a few hours a surprising
development. Now, it is remarkable that this medium is quite unsuited to the
life of the microbe of fowl cholera. Not only does it not develop, but
the microbe perishes in this liquid in less than forty-eight hours. May we
not connect this singular fact with that which is observed when a microscopic
organism proves innocuous in an animal which has been inoculated with it? It
is innocuous because it cannot develop itself in the body of the animal, or
because, its development being arrested, it cannot attain the vital
organs.
The decoction extracted from the muscles of the fowl is the
only medium which really suits the microbe of fowl cholera. It suffices to
inoculate the fowl with the hundredth, even the thousandth, part of a drop of
this mixture, to produce the disease and cause death. But here is a strange
peculiarity. If guinea-pigs are inoculated with this little parasite they are
hardly ever killed by it. Guinea-pigs of a certain age generally exhibit only
a local lesion at the point of inoculation, which ends in an abscess more or
less prominent. After opening spontaneously, the abscess closes again and
heals, while the animal preserves its appetite and its appearance of
health. These abscesses sometimes last several weeks. They are
surrounded by a pyogenic membrane and filled with a creamy pus, in which
the microbe swarms side by side with the pus globules. It is the life
of the microbe inoculated under the skin which causes the abscess.
The abscess, with the membrane which surrounds it, becomes for the
little organism a sort of closed vessel, which it is even easy to tap
without sacrificing the guinea-pig. The organism is mixed with the pus in
a state of great purity, and although it is localised its virulence
is extreme. When fowls are inoculated with the contents of the
abscess they die rapidly, while the guinea-pig, which has furnished the
virus, gets well without the least suffering. A curious instance this is
of the local evolution of a very virulent microscopic organism,
which produces neither internal disorders nor the death of the animal upon
which it lives and multiplies, but which can carry death to other species
inoculated with it. Fowls and rabbits living among the guinea-pigs suffering
from these abscesses might in a moment be smitten and perish, while the
health of the guinea-pigs remained unchanged. To produce this result it would
suffice that a little of the discharge from the abscess of a guinea-pig
should get smeared over the food of the fowls and rabbits. An observer
witnessing such deaths without apparent cause, and ignorant of this strange
dependency, would no doubt be tempted to believe in the spontaneity of the
disease. He would be far from supposing that the evil had originated in the
guinea-pigs, which were all in good health. In the history of contagia
what mysteries may some day be cleared up by even more simple solutions
than this one!
When some drops of the liquid containing this microbe
are placed on the food of fowls, the disease penetrates by the intestinal
canal. There the little organism increases in such great abundance
that inoculation with the excrements of the injected fowls produces
death. It is thus easy to account for the mode of propagation of this
very serious disease, which depopulates sometimes all the poultry yards
in the country. The only means of arresting the contagion is to
isolate, for a few days only, the fowls and the chickens, to remove the
dung heaps, to wash the yard thoroughly, especially with water
acidulated with a little sulphuric acid, or carbolised water with two grammes
of acid to the litre. These liquids readily destroy the microbe, or
at least suspend its development. Thus all causes of contagion
disappear, because, during their isolation, the animals already smitten die.
The action of the disease, in fact, is very rapid.
The repeated
cultivation of the infectious microbe in the fowl infusion, passing always
from one infusion to the next following, by sowing in the latter an
infinitely small quantity, so to speak, of the virus--as much, for example,
as may be retained on the point of a needle simply plunged into the
cultivation--does not sensibly lessen the virulence of the microscopic
organism. Its multiplication inside the bodies of fowls is quite as easy with
the last as with the first culture. In short, whatever may be the number of
the successive cultures of the microbe in the fowl infusion, the last culture
is still very virulent. This proves the microbe to be the cause of
the disease--a proof the same in kind as that which had already
enabled Pasteur to show that splenic fever and septicæmia are produced
by specific microbes.
Like the bacillus of splenic fever, the microbe
of the fowl cholera is an aerobic organism. It is cultivated in contact with
the air, or in aerated liquids. At the same time, though it is entitled to be
called an aerobic organism, it differs essentially in certain respects
from the parasite of splenic fever. If splenic fever blood filled
with filaments of the parasite be enclosed in a vessel protected from
the air--say, in a tube closed at its two extremities--in a few
days, eight or ten at the most, and much fewer in summer, the
parasite disappears, or rather is reduced to fine amorphous granulations,
and the blood loses all its virulence. If the same system of shutting
out the air be employed with the blood of a fowl charged with the
microbe of fowl cholera, this microbe will be preserved with its virulence
for weeks, months, even years. Pasteur has been able to keep for
three years tubes thus sealed, a drop of blood from which when
cultivated in fowl infusion, sufficed to infect the birds in the poultry
yard with cholera. And not only is the microbe preserved thus in the
blood contained in the tube; the same occurs if fowl infusion be put
into tubes and then sealed by the flame of a lamp.
When, in course of
time, such tubes lose their virulence, it is because the vitality of the
organism is extinct. The moment the contents of the tube cease to be
virulent, it is a sign that the contagium is dead. It is useless, then, to
attempt to cultivate it: the microbe cannot be revived.
Here, then, is
a third virulent disease, also produced by a microscopic organism. The
characteristics of fowl cholera are very different from those of splenic
fever and acute septicemia, and these three microbes do not in the least
resemble each other. But, glancing back over Pasteur's work, are not the
diseases of silkworms, pebrine and flacherie, also virulent diseases? Thus,
in so many things, through so many studies, the same connection holds good.
Each discovery of Pasteur's is linked to those which precede it, and is the
rigorous verification by experimental method of a preconceived
idea.
'Nothing can be done,' said he one day, 'without preconceived
ideas; only there must be the wisdom not to accept their deductions
beyond what experiments confirm. Preconceived ideas, subjected to the
severe control of experimentation, are the vivifying flame of
scientific observation, whilst fixed ideas are its danger. Do you remember
the fine saying of Bossuet? "The greatest sign of an ill-regulated mind
is to believe things because you wish them to be so." To choose a road,
to stop habitually and to ask whether you have not gone astray, that
is the true method.'
It is this method which conducted him in 1880 to
that wonderful discovery, the attenuation of contagia. What certain of these
contagia are, we have already seen. We shall now learn what they become in
the hands of Pasteur.
_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.'
_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 Ecole 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.
After all,
there is nothing surprising in the fact that the vital resistance of a
newly-born guinea-pig should differ from that of an adult one. But what is
very remarkable is, that if an older guinea-pig be inoculated with the blood
of one a day old; if a third, still older, be inoculated with the blood of
the second; and so on; the virulence of the microbe will be gradually
reinforced--that is to say, the usual habit of this parasite to develop
itself in the body of the animal will be restored. The process may be likened
to that of an animal or vegetable species, passing by successive stages and
long sojourns, from one region to another very distant one, subjected
to quite new conditions of climate, and gradually becoming acclimatised to
the last one. How great, then, must be the importance of the medium of
cultivation, with regard to the virulence of the microbes of communicable
diseases! Cultivating the microbe by passing it from one guinea-pig to
another, we soon arrive at a strength capable of killing guinea-pigs of a
week, a month, or several years old, until at last the smallest drop of the
blood of these guinea-pigs suffices to kill a sheep; and from the sheep we
may pass on to the ox.
The same is the case with the microbe of fowl
cholera. When it has ceased to have any effect upon fowls, its virulence can
be restored by inoculating small birds. Blackbirds, canaries, sparrows, all
die, if the virus has not been too much attenuated; and the effect is
similar on young chicks. Thus by several successive transitions from bird
to bird a virulence may be fostered capable of destroying full-grown
fowls.
These facts suggested to Pasteur certain inductions which may be
well founded. Is not the attenuation of the virus by the influence of
the air one of the factors in the extinction of great epidemics? And may not
the reappearance of these scourges be accounted for by the reinforcement of the
virulence? |
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