simply by lowering its temperature, is it not also possible to restore
to health a fowl so inoculated by warming it up again? A hen was
inoculated, subjected, like the first, to the cold-water treatment, and
when it became evident that the fever was at its height it was taken
out of the water, wrapped carefully in cotton wool, and placed in an
oven at a temperature of 35 degrees. Little by little its strength
returned; it shook itself, settled itself again, and in a few hours
was fully restored to health. The microbe had disappeared. Hens killed
after having been thus saved, no longer showed the slightest trace of
splenic organisms.
How great is the light which these facts throw upon the phenomenon
of life in its relation to external physical conditions, and what
important inferences do they warrant as to the influence of external
media and conditions upon the life and development of living contagia!
There have been great discussions in Germany and France upon a mode
of treatment in typhoid fever, which consists in cooling the body of
the patient by frequently repeated baths. The possible good effects of
this treatment may be understood when viewed in conjunction with the
foregoing experiment on fowls. In typhoid fever the cold arrests the
fermentation, which may be regarded as at once the expression and the
cause of the disease, just as, by an inverse process, the heat of the
body arrests the development of the splenic fever microbe in the hen.
_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, pébrine 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.