Part 19

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.