Louis Pasteur: His Life and Labours

by René Vallery-Radot

II.

The first time Pasteur went to Alais the silkworm epidemic was universally attributed to a single cause—pébrine. Pébrine was called the disease. This word expressed everything. It indicated the existence of a mysterious scourge, the origin and nature of which could not be traced, but which was ready to fall upon all the establishments devoted to the nurture of the worms. Whatever might happen, or whatever might be the cause of ruin in a silkworm nursery, the disease was held responsible. One of the most striking proofs that the evil was attributed to pébrine alone is found in the fact that a prize of 5,000 florins was offered by the Austrian Government in 1868, as a reward for the discovery of the best remedy for the prevention and cure of pébrine—'the epidemic disease which devastates the silkworm.'

A rapid glance at the principles which have just been established suffices to show that pébrine might now be regarded as vanquished. Pasteur had demonstrated that moths free from corpuscles never produced a single corpusculous egg; he had proved, moreover, that eggs brought up in a state of isolation, at a distance from contaminated eggs, produce no worms, chrysalides, or moths which are corpusculous. It was easy, therefore, to multiply cultivations free from pébrine. The production of silk and the production of eggs was thus secured. To make sure that the eggs were pure it was only necessary to have recourse to the microscopic examination of the moths which had produced them. These observations might be made by women, by young girls, even by children. It was sufficient to crush up a moth in a little water, and to put a drop of this mixture under the microscope, to see the corpuscles clearly, if they existed. It seemed, then, that the plague was got rid of. But Pasteur was not slow in recognising that the general belief in a single malady could not be justified. If the experiments of 1866 had demonstrated to him the full extent of the corpusculous malady, and had established the principles of a treatment proper for its prevention, the method he had adopted had also shown him that pébrine was far from being the only cause from which the silk culture suffered.

It was in 1867 that this result was obtained. From an experimental point of view, that year counted double for Pasteur. Influenced by a profound sympathy for the misery which he had witnessed during two successive years, and, at the same time, impatient to find the cause of the scourge, Pasteur, in the months of February, March, and April, in advance of the great industrial cultivations, commenced a series of experiments on worms hatched by artificial heat, and fed with mulberry leaves from a hothouse.

During these forced experiments Pasteur observed that out of sixteen broods derived from non-corpusculous parents, fifteen succeeded, while the sixteenth perished almost entirely between the fourth moulting and the climbing on to the brambles. After having exhibited a most healthful appearance, the worms died suddenly. In a cultivation of 100 worms, ten, fifteen, twenty dead ones were picked up daily: these turned black, and became putrid with extraordinary rapidity, often within the space of twenty-four hours. Sometimes they were soft and flabby, like an empty, crumpled intestine. Consulting the authors who had written upon silkworms, Pasteur could not doubt that he had before his eyes a characteristic specimen of the disease called morts-flats, or flacherie. Not only were these worms free from all pébrine spots, but no corpuscles were to be found in any part of their bodies. A still more significant fact was, that corpuscles were also absent from the chrysalides and the moths of those few worms which were able to spin their cocoons. Although this sample was confined to a single group of eggs derived from parents free from corpuscles, Pasteur continued to entertain doubts as to the existence of only a single disease, and also as to the necessary connection of pébrine with flacherie.

These suspicions were confirmed by his cultivations of April and May. Numerous cases of flacherie presented themselves. Uncertainty was no longer possible as to the mutual independence of the two maladies—pébrine and flacherie. The cultivations most seriously invaded by the last-mentioned disease came from eggs produced by parents free from corpuscles, and led on to reproducers also free from this parasite. On visiting a multitude of industrial cultivations, Pasteur discerned that what had passed in his own laboratory was of very general occurrence, and that, contrary to the received opinion, two distinct maladies divided between them the cause of all the misfortunes. Pébrine was evidently the most widely spread, but flacherie had also its share, and a very large share, in the calamity.

Here, once more, the microscope came to Pasteur's assistance. If, at the period of the rearing of the silkworms, when the mean temperature is always rather high, some mulberry leaves are crushed in a mortar and mixed with a little water, the liquid being left to itself, in twenty-four hours it will be found filled with microscopic organisms; some motionless, resembling little rods or spores joined end to end, like strings of beads, others more or less active, flexible, endowed with a sinuous movement like that of the vibrios found in nearly all organic infusions in process of decomposition. Whence come these microscopic organisms? The facts relating to spontaneous generation indicate that the germs of these organisms were on the surface of the pounded leaf, spread in the form of dust over the instruments used to triturate the leaf, possibly on the mortar, the pestle, or in the water added to the pounded leaves.

It is a curious fact, that if the intestinal canal of a worm in full process of digestion be opened, the pounded leaf which fills it from one end to the other will not show microscopic organisms of any kind, but only cells of parenchyma, green granules of the chlorophyl of the leaf, and remains of the air-vessels of the plant. Through the action of the liquids secreted by the glands which line the integuments of the intestinal canal, the germs of organisms are themselves digested or hindered in their development. The digestive functions of silkworms are so active that everything is carried away, destroyed in the same manner as the leaves themselves.

But if from any cause the digestion of the worms be impeded or suspended, then the germs introduced with the food into the intestinal canal will give rise to the multiplication of microscopic organisms which are always found in the artificially bruised leaf when mixed with a little water. How numerous are the causes which may check this digestive function of the worm—a function of such importance to a creature which in the space of one month passes from the weight of half a milligramme to that of five, six, seven, or even eight grammes! Pasteur proved that whenever a worm was attacked with flacherie, it always had, associated with the food in its intestinal canal, one or other of the microscopic organisms which are invariably to be met with among crushed mulberry leaves. Summing up in a kind of aphorism a series of observations, Pasteur observes: 'Every ver flat is one which digests badly, and, conversely, every worm which digests badly is doomed to perish of flacherie, or to furnish a chrysalis and a moth the life of which, through the injury produced by organised ferments, is not normally perfected.'

Thus, as in the case of pébrine, the morbid symptoms of flacherie are very variable. All depends on the intensity of the evil—that is to say, on the abundance and the nature of the parasites developed in the intestinal canal, and also on the period in the life of the worm when this fermentation begins to show itself. The most dangerous of all these ferments are those of the family of vibrios. If they exist in the first phases of the life of the worm, it dies quickly and very soon becomes putrid, sometimes resolving itself into an infected pus. The disease often manifests itself in a manner particularly distressing and disastrous to the cultivator. The worms have presented the most beautiful appearance up to the time of climbing the heather. The mortality has scarcely been two or three per cent., which is nothing; the moultings have been effected in a perfect manner, when suddenly, some days after the fourth moulting, the worms become languid, crawling with difficulty, and hesitating to take the leaves which are thrown upon their hurdles. If some few have mounted on to the heather, they stretch themselves on the twigs, their bodies swollen with food which they cannot digest. Sometimes they remain there motionless till they die, or, falling, remain suspended only by their false feet. The few moths which have succeeded in piercing their cocoons do not show any corpuscles. They can produce eggs, but these eggs, coming from parents weakened by disease, give rise the following year to a generation threatened with flacherie. It is in this sense that the disease may be regarded as hereditary, although the parasites of the intestinal canal to which flacherie is due do not transmit themselves to the eggs or to the worms which issue from them. The worms inherit weakly constitutions, and, being without power of resistance against anything that can derange their digestive functions, they are at the mercy of the accidents of their culture.

Too large an assemblage of worms in one nursery; too high a temperature at the time of moulting; a thunderous atmosphere, which predisposes organic matter to fermentation; the use of heated or wet leaves, especially if the wetting be caused by a fog or by the morning or evening dew, which deposits on the leaf the germs suspended in a great mass of air;—these are so many causes calculated to diminish the activity of the digestive functions of the worms, and to produce in consequence a fermentation of the leaf in the intestinal canal—the malady now under consideration. Often also flacherie depends upon mistakes committed by the husbandman while tending his precious 'kine,' to use an expression of the sixteenth century.

A Chinese book published on the rearing of silkworms contains a series of little practical counsels. 'The person who takes care of the silkworms,' says this guide to the perfect cultivator, 'ought to wear a simple garment, not lined. He must regulate the temperature of the spinning-house according to the sensation of heat or cold which he experiences; if he feels cold, he may conclude that the worms are cold, and he will increase the fire; if he feels hot, he will conclude that the worms are hot, and he will suitably diminish the fire.'

One point which had been ignored before the experiments of Pasteur was the contagious character of flacherie. This contagion may surpass that of pébrine itself as regards duration. In pébrine the dried corpusculous matter loses all virulence after the lapse of some weeks. The disease cannot, therefore, communicate itself from one year to another by the corpusculous dust of a rearing establishment. The germs, on the contrary, of the microscopic organisms which provoke fermentation in the mulberry leaves, especially the vibrios, retain their vitality for several years. The dust of a silkworm nursery infected by flacherie appears under a microscope quite full of cysts or spores of vibrios. These spores or cysts rest, like the sleeping beauty in the forest, until a drop of water falls upon them and awakens them into life. Deposited on the leaves which are to serve as nourishment, these germs of vibrios are carried into the intestinal canal of the worm, develop and multiply themselves, and completely disturb the digestive functions, unless the digestion is so strong that the germs are immediately arrested, and disposed of like the food itself. This is what happens when the worms are in full vigour. It is a struggle for life, in which the worms often gain the victory.

Giving to some very healthy worms a meal of leaves covered with the dry dust of a silkworm nursery, infected the year before by pébrine and flacherie, Pasteur reproduced flacherie, and not pébrine. Still more readily did he produce the first of these maladies, when he gave, as food, leaves polluted by the contents of the intestinal canal of worms which had died of the disease. As in the case of pébrine, the excreta of the worms attacked by flacherie, defiling the leaves, carry the mischief to the healthy worms, or add to the dangerous fermentation in the intestines of those which are already in part attacked.

To preserve silkworms from accidental flacherie, hygienic precautions are sufficient. As regards hereditary flacherie, or, to speak more correctly, that which develops itself easily on any diminution of vigour in the eggs and in the embryo, Pasteur again found a remedy by having recourse to the microscope. By means of the microscope it is possible to obtain information as to the health of the worms, the chrysalides, and the moths destined to produce the eggs. Every attention should be directed to the complete exclusion of ferments from the intestinal canal of the worms, and from the stomach-pouch of the chrysalides—a little pouch to which the intestinal canal of the worm is reduced, with its contents more or less transformed. But if there is not time to make this examination for parasitic ferments with the microscope, a simple inspection of the worms in their last stage will suffice. Pasteur laid great stress upon the observation of the worms when they climbed on to the heather.

'If I were a cultivator of silkworms,' he wrote in his beautiful work on the diseases of silkworms, 'I would never hatch an egg produced from worms that I had not observed many times during the last days of their life, so as to make sure of their vigour at the moment when they spin their silk. If you use eggs produced by moths the worms of which have mounted the heather with agility, have shown no signs of flacherie between the fourth moulting and mounting time, and do not contain the least corpuscle of pébrine, then you will succeed in all your cultivations.'