Pasteur's theory of the reason why any vaccine matter will have its prophylactic effect, is this: He believes there is in the blood of any animal subject to a disease caused by bacilli some substance which is necessary to the sustenance of those bacilli; and when the bacilli, having an attenuated virus, are introduced, they slowly consume all of this substance.
The substance being one which nature creates very slowly, no subsequent introduction of the bacilli, however virulent, can produce the disease until such time shall have elapsed that a new supply of the substance shall have been secreted. In this way he accounts for the fact that vaccination will protect from small-pox for a more or less defined period of time.
Pasteur hastened to apply his discovery of the attenuation of the virus of chicken cholera to the virus of splenic fever. Here, however, he was met with a serious difficulty. The microbes of splenic fever, if left in the flask for forty-eight hours, developed bright spots, and gradually into these spots the bacilli themselves seemed to be absorbed. Pasteur found these spots were the spores or seeds of the microbes, and he also found that, while the bacilli could be killed easily in various ways, the spores possessed a much greater resistance. They could be dried, for example, and preserved in that state indefinitely. It was apparent that the oxygenation which attenuated the venom of the bacilli of chicken cholera was impossible with those of splenic fever if the bacilli of the latter disappeared within a week, leaving the spores behind. But Pasteur had discovered before this that, unless the temperature of a fowl were lowered artificially, inoculation with the microbes of splenic fever would not produce the disease. From this he argued that, as the heat of the fowl's body was sufficient to resist the contagion, the bacilli themselves must be extremely sensitive to variations in temperature. He tried the experiment and found, by lowering the temperature of the flasks containing the cultures, he could prevent the formation of the spores. He then attenuated the venom of the splenic bacilli as he had that of the fowl cholera, tried it on guinea-pigs, found they became sick and recovered; inoculated them with the bacilli of full strength, but with no result. Pursuing his experiments, he discovered that he could by using vaccine-attenuated bacilli, of unequal strength, cause any degree of sickness he pleased.
In the early part of 1881 Pasteur agreed to hold a public exhibition of his vaccine for splenic fever, the animals to be supplied by the Society of Agriculture in Melun. The experiment was begun on May 5th. Pasteur inoculated twenty-four sheep, one goat, and six cows with six drops each of attenuated virus, and twelve days afterward he reinoculated them with a stronger virus. On May 31st he reinoculated the thirty-one animals with the strongest virus of splenic fever, and at the same time inoculated twenty-five sheep and four cows which had not been vaccinated as were the others. On June 2d over two hundred people assembled at the farm to see the result. The twenty-five sheep that had not been vaccinated all died before that evening. The non-vaccinated cows had intense fever and great swellings, and could scarcely stand up. On the other hand, the vaccinated sheep and cows were in full health and were feeding quietly. Pasteur had conquered splenic fever.
Having attenuated the virus of these bacilli, Pasteur began a series of experiments to determine whether the attenuated virus could be intensified until its former venom was obtained. This he succeeded in, and thus discovered what is probably the key to the solution of the problem of the periodicity of epidemics of contagious diseases, such as cholera. In 1882 Pasteur's attention was called to a new disease, swine fever (rouget), which was ravaging the herds of swine in France. He found the microbes, attenuated them, vaccinated the pigs, and secured the most favorable results. He also discovered that by passing the microbe of a disease through an animal not subject to that disease, he attenuated it so far as its effects on another were concerned.
It was in 1880 that Pasteur first began his experiments in hydrophobia. Securing the saliva of a child suffering from the disease, he inoculated rabbits with it and they died in thirty-six hours. He examined the saliva and the blood of the rabbits, and found in both a new microbe (a minute disk having two points). He established by repeated experiments that hydrophobia is a disease of the nerves, that a portion of the medulla oblongata, or of the spinal cord, is very much more certain to produce the disease, when introduced into the blood or placed on the brain, than is the saliva. He succeeded at last in isolating the microbe, in making cultures of it, and then attenuating it, and in May, 1884, he produced before a commission appointed by the Minister of Public Instruction the following results:
Of six dogs unprotected by vaccination, three died as the results of bites of a dog violently mad. Of eight unvaccinated dogs, six died after extra-venous inoculation of rabic matter. Of five unvaccinated dogs, all died after inoculation, by trepanning, of the brain with rabic matter. Of twenty-three vaccinated dogs, not one was attacked with the disease after inoculation, in any fashion, with the most virulent rabic matter procurable.
During his long and busy life Louis Pasteur has been honored after every fashion known to men. He has opened the gates of knowledge wider than they were ever opened before, and in his discovery of the germs of disease, and in his still more wonderful discovery of the possibility of attenuating those germs and converting them into vaccines, he has revolutionized all ideas of physiology. He is one of the greatest pioneers in science that has ever lived, and his work will make his name illustrious so long as men shall continue on this earth. The lesson of his life is the supreme value of experiment; for, as was once said of him by Professor Dumas, "Pasteur is never mistaken, because he never asserts anything he cannot show another man how to prove."[Back to Contents]
