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?
'The accounts which I have read,' Pasteur remarked some months ago, 'of the spontaneous appearance of the plague in Benghazi in 1856 and in 1858 tend to prove that this outbreak could not be traced to any original contagion. Let us suppose, guided by the facts now known to us, that the plague, a malignant disease belonging to certain countries, has germs of long duration. In all these countries its attenuated virus must exist, ready to resume its active form whenever the conditions of climate, of famine, of misery present themselves afresh. The condition of long duration in the vitality of the germs of evil is not even indispensable; for, if I may believe the doctors who have visited these countries, in all places subject to the plague, and in the intervals of the great outbreaks of the epidemic, cases may be met with of people attacked with boils, not fatal, but resembling those of the deadly plague. Is it not probable that these boils contain an attenuated virus of the plague, and that the passage of this virus into exhausted bodies, which abound only too freely in periods of famine, may restore to it a greater virulence?
'The same may be the case with other maladies which appear suddenly, like typhus in armies and in camps. Without doubt, the germs which are the authors of these diseases are everywhere scattered around, but attenuated; and in this state a man may carry them about him or in his intestinal canal without great damage. They only become dangerous when, by conditions of overcrowding, and perhaps of successive developments on the surfaces of wounds, in bodies enfeebled by disease, their virulence is reinforced.'