With reference to this subject, Professor Landouzy, in his fine work on serum therapeutics, quotes a passage from “The Pharsalia” of Lucan describing, in the year 60 A.D., the customs of the Psylli, a people encountered by the army of Cato during its sojourn in Africa. This passage is so interesting that I cannot refrain from reproducing it:—
“Alone unharmed of all who till the earth
By deadly serpents, dwells the Psyllian race.
Potent as herbs their song; safe is their blood,
Nor gives admission to the poison germ
E’en when the chant has ceased. Their home itself
Placed in such venomous tract and serpent-thronged
Gained them this vantage, and a truce with death,
Else could they not have lived. Such is their trust
In purity of blood, that newly born
Each babe they prove by test of deadly asp
For foreign lineage. So the bird of Jove
Turns his new fledglings to the rising sun,
And such as gaze upon the beams of day
With eyes unwavering, for the use of heaven
He rears; but such as blink at Phœbus’ rays
Casts from the nest. Thus of unmixed descent
The babe who, dreading not the serpent touch,
Plays in his cradle with the deadly snake.”[93]
The only scientific conclusion to be drawn from the facts and statements that we have just set before the reader is that, under certain circumstances, man can unquestionably acquire the faculty of resisting intoxication by snake-venom, by conferring upon himself a veritable active immunity by means of repeated inoculations of venom. We shall shortly see that the case is the same with regard to animals.
PART III.
ANTIVENOMOUS SERUM THERAPEUTICS.
CHAPTER XIII.
VACCINATION AGAINST SNAKE-VENOM—PREPARATION OF ANTIVENOMOUS SERUM—ITS PREVENTIVE PROPERTIES AS REGARDS INTOXICATION BY VENOM.
So long ago as the year 1887 it was shown by Sewall, in an important paper on “Rattlesnake-Venom,”[94] that it is possible to render pigeons gradually more resistant to the action of this venom by injecting them with doses at first very small, and certainly incapable of producing serious effects, and then with stronger and stronger doses. In this way, although these little animals are very sensitive, he succeeded in making them withstand doses ten times greater than the minimal lethal dose.
A little later Kaufmann[95] obtained the same result with the venom of French vipers. He did not, however, succeed in producing tolerance of doses more than two or three times greater than the lethal one.
In 1892, at the time of my first experiments with cobra-venom at Saigon,[96] I arrived at the conclusion that it was possible, by means of successive inoculations with heated venoms, to confer on animals a certain degree of resistance to doses invariably lethal to the controls.
From 1894 onwards, the investigations pursued simultaneously at the Paris Natural History Museum, by Phisalix and Bertrand, upon viper-venom, and at the Paris Pasteur Institute by myself, upon that of the cobra, and subsequently upon other venoms of various origins, led to much more definite results. These investigations show, on the one hand, that by vaccinating guinea-pigs or rabbits, and taking certain precautions, it is possible to confer upon these small animals a really strong immunity to venom; on the other hand, that animals vaccinated against cobra-venom are perfectly immune to doses of viper-venom or that of other snakes (Bungarus, Cerastes, Naja haje, Pseudechis) certainly lethal to the controls; and lastly, that the serum of the vaccinated animals contains antitoxic substances capable of transmitting the immunity to other animals.[97]
According to Phisalix and Bertrand, who, as we have stated, experimented only with viper-venom, the best method of vaccinating the guinea-pig consists in inoculating a dose of 0·4 milligramme of this venom heated for five minutes at 75° C., and, forty-eight hours afterwards, the same dose of non-heated venom. The latter is always lethal to the control guinea-pigs in from six to eight hours.
Vaccination against cobra-venom, which is much more toxic, is most surely effected by the method recommended by me, which consists in at first injecting small doses of this venom mixed with an equal quantity of a 1 per cent. solution of hypochlorite of lime. By degrees the quantity of venom is increased and that of the hypochlorite progressively diminished, and the injections are repeated every three or four days, while attentively following the variations in the weight of the animals. The inoculations are suspended as soon as emaciation supervenes, and resumed when the weight becomes normal again. After four injections of chloridated venom the chloride is omitted, and a direct inoculation made with one-half the minimal lethal dose of pure venom; then, three or four days afterwards, the injection is increased to three-fourths of the minimal lethal dose; and finally, after the lapse of another three or four days, a lethal dose is injected.