Fraser[89] (of Edinburgh) thinks that the repeated ingestion of small quantities of venom may suffice to confer immunity, and he mentions a certain number of experiments performed by him upon white rats and kittens, from which it would appear that the ingestion of venom, continued for a long time, finally renders these animals absolutely refractory to subcutaneous inoculation with doses of the same venom several times greater than the lethal one. He therefore concludes that this process of vaccination may probably be in use among snake-charmers.
I have submitted this hypothesis to the test of experiment. I succeeded in making adult rabbits, guinea-pigs, and pigeons absorb enormous doses of Cobra-venom by way of the alimentary canal. In this manner I have administered doses as much as a thousand times greater than the lethal one, yet I have never been able to prove that the serum of these animals became antitoxic.
On the other hand, I have succeeded in vaccinating very young guinea-pigs and young rabbits which were still being suckled, by making them absorb, every second day, minimal and certainly innocuous doses of very dilute venom. In the case of young animals, venom is not modified by the digestive juices, and a portion of it is absorbed by the mucous membrane of the intestine. When the dose ingested is suitably reduced they withstand it, and when these ingestions are repeated every second or third day during the first weeks of life, the animals become perfectly vaccinated against doses certainly lethal for controls of the same age and weight. But it is always difficult to push the vaccination far enough for the serum to acquire antitoxic properties, and I have never been able to prove the appearance of the latter.
I think, however, that it ought to be possible to arrive at this result by experimenting upon animals such as lambs, kids, calves, or foals, the intestine of which remains permeable to toxins for a sufficiently long period.
It may be that certain snake-charmers, who claim to possess family secrets which they transmit from father to son, employ an analogous method in order, in their infancy, to confer immunity to venoms upon those of their male children who are to inherit their strange and lucrative profession.
In Mexico, certain Indians called Curados de Culebras know how to acquire the privilege of being able to be bitten by poisonous snakes without the least danger to life, by inoculating themselves several times with the teeth of rattle-snakes.
Dr. Jacolot,[90] a naval surgeon, while staying at Tuxpan, made enquiries as to these Curados de Culebras, and was able to satisfy himself that their immunity is an actual fact.
The process of vaccination employed by the natives of Tuxpan is as follows:—A preparatory treatment is necessary. On the very day on which a man is to inoculate himself or get himself inoculated, he takes from 5 to 15 tubers of a plant known by the name of Mano de Sapo (i.e., Toad’s hand, Dorstenia contrayerva, Family Urticaceæ). These tubers must—and this is absolutely necessary—be administered on a Friday, and always in an odd number, 5, 7, 9, &c., up to 15, according to the tolerance of the subject.
If the plant be gathered on the first Friday in March it possesses its marvellous properties in the highest degree; in this case, even if it be dry, it is still excellent for the preparatory stage of the inoculation.
The physiological effects of mano de sapo are not very marked: the circulation is slightly diminished and a sensation of cold is experienced, but there are no nervous troubles. The subject frequently has attacks of vomiting or nausea. The inclination to vomit must be fought against, for if the plant should happen to be rejected it would be dangerous to submit to the inoculation.