If the animals prove resistant, the vaccination can thenceforth be pushed on rapidly, and the quantity of venom injected each time can be increased, testing the susceptibility of the organism by the variations in weight.
As a rule, three months are necessary for the vaccination of a rabbit against twenty lethal doses. In six months we can succeed in making it very easily withstand 100 lethal doses.
The serum of rabbits thus treated soon, i.e., after they have received from five to six lethal doses, exhibits antitoxic properties in vitro; these, however, are not very pronounced until after prolonged treatment. They gradually become just as intense as those observed in the case of animals vaccinated against diphtheria or tetanus.
In 1895 Fraser confirmed these results,[98] and on May 15 in that year exhibited before the Medico-Chirurgical Society of Edinburgh a rabbit vaccinated against a dose of cobra-venom fifty times lethal.
At once considering the possibility of obtaining serums highly antitoxic against snake-venoms, and of practical utility in the therapeutics of snake-bites, I prepared to vaccinate a certain number of large animals, horses and donkeys, in order to procure great quantities of active serum. I at first experienced some difficulties in providing myself with a sufficient store of venom. But thanks on the one hand to the obliging collaboration of some of my old pupils or colleagues, and on the other to the valuable co-operation of the Colonial Governments of Indo-China, the French Settlements in India, and Martinique, I soon received poisonous snakes and dried venom in abundance.
Fig. 93.—Vaccinating a Horse against Venom at the Pasteur Institute at Lille.
After this I was not long in pushing the vaccination of a few horses until I made them resist, in a single injection, 2 grammes of dry cobra-venom, a dose about eighty times lethal; for I was able to satisfy myself that about 0·025 gramme of cobra-venom was sufficient to kill fresh horses in from twelve to twenty-four hours.
The immunisation of horses to this very high degree of tolerance of venom is not obtained without difficulties; many animals succumb in course of treatment from endocarditis or acute nephritis; in the case of others, each injection of venom leads to the formation of enormous aseptic abscesses, which have to be opened and drained. It may be said that on an average an interval of sixteen months is necessary in order to obtain a serum sufficiently antitoxic.
