The venom of Viperidæ, on the contrary, especially that of Lachesis, is characterised by the almost total absence of neurotoxin, while its richness in hæmorrhagin is considerable. Consequently, heating for a few minutes at + 75° C. renders it almost entirely inactive, since hæmorrhagin is very sensitive to heat.
Given venom of some kind or other, the origin of which is unknown, it is therefore possible to ascertain whether the snake from which it was extracted belonged to the Colubridæ or Viperidæ, by determining its richness in neurotoxin resistant to heating at + 85° C.
Certain Viperine venoms, such as those of the European Vipera berus and Vipera aspis, the African Cerastes and American Crotalus contain at the same time a small proportion—varying greatly in amount according to the species—of neurotoxin, and a much larger proportion of hæmorrhagin. It is for this reason that these venoms, although greatly attenuated and deprived of their local action by heating, still remain toxic when injected in large doses into animals after having been heated to + 75° C.
On the other hand, some Colubrine venoms, such as those of Bungarus cæruleus, which are very rich in neurotoxin, contain a quantity of hæmorrhagin sufficient to differentiate their effects in appearance from those produced by cobra-venom, when they are injected, not beneath the skin, but directly into the veins. In this case their effects upon the blood are added to those of their neurotoxin.
It would seem, too, that the venoms of Australian Colubridæ (Hoplocephalus, Pseudechis) form a special group, which is richer in hæmorrhagin than are those of the Colubridæ of the Old World.[99]
On studying, in the case of these various venoms, the action in vitro and in vivo of a purely antineurotoxic antivenomous serum, such as, for example, that of an animal vaccinated against cobra-venom heated to + 75° C., it is found that this serum has a very decided effect upon cobra-venom, and likewise upon that of snakes belonging to allied species (Naja bungarus, Naja haje), and that its action upon the other venoms is less in proportion as they contain less neurotoxin. It prevents hæmolysis in vitro, and suppresses the effects of intoxication on the nervous system, but does not modify in any way the phenomena of coagulation or of proteolysis.
If this serum be made to act in vitro on those Viperine venoms that, when heated to + 75° C. and deprived of their hæmorrhagin, remain neurotoxic, like the venom of the common viper, it is found that it renders them entirely innocuous. Therefore, in the case of all species of poisonous snakes, and perhaps also in that of other poisonous animals (such as scorpions), it appears that the neurotoxic substance is one and the same, and always neutralisable by an antineurotoxic serum like that of animals vaccinated against cobra-venom.
Neurotoxin being the essentially active substance in venoms, and that to which the dangerous properties of poisonous snakes, as regards man and domestic animals, are especially due, it is the effects of this that it is most necessary to prevent. Consequently, the first quality that an antivenomous serum ought to exhibit, in order to be capable of being used in the therapeutics of poisoning, is the possession of an antineurotoxic power as high as possible. This antineurotoxic power is easily obtained by employing cobra-venom for the fundamental immunisation of the horses destined for the production of the serum.
Antineurotoxic serum thus prepared shows itself perfectly capable of preventing all effects of intoxication from cobra-bites, which are much the most frequent in India. In the same way it shows itself quite sufficiently efficacious with regard to Colubrine and Viperine venoms, the neurotoxic activity of which may cause death. But it does not possess any preventive action upon the local effects of hæmorrhagin, to which the noxiousness of certain Viperine venoms—such as those of Lachesis—are almost exclusively due.
In countries in which Viperidæ are very common, we must therefore not confine ourselves to vaccinating the animals that produce serum solely against the neurotoxin of cobra-venom, for instance; we must prepare these animals, after having immunised them to cobra-venom, by injecting them with progressively increasing doses of the various venoms derived from the snakes that are most frequently met with in the district.