CHAPTER XI.

_NATURAL IMMUNITY OF CERTAIN ANIMALS WITH RESPECT TO SNAKE-VENOMS._

It was long ago pointed out that certain warm-blooded animals, including the mongoose (_Herpestes ichneumon_, Family _Viverridæ_), hedgehog (_Erinaceus europæus_, Family _Erinaceidæ_), pig (_Sus scrofa_, Family _Suidæ_), and some herons (_Ajaja_, Subfamily _Plataleinæ_; _Cancroma_, Subfamily _Cancrominæ_; _Botaurus_, Subfamily _Ardeinæ_; _Mycteria_, Subfamily _Ciconiinæ_), known in Colombia under the names _Culebrero_ and _Guacabo_, exhibit a natural immunity with respect to snake-bites.

Pigs devour vipers with great readiness, and in the region of North America which adjoins the Mississippi and its tributaries they are even trained to destroy the young rattle-snakes and other poisonous serpents with which the valleys of these watercourses are infested.

During my stay in Indo-China I inoculated a young pig, beneath the skin of the back, with a dose of _Cobra_-venom (10 milligrammes) capable of killing a large-sized dog. The animal withstood the injection, but I am inclined to think that this is not a case of true immunity; it is probable that the pig owes its resistance to venom to the fact that its skin is lined with an enormous layer of adipose tissue, which is but very slightly vascular, and in which absorption takes place very slowly. This opinion is corroborated by my discovery that the serum of this animal is entirely destitute of any antitoxic substance. I mixed a dose of _Cobra_-venom, lethal for the rabbit, with 3·5 and 8 c.c. of pig-serum. These mixtures killed rabbits in the same time as the controls that received the venom diluted with equal quantities of rabbit-serum or physiological saline solution.

The natural immunity of the mongoose and the hedgehog rests upon more scientifically established facts.

My own experiments upon the immunity of the mongoose were made with six specimens of these little carnivores captured in Guadeloupe (French West Indies), an island in which no poisonous snakes exist; consequently their immunity could not have arisen from their having become accustomed to the bites of venomous reptiles.

I first introduced a mongoose into a cage containing a _Naja bungarus_ (_Ophiophagus_) of large size. The snake rose up immediately, dilated its hood, and struck savagely at the little animal, which, darting nimbly out of the way, escaped being seized and, frightened for a moment, took refuge in a corner of the cage. Its stupor, however, was but of brief duration, for at the very moment when the hamadryad was preparing to strike at it again, the mongoose, with open mouth and snarling, sprang upon the reptile’s head, bit it hard in the upper jaw and crushed its skull in a few seconds. This scene is in every respect reminiscent of the admirable description given by Rudyard Kipling, in his celebrated “Jungle Book,” of the great war that _Rikki-tikki_ (the Mongoose) fought with Nag (the Cobra) “through the bathrooms of the big bungalow in Segowlee cantonment”:--

“Nag was asleep, and Rikki-tikki looked at his big back, wondering which would be the best place for a good hold. ‘If I don’t break his back at the first jump,’ said Rikki, ‘he can still fight; and if he fights--O Rikki!’ He looked at the thickness of the neck below the hood, but that was too much for him; and a bite near the tail would only make Nag savage.

“’It must be the head,’ he said at last; ‘the head above the hood; and when I am once there, I must not let go.’

“Then he jumped. The head was lying a little clear of the water-jar, under the curve of it; and, as his teeth met, Rikki braced his back against the bulge of the red earthenware to hold down the head. This gave him just one second’s purchase, and he made the most of it. Then he was battered to and fro as a rat is shaken by a dog--to and fro on the floor, up and down, and round in great circles; but his eyes were red, and he held on as the body cart-whipped over the floor, upsetting the tin dipper and the soap-dish and the flesh-brush, and banged against the tin side of the bath. As he held he closed his jaws tighter and tighter, for he made sure he would be banged to death, and, for the honour of his family, he preferred to be found with his teeth locked. He was dizzy, aching, and felt shaken to pieces, when something went off like a thunderclap just behind him; a hot wind knocked him senseless, and red fire singed his fur. The big man had been wakened by the noise, and had fired both barrels of a shot-gun into Nag just behind the hood.”[83]

From the experimental point of view, these stirring battles between mongooses and cobras only show that a mongoose of the size of a large squirrel makes a plucky and victorious attack upon a venomous reptile of the most dangerous species and of very large dimensions; but it is impossible to tell with certainty whether the mongoose has been bitten.

I therefore inoculated a second mongoose with 2 milligrammes of venom, a lethal dose for 4 kilogrammes of rabbit. The animal did not experience the slightest malaise.

I then took blood from three other mongooses, by tying a carotid without killing the animals. This blood, mixed with venom or injected as a prophylactic into rabbits, exhibited an antitoxic power, which, though evident, was of little intensity, and insufficient in all cases as a certain preventative of death. All the rabbits that received a preventive dose varying from 2 to 7 c.c. of mongoose-serum succumbed to inoculation with venom, but with a considerable retardation (from two to five hours) as compared with the controls.

I endeavoured to determine the limit of tolerance of the mongoose with respect to venom. Two of these animals, which had never been inoculated, received doses of venom respectively four times and six times lethal for the rabbit. The first mongoose remained perfectly well; the second was ill for two days, and then recovered. A third mongoose, into which I injected a dose eight times lethal for the rabbit, succumbed in twelve hours.

It must be concluded from these facts that the West Indian mongoose is but little sensitive to venom; that it is capable of withstanding, without malaise, doses which are considerable in proportion to its size, but that its immunity is far from being absolute. If it is generally the victor in its combats with poisonous snakes, the result is mainly due to the extreme agility with which it is endowed.

A number of experiments have been made by Lewin,[84] and by Phisalix and Bertrand,[85] upon the immunity of the hedgehog to the venom of _Vipera berus_.

It had long been known that hedgehogs are inveterate hunters of vipers, which they devour with avidity. Thanks to the long and sharp spines by which their bodies are protected, they avoid being bitten and contrive to catch the reptiles very cleverly, but it occasionally happens that they do not escape being struck. However, even in these cases they rarely succumb.

Inoculation with fairly large quantities of venom does not make them ill: the dose of _viper_-venom lethal for these small animals is about forty times greater than that which kills the guinea-pig. Their power of resistance is therefore beyond doubt.

It may be asked whether this is due to their blood normally containing antitoxic substances. In order to elucidate this question, Phisalix and Bertrand first proved that the blood of normal hedgehogs is toxic to laboratory animals, especially to the guinea-pig. A mixture of this blood with viper-venom cannot therefore be harmless. But it sufficed to heat hedgehog blood to 58° C. to cause it to lose its toxicity, and it was found that it then became antitoxic. Guinea-pigs inoculated in the peritoneum with 8 c.c. of heated hedgehog-serum were able to withstand, immediately afterwards, twice the lethal dose of viper-venom.

It really seems, therefore, that the resistance of the hedgehog to venom is due to the presence of antitoxic substances in its blood. But, as in the case of the mongoose, there is no question here of genuine _immunity_.

The same is probably true with respect to the herons of Colombia, the _Culebrero_ and _Guacabo_, which eagerly search after young snakes for food. No investigations, however, have yet been made upon this subject.

These birds, moreover, are few in number; hunters pursue them for the sake of their brilliantly coloured plumage, and it is to be regretted that no attempt is made to prevent their destruction or to acclimatise them in countries in which poisonous snakes constitute a veritable calamity, such as Martinique, St. Lucia, or India.