_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.
[Illustration: FIG. 89.--MONGOOSE SEIZED BY A COBRA.
(For this illustration I am indebted to the kindness of M. Claine, late
French Consul at Rangoon.)]
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.