animals immune from snake poison was not an easy one, for the process
depends upon training the animal to gradually withstand larger and
larger doses of the venom; and considering the intensely toxic
character of the substance which had to be handled, the danger was
ever present of the animal succumbing to venom poison before its serum
had acquired the requisite pitch of protective power to render it of
service as an anti-toxin. Dr. Calmette tells us that he carried out a
very large number of experiments before he met with success. But it is
not necessary here to discuss his various efforts; suffice it to say
that at length his labours were rewarded, and the following extract
from one of his memoirs describes the methods which he adopted for
this purpose:--
"The best method of procedure for the purpose of vaccinating large
animals destined to produce anti-venomous serum consists in
injecting them from the outset with gradually increasing
quantities of the venom of the cobra mixed with diminishing
quantities of a one-in-sixty solution of hypochlorite of lime.[8]
The condition and the variations in the weights of the animals
are carefully followed, in order that the injections may be made
less frequently if the animals do not thrive well. Quantities
of stronger and stronger venom are in turn injected, first
considerably diluted, and then more concentrated; and when the
animals have already acquired a sufficiently perfect immunity, the
venoms derived from as large a number of different species of
snakes as possible are injected. The duration of the treatment is
of considerable length--at least fifteen months--before the serum
is sufficiently active to be used for the purposes of treatment."
[8] More recently the snake venom employed by Dr. Calmette
for the immunisation of his horses consists of a mixture of
colubrine and viperine poisons, the former making up about
80 per cent. of the mixture. A solution of this mixture is
heated at about 73° C. for half an hour and then filtered,
and injected into horses.
An immense number of animals have been vaccinated by this method at
the Pasteur Institute at Lille, where Dr. Calmette is now director;
and in one of his memoirs we are told that they have horses there
which have yielded during a period of eighteen months serum extremely
active against venom. These horses receive in a single inoculation,
without suffering the least inconvenience, doses of venom sufficient
to kill fifty horses fresh to the treatment.
Large quantities of this serum have been forwarded from the Lille
Institute to various parts of the world where venomous serpents are
most frequently met with, and already important evidence has been
collected as to its efficacy in cases of human beings bitten by
dangerous reptiles. So impressed with its importance are Indian
medical authorities, that its preparation has been included in the
work which the new great bacteriological institute at Agra is carrying
on.
The importance of the production _in situ_ of this anti-venomous serum
has been recently demonstrated by the experiments which have been
conducted in the Plague Research Laboratory, Bombay, by Mr. Lamb and
his colleagues, on the keeping properties of such serums in India.
From the careful investigations which have been made on this subject,
these gentlemen state that anti-venomous serum undergoes a progressive
and fairly rapid deterioration when stored in hot climates, and that
this deterioration is greater and more rapid the higher the mean
temperature to which it is subjected.
The protective potency of this horse-serum may be gathered from the
fact that it suffices to inject a rabbit, for example, with a quantity
amounting to about one two-hundred-thousandth of its weight to ensure
the latter acquiring complete immunity from a dose of venom capable of
otherwise killing it in twelve hours.
The rapidity with which it acts is also extremely remarkable. Thus, if
a rabbit receive two cubic centimetres (about fifty drops) of
anti-venomous serum in the marginal vein of one of its ears, it will
suffer with absolute impunity an injection of venom into the marginal
vein of the other ear capable of killing it under ordinary
circumstances in a quarter of an hour. Its curative powers are not
less remarkable, for it is possible to inject venom sufficient to kill
an animal in two hours, and to let one hour and three-quarters elapse
before administering the antidote, and yet at this late stage to save
the victim's life, although it is necessary where such a long interval
has occurred between the respective venom and serum injections to
employ the latter in larger quantities than is usually required. Dr.
Calmette believes that the anti-toxin may be applied at an even more
advanced stage of the disease if it is employed in yet larger doses.
Another novel and important feature about this anti-venomous serum is
the fact that it not only protects animals from one species of very
active venom, such as that of the cobra and other poisonous snakes,
but it also affords protection from the dreaded venom of scorpions.
This is a very remarkable and significant discovery, for hitherto the
opinion has been stubbornly held that each toxin requires its specific
anti-toxin for its correction. Dr. Calmette has, however, frequently
indicated by his researches that this view cannot be considered so
completely proven as is claimed by its supporters, and his latest
investigations support the theory that particular toxins may be
counteracted by several anti-toxins of different origin. Thus it has
been shown by Calmette and Roux that rabbits hyper-vaccinated against
rabies acquire the power of resisting venom-poison, and that the serum
of horses vaccinated against tetanus or lock-jaw also nullifies the
action of serpent venom.
The practical bearing of this discovery is obvious, and the hope is
justified that the at present cumbrous appliances required for the
elaboration of anti-toxins of such varied origin will ultimately give
way to simpler and less costly methods, which will admit of these new
antidotes being more widely circulated and applied.
We have seen that although most animals fall an easy prey to serpent
venom, yet there are a few notable exceptions, amongst which may be
mentioned hedgehogs, swine, and the mongoose. Now the very natural
question arises why, if these animals are already in such a high
degree immune from this poison, should not they be employed to furnish
forth protective serum, instead of laboriously training up susceptible
animals to become artificially immune and supply this venom
anti-toxin?
This brings us face to face with one of the many problems connected
with the subject of immunity which so far have successfully eluded all
attempts made to solve them. Experience has shown repeatedly that
although _artificially_ acquired immunity from a particular poison can
be handed on by means of an animal's serum, yet the _natural_ immunity
from a given poison enjoyed by one species of animal cannot be
similarly transferred to less-favoured varieties.
This fact has long been recognised in the case of poisons of bacterial
origin. Thus, white rats are absolutely immune from diphtheria, but
Wassermann showed some years ago that the serum of these animals has
no power whatever to counteract the action of diphtheria-toxin in
other animals. Guinea-pigs were inoculated with fatal doses of
diphtheria toxin along with white-rat serum; but although other
guinea-pigs treated with the same toxin mixed with the ordinary
artificially elaborated anti-diphtheritic serum survived, those which
received the rat serum died in every case.
Now very similar results have been obtained by Calmette in respect to
the serum of animals naturally immune from serpent venom. The serum of
the refractory little mongoose, as well as that of the hedgehog, is
wholly unable to save other animals from the lethal effect of venom
poison, and similar results have been noted in respect to swine serum.
But a very curious fact has also been discovered by Calmette--_i.e._
that these so-called naturally immune animals very frequently are
quite incapable of being artificially trained to elaborate a serum
possessing protective powers which can be transferred to another
animal.
How splendid a domain for beneficent research lies before the
scientific investigator is apparent to all, and the important work
already accomplished is but an augury of yet greater discoveries
awaiting the labours of such leaders as Calmette. It is not
surprising, therefore, that the scientific interest in toxins and
anti-toxins shows no signs of abatement. On the contrary, the
competition for obtaining and working the new "claims" which pioneer
research enthusiasts are constantly engaged in "pegging out" remains
as keen as ever.
Despite, however, the extraordinary interest which this subject has
aroused in scientific circles all over the world, nearly ten years
elapsed before any notice was taken of the curious discovery made by
two brothers that the blood of eels contained a highly poisonous
principle, and the memoir containing this remarkable announcement
remained until comparatively recently buried in the Italian journals
where it was first published.
Calmette was, we believe, the first to call attention to this
discovery of the brothers Mosso and give it the prominence it
deserves, and both he and other investigators have not only fully
confirmed it, but have greatly added to our knowledge concerning the
character of the poison contained in eel serum.
Now the venerable Izaak Walton, in one of his quaint and most
fascinating discourses, which although written more than two centuries
ago have a freshness as if penned but yesterday, waxes enthusiastic
over the eel, and supplies an elaborate recipe for its preparation for
the table, telling us "it is agreed by most men that the eel is a most
dainty fish; the Romans have esteemed her the Helena of their feasts,
and some the queen of palate-pleasure." The announcement that the
blood of eels is poisonous will hardly, despite its scientific
interest, form a comfortable subject for reflection to the modern
votaries of this novel Helena. Indeed, in the present timid temper of
the public, this article of diet would not improbably share the
ill-odour which befell the unfortunate oyster and be practically
banished from our tables; but although the oyster is perhaps
justifiably at present ostracised from our _menus_, taking the
majority of its breeding-grounds into consideration, it would be the
height of injustice to measure out similar drastic treatment to the
eel.
That the oyster bred in sewage-contaminated beds may revenge itself
upon its consumer by infecting him with the germs of typhoid has been
repeatedly contended, but that the eel, although its unsavoury
surroundings are proverbial, can be held responsible for poisoning
those who eat it has never, we believe, been seriously maintained,
although there is an old Italian saying which bids us "give eels and
no wine to our enemies."
Public confidence, however, in the eel as an article of food need not
be shaken, for it is satisfactory to learn that researches which, on
the one hand, condemn eels as living generators of a highly poisonous
substance, on the other hand allay any alarm which they may have
reasonably raised by showing that this toxic principle is entirely
destroyed in the processes of digestion, and that, therefore, taken
through the mouth it is rendered harmless, and only when introduced
into the system by inoculation beneath the skin or injected into the
peritoneum can it assert its dangerous properties. That the blood of
eels is, however, justifiably to be in future classed amongst the
toxins, the number of which has of late been so increased, is at once
apparent when we learn that about a dozen drops inoculated into a dog
weighing about fourteen pounds will destroy the latter in less than
ten minutes, whilst pigeons, rabbits, and guinea-pigs similarly
treated, only with smaller quantities, also invariably succumb to its
lethal action.
Quite recently an endeavour has been made to determine precisely the
degree of toxicity possessed by eel's blood, or, in other words, to
standardise the poisonous principle contained in it, so as to afford a
guide to those experimenting on the subject; and it has been asserted
that one cubic centimetre, or about twenty drops, injected into the
veins of a rabbit weighing four pounds, may be regarded as a fatal
dose for such an animal. But many difficulties surround such an
attempt to exactly define the degree of toxic action possessed by such
a substance, for, in the first place, the blood varies in respect to
this property in different eels, whilst it also differs widely in
character at different stages of the life of the fish. This seasonable
variation in toxic character has been noticed in the case of viper
venom, which it will be remembered was shown to be far more lethal in
action when collected from snakes in the spring of the year than in
the winter months.
The toxic substance contained in eel serum was originally called by
its discoverers, the Mosso brothers, _ittio-tossina_; and they record
the fact that the blood of rabbits and frogs, which animals had
succumbed to its action, did not coagulate after death, whilst,
curiously, in the case of dogs this abnormal phenomenon was not
observed.
There are various means which may be resorted to for destroying the
poisonous principle contained in eel blood, and from a dietetic point
of view it is satisfactory to know that heat-exposure for a quarter of
an hour to a temperature of from 57·7° to 77·7° Cent. entirely removes
it, whilst its virulence is greatly modified by submitting it for a
longer period, twenty-four hours, to a much lower temperature, _i.e._
37° Cent. It also gradually loses its toxic properties eight days
after it has been collected, even when carefully shielded from light,
a feature which contrasts favourably with viper venom, which can be
kept for more than a year and remains as active as when first derived
from the snake. We have seen also that its toxic properties invariably
succumb to the processes of digestion, so that even if fashion or fad
or advertising speculators, backed by scientific names, were to decree
that a wealth of nourishment and support was contained in raw eel
"juice," and the edict went out that it was a desirable and highly
important article of invalid diet, the general public may, according
to its wont, innocently accept the edict and in this case suffer no
evil consequences.
But another and very remarkable method of mitigating the virulence of
eel blood, and one which so far has received no explanation, is
mentioned by Dr. Wehrmann, of Moscow, who has been lately studying the
character of this fish's blood in Dr. Calmette's laboratory at the
Pasteur Institute at Lille. Dr. Wehrmann found that if blood serum be
taken from animals previously rendered artificially immune to the
action of serpent venom, and if some of this so-called anti-venomous
serum be injected under the skin of eels some hours before they are
killed, the lethal properties of their blood after death are
considerably reduced. Thus, an eel weighing about six ounces received
subcutaneous injections of five cubic centimetres of anti-venomous
serum; after the lapse of four-and-twenty hours it was killed and
bled, and its serum inoculated into animals in the usual way. But
whereas two cubic centimetres of normal eel blood sufficed to kill a
guinea-pig, this eel's blood had to be administered in _twice_ that
quantity to produce a fatal result, so that its toxic character had
been reduced to a very appreciable extent. The readiness with which
eel serum parts with its lethal properties, and the restricted
conditions under which they can operate, sufficiently assure us that
in the present state of our knowledge there is no danger to be
apprehended from this fish, and in the absence of any experiments to
show what is the effect on human beings of subcutaneous inoculations
of such blood, there is no call for this substance to be scheduled
under the Poisons Act. We have, however, by no means exhausted the
extremely curious properties which characterise this material, and
these properties are brought to light in a remarkable manner in
connection with the investigations which have been carried out to
artificially protect animals from its lethal influence, and also in
some interesting experiments which have been made to compare the
toxicity of eel blood with that of vipers.
It is far from an easy matter to secure for experimental purposes an
adequate supply of eel serum, for even a big fish weighing nearly five
pounds is not capable of yielding more than about twenty-five cubic
centimetres of blood, and from this only from ten to twelve cubic
centimetres of serum are obtainable. Calmette has shown that not only
the venom glands of reptiles contain toxic substances, but that the
blood of such snakes also possesses lethal properties, only in a far
less degree. Curiously, the serum of eels is no less than three times
as toxic as the serum of the most vicious viper, and, moreover,
produces far more discomfort and pain to the animals into which it is
introduced than accompanies the injection of viper blood. In the case
of viper blood its introduction is followed by no symptoms of
discomfort, the animal remains quite quiet, growing more and more
somnolent, a condition which is followed by an abnormal fall of
temperature, ultimately ending in complete collapse, symptoms which in
a much more modified degree characterise the injection of _heated_ eel
serum into animals. This heated eel serum, which we have seen is
deprived of the objectionable characteristics of ordinary eel serum,
produces but very transitory symptoms in animals, occasioning some
degree of somnolence, and now and again a reduction in temperature, a
condition from which, however, the animals rapidly recover in from two
to three hours. Animals, however, treated with this heated eel serum
acquire a power of resisting the lethal action of unheated or ordinary
eel serum, and this artificially induced condition of immunity
continues for about three days after the completion of the treatment.
The protective properties of this heated serum are not restricted to
animals subsequently inoculated with eel serum, but are extended also
to animals which afterwards receive injections of viper serum; but of
much greater interest and importance is the remarkable fact that
heated eel serum, as well as weak doses of the latter not heated but
diluted with water, are capable of protecting animals from the fatal
consequences of the far more potent viper venom.
It is interesting to note that, although diluted eel serum can protect
an animal from so deadly a poison as viper venom, the serum of vipers
is quite unable to afford any such service in the case of animals
inoculated with ordinary eel serum. The full complement of protective
power obtainable from this treated eel serum is only able to slowly
assert itself, for it is necessary for a period of as long as
twenty-four hours to elapse after its introduction to ensure the
animal's system being thoroughly impregnated with it and enable it to
withstand a lethal dose of viper venom.
In this respect, what may be designated treated or protective eel
serum differs very markedly from anti-venomous serum, which we have
seen is serum derived from animals trained up to withstand fatal does
of serpent venom, for anti-venomous serum acts immediately, and at
once confers immunity on an animal from the lethal effects of such
venom.
The rapidity with which it acts is indeed one of the most astonishing
properties of this particular anti-toxin. Thus if two cubic
centimetres of anti-venomous serum be inoculated into the marginal
vein of a rabbit's ear, it at once confers upon the latter complete
immunity from snake poison. Immediately after the injection of the
serum, venom sufficient to destroy an ordinary rabbit in a quarter of
an hour may be injected with impunity into the vein of the other ear.
But not only are the _protective_ powers of this serum so remarkable
in their degree, but its _curative_ powers, a much more difficult
property to establish in a substance, are extraordinarily intense, as
may be gathered from the following example. Four rabbits were
inoculated with a quantity of venom calculated to destroy them in the
space of two hours; one of these four animals was abandoned to its
fate, but the other three received, practically at the eleventh hour,
viz. just fifteen minutes before the expiration of the calculated two
hours' respite, an intravenous injection of a small quantity of
anti-venomous serum, only amounting to one four-hundredth part of the
weight of each animal respectively. The rabbit which received only the
venom died at the end of two hours, whilst the other three remained in
perfect health.
But although eel serum can be persuaded to part with its poisonous
character and even exercise protective powers over otherwise doomed
victims, it is not able to stretch forth a healing hand to the
afflicted, for, when once the poison has been introduced, whether it
be eel or viper blood, or the venom of snakes, it is absolutely
powerless to mitigate or stop in any way the deadly progress of the
toxin. Thus whilst eel blood may acquire _protective_ properties it
cannot acquire _curative_ properties, and, therefore, treated eel
serum cannot be legitimately enrolled with the anti-toxins which have
been elaborated, as, for example, anti-venomous serum, for, to be
worthy of such rank, a substance must be capable of wielding both
protective and curative powers.
But, although eel serum may under certain conditions protect from the
lethal action of serpent venom, eels are not themselves under ordinary
circumstances endowed with any power to withstand the influence of
this poison, for a good-sized eel will succumb to a dose of venom
which is sufficient to kill a guinea-pig.
Considerable interest is attached to the fact that anti-venomous serum
not only acts as an anti-toxin towards serpent venom, but also towards
a poison of quite a different character, such as that present in the
normal blood of eels, for this fact tends to confirm the view upheld
by some authorities, that specific toxins do not necessarily only
yield to specific anti-toxins, and that a particular anti-toxin may
act as such towards divers toxins of varied origin and character.
Calmette has brought this point out very clearly in his later
investigations on the vegetable poison abrine, a very powerful toxin,
furnished by the active principle of the seeds or beans of a
leguminous plant common in India and South America, and frequently
used, as already mentioned, by the natives in India to revenge
themselves on their enemies in poisoning their cattle. Immunising
serums of various kinds were selected for testing their protective
action on animals poisoned with abrine, and it was found that
anti-tetanic, anti-diphtheritic, anti-anthrax, and anti-cholera serums
all individually exerted a decided immunising action with regard to
this powerful vegetable poison. The hope is, therefore, perhaps not
beyond the realm of possibility, that at some future time the
complexity of drugs which now figure in the chemists' pharmacopoeia
may be replaced by a few substances the application of which will come
within the means and understanding of all. So far we have not dealt
with the artificial immunisation of an animal from the action of eel
poison, but this apparently offers very little difficulty, and is
accomplished by introducing very small and gradually increasing doses
of eel serum into the system, care being taken to proportion the
quantity given according to the weight and general condition of the
animal to be immunised. A rabbit, for example, treated in the above
manner, subsequently yielded a serum which was proved to possess both
preventive and curative powers in respect to both eel poison, and
viper venom and blood, entitling this so called anti-eel serum to take
its place amongst the anti-toxins, and furnishing yet another instance
of a substance exercising its immunising influence over various
toxins.