FLEAS AND PLAGUE
IN order to understand the part played by fleas in the transmission
of plague it is necessary to have some clear elementary knowledge of
the nature of that disease. Plague is an infectious fever caused by
a specific bacterial organism. _Bacillus pestis_ was first identified
in 1894 by Kitasato, a Japanese, and immediately afterwards, but
independently, by Yersin. It is an exceedingly minute, short,
moderately thick, oval bacillus, with rounded ends. It has the most
astounding power of rapid multiplication. Nothing is, at present, known
of its natural history outside the body of the sufferer, but it can
be cultivated. Little is known of its toxic action, but a weak toxin
has been got from cultures. The bacillus itself is not of a resistant
nature and is easily killed by heat and ordinary germicides. Acids
appear to be fatal to it.
In ordinary cases the bacillus is found in buboes. A _bubo_ is nothing
more than an inflamed gland. In so-called septicæmic cases it is found
in the blood of the animal afflicted by the disease. In pneumonic
cases the bacillus may be found in the sputum of the patient. It is
the custom to speak of (_a_) bubonic plague, (_b_) septicæmic plague,
(_c_) pneumonic plague, as though they were three diseases. This is
inaccurate: for they are only forms, with varying symptoms, of one and
the same disease caused by the same bacillus.
The disease which we call plague is, in truth, really a fight between
the afflicted animal and the invading bacillus. It may be inferred
from the fact that bacilli are scarcely ever found in the blood in
bubonic cases that the invaders are stopped by the lymphatic glands
next above the point of inoculation. In such cases the fight, which is
the illness, takes place chiefly in the bubo. In non-bubonic cases the
fight goes on in the blood-vessels or in the lungs as the case may be.
Whether the plague is primarily a disease of rats would be difficult to
say; but rats and other rodents are very susceptible to it. It has also
been transferred to mice, rabbits, guinea-pigs, squirrels, pigs, sheep,
goats, cattle and horses. Men and monkeys are equally susceptible. Cats
and dogs have been known to die of it and during the Great Plague of
London many were destroyed under the belief that they were bearers of
infection.
That plague among human beings was associated with mortality among rats
and mice, is an observation of great antiquity. The student of the
Hebrew scriptures will remember the Book of I Samuel vi. 4: “Then said
they, What shall be the trespass offering which we shall return to him?
They answered, Five golden emerods [buboes] and five golden mice [rats]
according to the number of the lords of the Philistines: for one plague
was on you all and on your lords.”
Eastern authors, of a later date, refer in several places to rats, in
times of plague, staggering about as though they were drunk. The Mogul
Emperor Jehangir in his diary of the plague at Agra (1618) mentions
the unusual mortality of the rats. In India it seems long to have been
a custom, dictated by experience and caution, to leave houses when
rats began to die. In Europe, during the middle ages, the mortality of
rats when the plague was raging does not seem to have impressed the
chroniclers and during the recent outbreak at Glasgow (1900) none was
detected.
As an illness of mankind, the plague reached Europe from the East. We
have no evidence of any outbreak in Europe before the reign of the
Emperor Justinian. When it raged for the first time at Constantinople
(A.D. 542) the mortality was enormous. Ten thousand persons are said to
have died in a day with all the symptoms of bubonic plague.
It spread swiftly through the Roman Empire. In the fourteenth century
the same disease under the name of the Black Death again ravaged
Europe. Again the mortality was enormous. Millions perished little
suspecting that fleas could be connected with their fate. Everywhere
popular tradition reported the plague as the most highly contagious of
all diseases.
In the history of science the plague epidemics in Egypt between 1833
and 1845 are of importance, because the disease was, for the first
time, seriously studied by skilful French physicians. Some of the
French medical school went so far as to deny contagion altogether.
The modern view is that aerial infection may be put aside as almost
impossible except in pneumonic cases; but that plague may be
transmitted by any method which inoculates the blood with _Bacillus
pestis_.
Our modern knowledge dates from the year 1894 when the plague reached
Hong Kong. Its existence as a rat disease was recognised. In the
autumn of 1896, when plague broke out in India, the men of science,
who made careful observations on the spot, were struck by the fact
that infection spread from house to house in a fashion that seemed
inexplicable, unless the bacillus was carried by an animal.
We pass now from rats to fleas. That fleas might be connected with the
spreading of plague was suggested in the year 1897 when Ogata first
found bacilli in fleas. He obtained fleas from plague-sick rats. These
he crushed, and injected the liquid into a couple of mice. One of
these died of plague in three days. The German Plague Commission in
Bombay found plague bacilli in fleas, but, for various reasons, did not
consider that the bite of the flea was the means by which the disease
was transmitted.
The real credit is due to Simond, a Frenchman, who worked during the
Indian epidemics. He took fleas from infected animals and observed in
their stomachs bacilli identical with _B. pestis_. He suggested that
the bacillus was carried from rats to men; and he brought forward some
evidence tending to show that infected fleas could transmit infection
by biting. But Simond was not able to bring forward conclusive
proof. He pointed out a line of research to others which has proved
exceedingly fruitful. In the same year (1898) Hankin suggested that
some biting insect might be the means of transmission from rats to man.
The bacillus of plague has now been identified in ants, bugs, and flies
as well as fleas. It seems likely that any suctorial insect which feeds
on a plague-stricken rat will take numbers of the bacilli into its
stomach.
The points which Simond wished to establish were that plague-stricken
rats with fleas are exceedingly infective, that they cease to be
infective when they have been deserted by their fleas, and that fleas
which infest rats will transfer themselves to man. Since 1905 an
elaborate series of observations and experiments have been carried out.
Post-mortems have been made of countless rats. Numberless fleas have
been collected and dissected. But this summary would be very incomplete
if it did not mention the work of Verjbitski, a Russian doctor at
Cronstadt, whose labours remained almost unnoticed although he made his
experiments as long ago as 1902-1903. His thesis, written in Russian,
was not published in any scientific journal. But his ingenious and
careful experiments showed that fleas could transmit plague from animal
to animal. He found that the commonest flea captured off rats at
Cronstadt was _Leptopsylla musculi_, the usual host of which in other
places is the mouse. Now this flea does not, except very rarely, bite
human beings, and the real significance of the facts discovered was not
appreciated.
The common rat-flea in most parts of Europe is _Ceratophyllus
fasciatus_ and in India and sub-tropical countries _Xenopsylla
cheopis_. This last species has acquired the title of “the plague
flea,” or, more accurately, the oriental rat-flea.
During the plague investigations in India many careful experiments have
been made proving beyond doubt that the disease may be transferred from
rat to rat by the transference of fleas from a septicæmic to a healthy
animal. It was first shown that when fleas were present the plague
could be transferred from rat to rat, kept in proximity, but carefully
screened so as to avoid any possibility of contact. Next, fleas were
collected from rats dead or dying of septicæmic plague and transferred
to healthy rats living in flea-proof cages. More than half of the
healthy rats contracted plague. It was shown that if fleas are present,
the disease once started spreads from animal to animal; and it would
seem that the rate of progress was in direct proportion to the number
of fleas present.
The blood of a plague-infected rat may contain an enormous number of
plague bacilli. Although such figures do not convey any very clear
idea of numbers, as many as a hundred million bacilli have been found
in a cubic centimetre of rat’s blood. A rat-flea, with a stomach of
average size, might receive therefore as many as 5000 germs into its
stomach; and it is clear that fleas feeding on a large proportion of
plague-infected rats just before death would be almost certain to
imbibe at least some plague bacilli. There is, moreover, good evidence
for believing that multiplication of the plague bacilli may take place
in the flea’s stomach. Nor does the blood imbibed by the flea cease to
be infective when it passes from the stomach. Both the contents of the
rectum and the excrements of fleas taken from plague rats often contain
abundant and actively virulent plague bacilli. A number of infected
fleas are put into a test-tube: the mouth of the tube is covered with
a glass slide, and the mouth is turned upside down. The fleas are
then seen to run over the slide, and, in a short time, they deposit
an appreciable amount of fæcal matter on the surface. This under the
microscope is seen to be covered with plague bacilli; and a large
percentage of guinea-pigs, who have an emulsion of the fæcal matter
injected into them, contract plague.
It is remarkable that, so far as we know at present, the plague
bacillus is confined to the flea’s alimentary canal. On rare occasions
it is found in the gullet when fleas have been killed immediately after
feeding on septicæmic blood. But no plague bacillus has been found in
the body-cavity or in the salivary glands.
In the stomach of the flea, plague bacilli have been found in vast
numbers twelve and even twenty days after the insect has imbibed
septicæmic blood. It is naturally of great practical importance to
know how long fleas taken from plague-infected rats remain infective:
that is to say, are capable of transmitting the infection to healthy
animals. Two series of careful experiments, made during the epidemic
plague season in India, have shown that fleas could remain infective
for as long as fifteen days. In a third series of experiments, made
during the non-epidemic season, it was found that the fleas remained
infective for only seven days.
It has been ascertained that both the male and the female oriental rat
(_X. cheopis_) flea can transmit plague.
We come now to one of the most interesting questions of all: namely,
the method by which the rat-flea transmits plague to a healthy animal.
A variety of suggestions have been made, several of which can be
shortly dismissed. It was thought, at one time, that infection might
be conveyed by the animal eating the infected fleas. But it is very
improbable that this means of infection is of any real importance, even
if it may sometimes occur. Experiments in feeding have shown that an
animal is unlikely to become infected by swallowing material containing
plague bacilli, unless the amount is considerable. Moreover we know
that infected fleas confined in test-tubes readily convey the disease
when allowed to bite an animal. In such cases the situation of the
primary bubo corresponds with the area of skin upon which the fleas are
placed. That the transmission of plague is due to the _bite_ of the
flea seems abundantly clear.
It has also been suggested that the proboscis of the flea acts as a
mechanical instrument for the transference of the bacilli. No doubt
the outside surface of the flea’s proboscis must become contaminated,
when it sucks the blood of a plague-stricken rat; but it is difficult
to suppose that contamination of the proboscis can explain cases of
continued infectivity during which the flea has been feeding regularly
upon healthy animals.
Next, there is a hypothesis that the salivary glands of the flea become
infected and that the bacilli are inoculated along with the saliva.
The reader will remember that when a flea sucks, a stream of saliva is
pumped down the mandibles into the puncture. But this hypothesis is
shattered by the fact that plague bacilli are apparently confined to
the alimentary canal of the flea, and that they have never been found
in the salivary glands.
An apparently more probable explanation, that the contents of the
stomach (in which as we know the bacilli may multiply) are regurgitated
and transferred to the wound by the mouth-parts, is rendered less
credible when we remember that there is a valvular arrangement at
the opening of the flea’s stomach which seems to make such a thing
impossible.
Lastly, there remains the only theory on which we have positive
evidence. It is the theory that the bacilli contained in the fæces of
the flea are deposited on the skin and then find their way into the
wound made by the piercing organ. They may be helped in this by the
rubbing and scratching which follow on the bite of the flea. We know,
of course, that plague bacilli are present in abundance in the fæces of
fleas taken from plague-sick rats, and that such fæces are infective
to guinea-pigs both by cutaneous and by subcutaneous inoculation.
Experiments were made to discover whether the pricks made by fleas were
of sufficient size to allow plague bacilli to enter the body, no other
damage to the skin being done. Healthy fleas, confined in a test-tube,
were allowed to feed on a small part of a guinea-pig’s abdomen,
the hair of which had been cropped close without injuring the skin
Immediately afterwards a few drops of the septicæmic blood of a rat
which had died of plague, or of a virulent culture of plague bacillus,
were lightly spread over the part. Many successful infections were
obtained in this way.
Similar experiments were made in which the plague culture was first
spread on the skin, and, afterwards, healthy fleas were allowed to feed
on the same spot. Successful infections were also obtained by this
means.
Two facts then seem to be demonstrated beyond doubt: first, that the
puncture made by a flea will allow the bacillus to gain access to an
animal’s body and to infect it; secondly, that there is a possibility
of infection by the fæces of fleas.
As to whether this is the usual process the highest authorities are not
ready to express any opinion. The safest course appears to be to kill
fleas but to avoid rubbing them in.
Good work was done during the recent outbreak of plague in San
Francisco when the energies of an army of men were directed to
controlling and destroying the rat population. Enormous numbers of
rats were killed, their breeding places were destroyed and everything
was made as uncomfortable for them as possible. Men of science were
at the same time engaged in collecting and examining the fleas from
many thousands of rats. The great success of the work confirmed the
soundness of the theory on which it was based. The spread of the
most terrible of epidemic diseases was controlled and prevented by
knowledge. At San Francisco the fleas of man, rats, mice, dogs, cats,
ground-squirrels and gophers were studied. It was found there, as
elsewhere, that while each species of flea has its particular host few
are unwilling or unable to attack man and other animals when the host
dies.
There is good reason to believe that during the last outbreak of
plague in Manchuria the fleas carried the bacillus from the marmots
(_Arctomys_) to man.
Plague can be transmitted by the human flea; but it may be doubted
whether this often occurs under natural conditions. The rat-fleas seem
inclined to take more readily to mankind than the human fleas do to
rats. Experiments at Bombay seemed to show that, though the human flea
was able to transmit the plague infection, it does not transmit it as
readily as the oriental rat-flea. An explanation of this was obtained
when it was discovered that _Pulex irritans_ does not live well either
on rats, or on guinea-pigs, which were the subjects of the experiments.
A count of the fleas was made, each day, in a number of experimental
cages, in which live human fleas were placed in company with wild
Bombay rats. A great number of human fleas were put into a flea-proof
cage along with a rat. Each day a census was taken of the fleas still
alive. After twenty-fours hours it was found that little more than one
per cent. of the fleas put in could be recovered, and no fleas were
ever found alive after the fifth day.
The European rat-flea (_Ceratophyllus fasciatus_) seems to be quite as
readily able to transmit plague as the oriental insect. How far other
fleas are able to transfer infection we have little or no knowledge.
But twenty-seven experiments to transmit plague from animal to animal
by means of cat-fleas (_Ctenocephalus felis_) were once made and none
of these were successful. The reason for the failure we do not know.
If infected fleas are kept in captivity after they have fed on
septicæmic blood, it is found that, after a while, they are no longer
able to convey infection. On being dissected no bacilli are found in
them. A clearing process, therefore, evidently goes on. If a number
of fleas be fed on a septicæmic rat and, subsequently, be kept under
observation and nourished on healthy animals, the proportion found to
be infected steadily diminishes day by day. It is remarkable that the
existence of numerous plague bacilli in the stomach of a flea does not
seem materially to affect the insect’s life. Fleas, in other words, do
not suffer from plague though they can transmit it.