FLEAS AND PLAGUE
Plague has always been one of the most dreaded diseases, and when we
read of its ravages in the old world and the utter helplessness of the
people before it we do not wonder that the very word filled them with
horror. One of the greatest scourges ever known began in Egypt about
A.D. 542, and spread along the shores of the Mediterranean to Europe and
Asia. It lasted for sixty years, appearing again and again in the same
place and decimating whole communities.
Another great pandemic, beginning in 1364, spread over the whole of the
then known world and appeared in its most virulent form. On account of
diffuse subcutaneous hemorrhages it came to be known as the "black
death" and of course spread terror in all the communities where it
appeared. Whole villages and districts were depopulated. The death-rate
was very high, one authority placing the total mortality at twenty-five
million.
During this time new centers of infection were established, and since
then it has been carried by the commerce of the nations to all parts of
the world. It is not restricted, as many other epidemic diseases, to the
tropics or semi-tropics, although as a matter of fact we find it is more
prevalent in these regions on account of the sanitary conditions.
HOW PLAGUE WAS CONTROLLED IN SAN FRANCISCO
Attention is called to these things in order that we may compare past
conditions with present. During the last few years San Francisco has
been fighting an outbreak of plague that in other days would have been
nothing less than a national calamity. But with modern methods of
handling it, based on knowing what it is, what causes it and how it is
spread, the authorities there have been able not only to hold the
disease in check, but practically to stamp it out with the loss of
comparatively few lives.
Dr. Blue of the Public Health and Marine Hospital Service and his
co-workers directed their whole energy toward controlling the rats. A
small army of men were employed, catching rats in every quarter of the
city. Dr. Rucker reports that fully a million rats were slain in this
campaign. Their breeding-places were destroyed by making cellars,
woodsheds, warehouses, etc., rat-proof and removing all old rubbish.
Garbage cans were installed in all parts of the city, as it was required
that all garbage be stored where rats could not feed upon it, and
altogether every effort was made to make it as uncomfortable as possible
for the rats.
The marked success attending this work abundantly confirms the soundness
of the theory upon which it was based, and serves as another example of
the way in which science is teaching us how to prevent or control many
of our most serious diseases.
THE INDIAN PLAGUE COMMISSION
In 1896, what proved to be a very serious outbreak of plague, occurred
in Bombay and spread to other parts of India. In 1898, a commission was
appointed to inquire into the origin of the different outbreaks, the
manner in which the disease is communicated, etc. This was known as the
Indian Plague Commission, and its exhaustive report, together with the
minutes of the evidence presented to the committee, represents a
stupendous amount of work on this subject and is the basis for much of
the later investigation that has been undertaken.
After the consideration of the evidence from various sources the
commission decided that the principal mode of infection both for man and
rats was through some sort of an abrasion in the skin, although it
recognized also the possibility of infection through the nose and
throat, and possibly, very rarely, through the intestinal tract or other
places.
Considerable time was spent in considering Dr. Simond's claim, made in
1898, that fleas which have been parasitic on plague-infected rats
migrate on the death of their hosts and convey the infection to healthy
men and rats. Dr. Simond sought to establish the following:
"Firstly, that plague rats are eminently infective when infected
with fleas and that they cease to be infective when they have been
deserted by their parasites: Secondly, that living plague bacilli
are found in association with fleas which are taken from
plague-infected rats: Thirdly, that plague can pass from infected
rats to other animals which have not come directly in contact with
them or with their infected excretions: Fourthly, that fleas which
infest rats will transfer themselves as parasites to men."
After reviewing the experiments which had been made to establish these
claims the commission believed that sufficient precaution had not been
taken to prevent infection from other sources and that not enough
definite evidence was produced. Against this claim much negative
evidence was considered and the final conclusion was "that suctorial
insects do not come under consideration in connection with the spread of
plague."
In 1905 another body of men known as the Advisory Committee was
appointed to arrange for further studies in India and other places,
particularly in relation to the mode of dissemination of the disease.
They at once appointed a new working commission who immediately began
their studies and experiments. The preliminary reports of their work,
which are still known as the Reports of the Indian Plague Commission, as
well as the reports of contributing investigations that are being made
from time to time, have served to establish entirely Dr. Simond's claims
and have completely revolutionized the methods of fighting plague.
There are several different types of plague, seeming to depend largely
on the manner of infection. The most common type is that known as the
bubonic plague which is characterized by buboes or swellings in various
parts of the body. This form of infection is usually received through
the skin in some manner or other. Only rarely does direct man-to-man
infection occur though there is always the possibility of it. The
investigations have shown that the flea is the most common agent in
transferring the disease from rat to rat or from rat to man. This may be
accomplished by the flea transferring the bacilli directly from one host
to another on its proboscis, or they may be carried in the alimentary
canal of the flea and gain an entrance into the skin through an abrasion
of some kind when the flea is crushed as it is biting, or when some of
the bacilli are left on the skin in the excreta of the insect.
RESULTS OF VERJBITSKI'S EXPERIMENTS
A very important series of experiments bearing directly on this subject
was made in 1902 and 1903 by Dr. D.T. Verjbitski. The paper giving the
results of this work was not published in any scientific journal until
1908 when the Advisory Committee published it in one of their reports.
The experiments were so well planned and executed and the results so
definite that I think it is worth while to give in full his summary of
results. The bugs referred to are bedbugs.
"(1) All fleas and bugs which have sucked the blood of animals
dying from plague contain plague microbes.
"(2) Fleas and bugs which have sucked the blood of animals which
are suffering from plague only contain plague microbes when the
bites have been inflicted from 12 to 26 hours before the death of
the animals, that is, during that period of their illness when
their blood contains plague bacilli.
"(3) The vitality and virulence of the plague microbes are
preserved in these insects.
"(4) Plague bacilli may be found in fleas from four to six days
after they have sucked the blood of an animal dying with plague. In
bugs, not previously starved or starved only for a short time (one
to seven days), the plague microbes disappear on the third day; in
those that have been starved for four to four and one-half months,
after eight or nine days.
"(5) The numbers of plague microbes in the infected fleas and bugs
increase during the first few days.
"(6) The fæces of infected fleas and bugs contain virulent plague
microbes as long as they persist in the alimentary canal of these
insects.
"(7) Animals could not be infected by the bites of fleas and bugs
which had been infected by animals whose own infection had been
occasioned by a culture of small virulence, notwithstanding the
fact that the insects may be found to contain abundant plague
microbes.
"(8) Fleas and bugs that have fed upon animals which have been
infected by cultures of high virulence convey infection by means of
bites, and the more certainly so the more virulent the culture with
which the first animal was inoculated.
"(9) The local inflammatory reaction in animals which have died
from plague occasioned by the bites of infected insects is either
very slight or absent. In the latter case it is only by the
situation of the primary bubo that one can approximately identify
the area through which the plague infection entered the organism.
"(10) Infected fleas communicate the disease to healthy animals for
three days after infection. Infected bugs have the power of doing
so for five days.
"(11) It was not found possible for more than two animals to be
infected by the bites of the same bugs.
"(12) The crushing of infected bugs in situ during the process of
biting, occasioned in the majority of cases the infection of the
healthy animal with plague.
"(13) The injury to the skin occasioned by the bite of bugs or
fleas offers a channel through which the plague microbes can easily
enter the body and occasion death from plague.
"(14) Crushed infected bugs and fleas and their fæces, like other
plague material, can infect through the small punctures of the skin
caused by the bites of bugs and fleas, but only for a short time
after the infliction of these bites.
"(15) In the case of linen and other fabrics soiled by crushing
infected fleas and bugs on them, or by the fæces of these insects
the plague microbes can under favorable conditions remain alive and
virulent during more than five months.
"(16) Chemical disinfectants do not in the ordinary course of
application kill plague microbes in infected fleas and bugs.
"(17) The rat flea _Typhlopsylla musculi_ does not bite human
beings.
"(18) Human fleas do bite rats.
"(19) Fleas found on dogs and cats bite both human beings and rats.
"(20) Human fleas and fleas found on cats and dogs can live on rats
as casual parasites, and therefore can under certain conditions
play a part in the transmission of plague from rats to human
beings, and vice versa."
RESULTS OF VARIOUS INVESTIGATIONS
Various other plague commissions from other countries as well as many
individuals have investigated the same subject, and the results all
point conclusively to the fact that the rats and the fleas are at least
the most important factors in the spread of the disease. The evidence
from many sources and from many experiments may be briefly summed up as
follows: The disease is caused by the presence in the system of minute
bacteria, _Bacillus pestis_. It is probable that plague is primarily a
disease of rats and only secondarily and accidentally, as it were, a
disease of man.
Rats are subject to the plague and are often killed by it in great
numbers. An outbreak of plague among men is often preceded by a very
noticeable outbreak among rats.
Rats dying of the plague have their blood filled with the plague
bacillus. Fleas or other suctorial insects feeding on such rats take
myriads of these bacilli into their stomach and get many on their
proboscis.
The fleas usually leave a rat as soon as it dies and of course seek some
other source of food. When such infected fleas are permitted to bite
other rats or guinea-pigs these animals often develop the disease.
Several of the species of fleas that infest rats will bite man also, and
in the cases of many plague patients it can be definitely shown that
they had recently been bitten by fleas.
STRUCTURE AND HABITS OF FLEAS
A study of the structure and habits of fleas shows that in many respects
they are particularly adapted for spreading such a disease as bubonic
plague. The piercing proboscis consists of three long needle-like
organs, the epipharynx and mandibles, and a lower lip or labium. The
mandibles have the sides serrate like a two-edged saw. The labium is
divided close to its base so that it really consists of two slender
four-segmented organs which lie close together and form a groove in
which the piercing organs lie. When the flea is feeding, the epipharynx
and mandibles are thrust into the skin of the victim, the labium serving
as a guide. As the sharp cutting organs are thrust deeper and deeper the
labium doubles back like a bow and does not enter the skin. Saliva is
then poured into the wound through minute grooves in the mandibles, and
the blood is sucked up into the mouth by the sucking organ which lies in
the head at the base of the mouth-parts. Just above this piercing
proboscis is a pair of flat, obtuse, somewhat triangular pieces, the
maxillary blades or maxillæ. When the proboscis is fully inserted into
the skin the tips of these maxillæ may also be embedded in the tissue
and perhaps help to make the wound larger. Attached to these maxillæ is
a pair of rather stout, four-jointed appendages, the palpi. They
probably act as feelers.
If the flea chances to be feeding on a plague-infected rat or person
many of the plague bacilli will get on the mouth-parts and myriads of
them are of course sucked up into the stomach with the blood. Those on
the proboscis may be transferred directly to the next victim that it is
thrust into, and those in the stomach may be carried for some time and
finally liberated when the flea is feeding again or when it is crushed
by the annoyed host. The latter is probably the most common method of
infection, for the bacilli that are liberated when the flea is crushed
may readily be rubbed into the wound made by the flea bite or into
abrasions of the skin due to the scratching. Kill the flea, but don't
"rub it in."
[Illustration: FIG. 105--Rat-flea (_Læmopsylla cheopis_); male.]
[Illustration: FIG. 106--Rat-flea (_L. cheopis_); female.]
[Illustration: FIG. 107--Head of rat-flea showing mouth-parts.]
[Illustration: FIG. 108--Human-flea (_Pulex irritans_); male.]
During the recent outbreak in San Francisco many thousand fleas that
were infesting man, rats, mice, cats, and dogs, squirrels and other
animals have been studied and it has been found that while each flea
species has its particular host upon which it is principally found, few
if any of them will hesitate to leave this host when it is dead and
attack man or any other animal that may be convenient.
COMMON SPECIES OF FLEAS
Throughout India and in all the warm climates where plague frequently
occurs the most common flea found on rats has come to be known as the
plague flea (_Læmopsylla cheopus_) (Figs. 105, 106), and is doubtless
the principal species that is concerned in carrying the disease in those
climates. It now occurs quite commonly on the rats in the San Francisco
Bay region and is occasionally found there on man also. In the United
States, Great Britain and other temperate regions another larger
species, _Ceratophyllus fasciatus_ is by far the most common flea found
on rats, and is commonly known as the rat flea. It occurs on both the
brown and the black rats _Mus norvegicus_ and _M. rattus_, on the house
mouse and frequently on man. It has also been taken in California on
pocket gophers and on a skunk.
The common human flea (_Pulex irritans_) (Figs. 108, 109), is found in
all parts of the inhabited world. Although we regard it primarily as a
pest of human beings it often occurs very abundantly on cats, dogs, mice
and rats as well as on some wild mammals such as badgers, foxes and
others and has occasionally been found on birds.
Most entomologists regard the fleas commonly found on cats and dogs as
belonging to one species _Ctenocephalus canis_. Others believe them to
be distinct species and call the cat flea _Ctenocephalus felis_. So far
as our personal comfort and safety is concerned it makes but little
difference to us whether the flea that bites us is called _canis_ or
_felis_ for they both look very much alike, and act alike and the bite
of one hurts just as much as the bite of the other. Although cats and
dogs are their normal hosts they are very often troublesome household
pests, sometimes making a house almost uninhabitable. They are
frequently found on rats, and therefore may carry the plague bacillus
from rat to rat or from rat to man.
GROUND-SQUIRRELS AND PLAGUE
As early as 1903 Dr. Blue, in charge of the plague suppressive measures
in San Francisco, became impressed with the possibility of the common
California ground-squirrels (_Otospermophilus beecheyi_), acting as an
agent in the transmission of plague. It was rumored at that time that
some epidemic disease was killing the squirrels in some of the counties
surrounding San Francisco Bay, notably in Contra Costa County. None of
the squirrels were examined at that time, but since then many thousand
have been carefully studied and it has been definitely shown that many
of them are plague-infected. Just how the plague got started among them
will probably never be really known. There is little doubt, however, but
that it was transferred in some way from the rats to the squirrels. The
trains and the bay and river steamers running out from San Francisco
would afford abundant opportunity for the rats to go from the city to
the warehouses all along the shore. Once there they would use the same
runways as the squirrels about the warehouses and in the near-by fields.
In harvest time the rats migrate to the fields and make constant use of
the squirrel holes. The farmers in some sections report that they
frequently catch more rats than squirrels in traps set in squirrel holes
at that season of the year.
This close association of the rats and the squirrels affords a good
opportunity for the fleas infesting them to pass from one host to the
other.
So far only two species of fleas have been recorded from the
ground-squirrels. One, _Ceratophyllus acutus_, is very common, sometimes
literally swarming over the squirrels, particularly if a squirrel is
sick or weak from any cause. The other species, _Hoplopsyllus anomalus_,
is less abundant but still quite common. Both of these species infest
rats also, so the chain of evidence is practically complete. We have
only to assume that at sometime one or more of the plague-infected rats
found their way into the region where the squirrels were, and the fleas
passing from the rats to the squirrels would carry the plague with them.
The fact that the plague already has such a start among the squirrels
opens a new and very serious phase of the problem of suppressing the
disease. All who have hunted the ground-squirrels will testify to the
readiness with which the fleas from them will bite those who are
handling them. As it is the sick or weak squirrels that are most often
taken there is always a chance that plague may be transferred from them
to human beings. The records of the plague cases in California show at
least three cases in which there seems to be very little doubt that the
disease resulted from handling plague-infected squirrels.
[Illustration: FIG. 109--Human-flea (_P. irritans_); female.]
[Illustration: FIG. 110--Mouse-flea (_Ctenopsyllus musculi_); female.]
A still more serious thing is the possibility of the disease remaining
in a more or less virulent form among the squirrels for some time,
possibly for years, and then breaking out again in some locality where
the rats or men may become infected. As long as there is a trace of the
disease among the squirrels there is always the chance of it spreading,
so that new areas may become infested. Those in charge of the
plague-suppressive measures are fully aware of these dangers and are
making a careful study of the situation and will doubtless be able to
cope with it successfully. It may be that the squirrels will have to be
exterminated in the infected regions. This would be a long and difficult
task, but the success attending the fight against the rats in a great
city shows what can be done when the determination to do it is there.
REMEDIES FOR FLEAS
We have seen how a great city set to work to rid itself of the
plague-sick rats. As a matter of fact it was not the rats that they
were after primarily. If the rats had not harbored fleas the city would
have been glad to let the disease take its course and destroy as many
rats as possible. But it was found that the only way to get rid of the
fleas that might possibly be infected with the plague was to kill their
rat hosts.
General cleaning-up measures will of course very materially lessen the
number of fleas about the private dwellings, but there often remains a
number of fleas in the house that are a source of great annoyance even
if the danger is eliminated.
Particularly is this apt to be so in places where cats or dogs are
members of the household. These animals almost always harbor at least a
few fleas, and where there are a few there is always a possibility, even
a great probability, that there will be many more unless an effort is
made to get rid of them.
In some sections of the country it is the cat and dog flea that is the
most troublesome to man. The minute white eggs of the fleas are usually
laid about the sleeping-places of these animals and the slender active
larvæ that hatch from them feed upon any kind of organic matter that
they can find in the dust or in the cracks and crevices. About eight or
ten days after hatching the larvæ spin delicate brownish cocoons in
which they pass the pupal stage, issuing a few days later as the adult
fleas.
It will at once appear, then, that it is important to provide the cats
and dogs with sleeping-places that can be kept clean. If they have a mat
or blanket to sleep on this can be taken up and shaken frequently and
the dust swept up and burned. In this way many of the eggs or larvæ may
be destroyed. Very often the dust under a carpet that has not been taken
up and dusted for some time will be found to be harboring a multitude of
fleas or their larvæ. In such cases a thorough cleaning of the carpet
and the floors will bring relief. Houses that are unused for some time
during the summer months are often found to be overrun with fleas in the
fall, for the fleas have had an unmolested opportunity to breed and
multiply. Such rooms of course require a thorough cleaning or it is
sometimes possible to kill the fleas by a liberal use of pyrethrum
powder or benzine or to fumigate. In this connection, Dr. Skinner's note
in the _Journal of Economic Entomology_ is worth repeating.
"In the latter part of last May (1908) I moved into a house that
had not been previously occupied. No carpet was used and being
summer only a few rugs were placed on the floors. A part of the
household consisted of a collie dog and three Persian cats. Very
soon the fleas appeared, the dog and cat flea, _Ctenocephalus
canis_. I did not count them and I can't say whether they numbered
a million or only a hundred thousand. On arising in the morning and
stepping on the floor one would find from three to a dozen on the
ankles. The usual remedies for fleas are either drastic or somewhat
unsatisfactory. The drastic one is to send the animals to the
institutions, where they are asphyxiated, or take the other advice,
'Don't keep animals.'
"I tried mopping the floors with rather a strong solution of
creolin but it did little good. Previous experience with pyrethrum
was not very satisfactory. Knowing the volatility of naphthalene in
warm weather and the irritating character of its vapor led me to
try it. I took one room at a time, scattered on the floor five
pounds of flake naphthalene and closed it for twenty-four hours. On
entering such a room the naphthalene vapor will instantly bring
tears to the eyes and cause coughing and irritation of the air
passages. I mention this to show how it acts on the fleas. It
proved to be a perfect and effectual remedy and very inexpensive,
as the naphthalene could be swept up and transferred to other
rooms. So far as I am concerned the flea question is solved and if
I have further trouble I know the remedy. I intend to keep the dog
and the cats."