commonly found on rats, but no doubt if search be made they could be
found after feeding on a septicæmic plague rat. These mites, however,
always confine themselves closely to their particular host and are not
known to bite man. The tiny itch mite (_Notoedres alepis_, Railliet
and Lucet) producing rat scabies has, according to Schumann,[202] been
known to cause a cutaneous lesion in man, but this mite need not be
considered from a plague standpoint.
The flea, on the other hand, lives but part of the time on its host,
its eggs developing in the nests or runs of the animal. Again, this
insect does not confine itself to one particular species of host only,
as frequently the flea of one animal is found on an animal of an
entirely different species. Unlike the lice, they are very active and
can readily move from place to place. Not only that, but it has been
frequently demonstrated that the fleas of rats and of other animals
would readily take to man, especially if their natural host was
scarce. That rat fleas will bite man has been demonstrated by Gauthier
and Raybaud, working with the _Leomopsylla cheopis_; Tidswell,
_Lœmopsylla cheopis_ and _Ceratophyllus fasciatus_; Liston,
_Lœmopsylla cheopis_; Tiraboschi, _Lœmopsylla cheopis_; Indian Plague
Commission, _Lœmopsylla cheopis_; and McCoy and Mitzmain[203],
_Lœmopsylla cheopis_, _Ceratophyllus fasciatus_, and _Ctenopsyllus
musculi_. It has generally been considered that the _Ctenopsyllus
musculi_, above all others, would not bite man, but the last-named
observers showed that it would occasionally feed, although it would
not live long, in captivity. One of the fleas, a _Ceratophyllus
fasciatus_, was kept alive by Mitzmain for over four months on man’s
blood alone.
EXPERIMENTS PROVING THAT FLEAS CAN TRANSMIT PLAGUE.
By a series of experiments carried out in specially constructed cages
and go-downs where healthy rats in the absence of fleas were brought
in contact with plague-infected rats, the Indian Plague Commission
showed that the healthy rats would not contract the disease,
notwithstanding the fact that they were not only in intimate contact
with the sick rats, but also with the contaminated food and excreta of
the sick rats. They then showed that if fleas were introduced the
healthy rats would contract plague, the rate of progress of the
epizootic being in direct proportion to the number of fleas present.
By hanging cages containing healthy rats in cages holding infected
rats, but above the jumping distance of a flea, it was shown that the
healthy rats would remain well, while those in cages hung within 2
inches from the ground would contract plague. Thus they excluded
aerial infection. They also found that if fleas were excluded young
rats could suckle a plague-infected mother without contracting the
disease.
Guinea pigs were allowed to run in houses where cases of human and of
rat plague were known to have occurred and where many fleas were
present. These rodents served as good traps for the fleas and 29 per
cent of them contracted plague.
Most of the experiments of the Indian Plague Commission were done with
the Indian rat flea, the _Lœmopsylla cheopis_, but they also performed
27 experiments with the cat flea, _Ctenocephalus felis_, with negative
results; 35 experiments with the human flea, _Pulex irritans_, 3 of
which were successful; and 2 experiments with the _Ceratophyllus
fasciatus_, the common rat flea of Europe and North America, both of
which were successful.
In San Francisco a few experiments under purely experimental
conditions have been carried on by McCoy to determine the ability of
the squirrel flea, the _Ceratophyllus acutus_, to transmit plague.
Fleas that had been previously fed on the blood of a septicæmic
plague-infected squirrel were then allowed to feed from test tubes on
healthy guinea pigs. While the feces of some of these fleas up to four
days, when inoculated into guinea pigs, were proven to be infective,
none of those guinea pigs on which the fleas were allowed to feed
contracted plague. It might be said, however, that in no case were
they seen to eject feces while feeding, the significance of which will
be apparent later.
THE BACILLUS IN THE FLEA.
The Indian Plague Commission found that the average capacity of the
rat flea’s stomach (_Leomopsylla cheopis_) was 0.5 cubic millimeter,
and that it might receive as many as 5,000 germs while imbibing blood
from a plague rat. They further found that the bacillus would multiply
in the stomach of a flea and that the percentage of fleas with bacilli
in the stomach varied with the season of the year. In the epidemic
season the percentage was greatest for the first four days, and on one
occasion the stomach was found filled with _Bacillus pestis_ on the
twentieth day. In the nonepidemic season no plague bacilli were found
in the stomach after the seventh day. They also found that in the
epidemic season fleas might remain infective up to fifteen days, while
in the nonepidemic season but seven days, and in the latter case the
percentage of infection in animals was much less than in the epidemic
season. They showed that while one flea was occasionally able to carry
the infection this was not usual. It was found that both the males and
the females were capable of transmitting the disease.
After a number of dissections they were unable to demonstrate the
presence of bacilli anywhere but in the stomach and rectum. At no time
was anything found in the body cavity or salivary glands and but
rarely in the œsophagus, and then only when the flea was killed
immediately after feeding.
We have in San Francisco examined quite a number of serial sections of
plague-infected fleas with the same result as obtained by the Indian
Plague Commission. The bacilli are readily demonstrated, sometimes in
enormous numbers, in the gizzard, stomach, and in the rectum, but at
no time have they been found in the body cavity, the salivary glands,
or the ovary. In fact, as we are dealing with a vegetable organism and
not an animal organism, like the _Plasmodium malariæ_, we could hardly
expect to find any biologic change, except simple multiplication,
occurring in the intermediate host.
HOW THE FLEA CLEARS ITSELF OF BACILLI.
Some explanation is necessary as to why the bacilli eventually
disappear from the flea, although they seem to multiply during the
first few days. It is evident that the peristaltic action of the
stomach during the course of digestion forcing the blood at the proper
time into the rectum, finally to be ejected from the body, would in
itself cause many bacilli to be discharged, but naturally a few would
remain to multiply indefinitely. The bacteriacidal action of the blood
is soon lost after entering the flea’s stomach, but it has been shown
by proper staining that the leucocytes after the first feeding with
healthy blood contain numbers of _Bacillus pestis_, and it seems
probable that this phagocytic action is important in the cleansing
process. It has been shown that after successive feedings on the blood
of noninfected animals the power of phagocytosis is increased, and
that successive feedings on the fresh blood of animals that have been
immunized against plague still further assists and hastens the
process. When there is a frequent introduction of fresh normal or
immunized blood its bactericidal action is also instrumental in the
cleansing process.
REGIONAL DISTRIBUTION OF FLEAS ON RATS.
The location of the primary bubo in a case of plague, human or rodent,
depends upon the site of inoculation, for that group of glands will
first enlarge which has direct lymphatic connection with the area
through which the _Bacillus pestis_ enters the animal organism. The
British Indian Plague Commission found that 72 per cent of their
naturally infected rats and 61 per cent of the rats experimentally
infected by fleas had cervical buboes, while in no instance in over
5,000 plague rats was a mesenteric bubo encountered. On the other
hand, where plague was induced through feeding healthy rats with the
carcasses of plague rats a mesenteric bubo was found in 74.5 per cent
of those infected and a cervical bubo in 36 per cent. In San Francisco
in naturally infected rats a primary mesenteric bubo has never been
seen, and a cervical bubo has been seen but once. These figures show
conclusively that naturally infected rats are not infected by feeding.
It is curious, as has been pointed out by McCoy[204], that such a
large percentage of cervical buboes should be found in India, while a
cervical bubo has been seen but once in naturally infected animals in
San Francisco. Here the axillary and inguinal buboes are the rule. The
Indian Commission found that the commonest situation to find fleas on
guinea pigs was the head and neck. They combed 53 guinea pigs to
determine the regional distribution of fleas, and found that 65.3 per
cent were taken from the neck and head. This would account for the
preponderance of cervical buboes in guinea pigs observed in their
work, and inferentially for the preponderance of cervical buboes found
in naturally infected rats. Thinking that the predominating rat flea
in San Francisco, the _Ceratophyllus fasciatus_, might be the carrier
of the infection and that it might prefer a different part of the body
than the _Lœmopsylla cheopis_, McCoy and Mitzmain carried on a series
of investigations to determine the regional distribution of fleas on
the rat’s body, but this has shown that while the _Ctenopsyllus
musculi_ seems to be generally confined to the head and neck, the
_Ceratophyllus fasciatus_ and _Lœmopsylla cheopis_ are almost
invariably taken from the body, especially from the pelvic region.
ANATOMY OF THE MOUTH PARTS OF THE CERATOPHYLLUS FASCIATUS.
The following description differs somewhat from that given by
Wagner[205] and the description found in the Journal of Hygiene, both
of which, however, refer to different species of Siphonaptera.
The mouth parts may be divided into those inside and those outside of
the head.
OUTSIDE THE HEAD.
The epipharynx, or pricker, is a long, slender, hollow organ. Its
cavity is closed distally, and proximally connects with the hoemocoel.
It is made up of a dorsal and a ventral portion. Its dorsal portion
ends just within the head. Its ventral portion is grooved and is
continuous with the posterior wall of the aspiratory pharynx. Its
distal extremity is slightly expanded, forming a stylet for piercing,
while the little papillæ seen along the anterior surface in many
species are absent in this one. Laterally there is a membranous
expansion which interlocks with a similar expansion on the mandibles,
forming a tube, through which the blood is sucked.
The mandibles are two in number, articulating just within the head, so
that they are capable, of independent movement. They are serrated at
their distal extremities. Above, within the head, the anterior portion
of the mandibles ends just behind the beginning of the hypopharynx, to
which it is connected, becoming practically continuous with that
organ. The posterior portion is attached to its basal element. Each
mandible contains a groove, forming practically a closed canal, which
becomes continuous with the exit duct of the salivary pump.
The rostrum (labial palpi) forms a protection and guide to the
mandibles and epipharynx. Its first portion is unpaired and
articulates within the head, with its basal element. At the apex of
its first portion it bifurcates, forming a paired organ, which is
divided into a varying number of pseudojoints, depending on the
species of the flea. As it is a chitinous structure, these
pseudojoints, areas in which there is little chitin, enable it to
double up as the mandibles and epipharynx are inserted into the skin.
At the apex of the rostrum are some tactile hairs.
The maxillæ are triangular chitinous plates situated on either side of
that portion of the head where the biting organs emerge. These
structures serve to protect the origin of the epipharynx and
mandibles, rest upon the cutaneous surface in the act of biting,
thereby steadying the head and serving as a fulcrum when the flea
withdraws its biting apparatus when through feeding. The maxillæ have
their palpi, which are four jointed, paired organs coming out at the
anterior lower angle of the head. Their function is sensory.
INSIDE THE HEAD.
The hypopharynx is a chitinous plate forming part of the floor of the
aspiratory canal. To its under surface are attached the muscles which
operate the salivary pump. Its lower portion is connected to the
mandibles, while its upper portion is connected to the posterior
portion of the floor of the aspiratory pharynx by a membranous
ligament.
The aspiratory pharynx extends from the connection of the hypopharynx
with the mandibles to the œsophageal commissure. In a general way it
first passes upward and then turns, passing backward. Its roof is
formed by the continuation of the ventral surface of the epipharynx,
while its floor is formed by the hypopharynx below and above by the
chitinous layer which is continuous with the œsophagus. The anterior
end of this particular portion curves strongly downward, where it is
attached to the upper portion of the hypopharynx by a membranous
ligament. In a general way it may be divided into a vertical and
longitudinal portion. The longitudinal portion expands laterally, so
that its capacity is greatly increased when dilated. Into the floor of
this longitudinal portion empties the vertical part of the aspiratory
pharynx, and at the junction of the two there seems to be a valvular
arrangement, preventing blood from escaping after it has entered the
upper part of the aspiratory canal. The œsophagus starts at the
œsophageal commissure and ends in the gizzard. It is not expanded as
in some insects, forming a gullet, but is practically the same
diameter throughout its entire extent. It is lined with chitin,
surrounded by a delicate basement membrane.
The gizzard is a mushroom-shaped organ, opening into the stomach and
receiving the contents of the œsophagus and the aspiratory pharynx.
From its anterior concave inner surface project a number of
finger-like processes that arise from a basement membrane. They are
lined with chitin, and each one near its base contains an elongated
nucleus. These processes reach to the center of the gizzard and in a
general way point towards the opening into the stomach. The gizzard is
surrounded by circular bands of muscle fibers. Its function is not
entirely understood. Wagner[205] has pointed out that these processes
may act as whips to defibrinate the blood. It is more probable that
their action is mainly valvular, preventing regurgitation of blood
from the stomach.
The stomach of a flea is large and is capable of great distention. It
is composed of a layer of secretory cells, resting on a basement
membrane, the organ being surrounded by muscle fibers passing in
different directions. The epithelial surface is thrown into little
projections like villi. As absorption occurs in the stomach, these
villi, or projections of the epithelial cells, may serve to increase
the absorptive surface as well as serving a glandular function. At the
anterior end of the stomach are the cecal glands.
The intestine is short, receives the excretion from the Malpighian
tubules, and ends in the rectum. In the rectum may be seen the
so-called “rectal glands.” All of the alimentary canal, with the
exception of the stomach, is lined with chitin. The stomach and the
rectum are capable of peristaltic movement.
The salivary glands, four in number, two on each side of the anterior
part of the stomach, are simple acinous glands, lined with a single
layer of secreting cells. The lumen of the glands is large and acts as
a reservoir for the salivary secretion. The ducts from these glands
unite to form a single duct which passes beneath the subœsophagal
ganglion and empties into the salivary pump. This duct is lined on its
inner surface by a spiral arrangement of chitin, giving it a very
characteristic appearance.
DESCRIPTION OF FIGURE SHOWING MOUTH PARTS.
1. Epipharynx.
2. Mandibles.
3. Rostrum, paired portion.
4. Rostrum, unpaired portion.
5. Maxilla.
6. Maxillary palpus.
7. Salivary grooves.
8. Basal element of rostrum.
9. Basal element of mandibles.
10. Salivary pump.
11. Salivary duct.
12. Vertical portion, aspiratory canal.
13. Longitudinal portion, aspiratory canal.
14. Œsophagus.
15. Œsophageal ganglia.
16. Muscles operating aspiratory canal.
17. Hypopharynx.
18. Muscles operating salivary pump.
19. Ligament connecting hypopharynx with floor of aspiratory canal.
[Illustration:
PLATE I.
MOUTH PARTS OF CERATOPHYLLUS FASCIATUS.
]
THE ACT OF BITING.
The epipharynx, or pricker, makes an opening into the skin, through
which the mandibles are inserted. These organs, by means of their
serrations and independent movement, then enlarge the opening as with
a saw, permitting them, with the epipharynx, to pass deeper and deeper
until the points of the maxilla rest upon the cutaneous surface. The
labial palpi serve as a protective case when the organs are not in
action. When in action they serve as a guide to the piercing organs,
but are not inserted into the skin. They double up like a bow, on each
side, the bend of the bow becoming greater and greater as the biting
apparatus passes deeper and deeper. Mitzmain[206] has pointed out that
the spring-like action of this bow may assist the flea to withdraw the
mandibles and epipharynx.
During the process of penetration the salivary pump receives saliva
from the salivary glands and pumps it down, through the channel in the
mandibles, into the wound. It will be seen that the hypopharynx, being
attached above by a membranous ligament and connected intimately with
the mandibles below, moves downward with these organs as they pass
through the skin. At the same time the muscles attached to its under
surface and the salivary pump contract, enlarging the lumen of the
pump. When the mandibles are retracted the salivary pump collapses,
thereby forcing the saliva out with the movement upward of the
mandibles. At the proper time the muscles operating the aspiratory
pharynx contract, drawing the canal open and aspirating blood through
the canal made by the approximation of the epipharynx and mandibles
and into the aspiratory pharynx. When full, the muscles relax from
before backward and the pharynx, by means of the elastic reaction of
its chitinous lining, contracts and forces the blood backward through
the gizzard and into the stomach. It has already been pointed out that
the finger-like processes in the gizzard probably act as valves to
prevent regurgitation from the stomach.
HOW THE FLEA INFECTS ITS HOST.
The exact method by which the flea can transmit plague from animal to
animal has, in our opinion, never been satisfactorily explained. There
have been several explanations offered: First, that the rat may eat
the flea. Miller[207] has found that the _Hepatazoon perniciosum_ is
transmitted from rat to rat through the rat eating the mite, _Lelaps
echidninus_, which acts as the intermediate host. We know, however,
that when a rat is fed on plague material a mesenteric bubo is the
rule, while in naturally infected rats a mesenteric bubo is a rare
condition. This, then, negatives the possibility of plague being
contracted through eating the flea.
Another explanation is that the infection comes from the saliva
injected at the time of biting. We have already stated that after
repeated examinations, both by dissecting out the salivary glands and
by serial sections of the entire flea, plague bacilli have never been
demonstrated in these glands or anywhere outside of the alimentary
tract.
Another explanation has been advanced, that the bacillus is introduced
by the contaminated mandibles. It is not possible to exclude this as a
means of infection, although the Indian Plague Commission made
numerous investigations and was unable to demonstrate the bacillus on
the mandibles.
The possibility of infection taking place by regurgitation from the
stomach has also been considered. As the stomach is guarded by the
finger-like processes in the gizzard which seem to act as competent
valves, and as the movement of the blood aspirated by reason of the
mechanism already explained is in a backward direction, it would seem
improbable that there is any regurgitation from the stomach.
The most plausible explanation that has been advanced has been based
on an observation that blood-sucking insects at the time of biting
frequently eject a drop of blood from the rectum. We know that the
rectum may contain numerous plague bacilli, and it is supposed that
this blood ejected in the vicinity of the bite is either brought in
contact with the slight wound by the feet or mandibles of the flea
itself or is rubbed in as a result of scratching. Verjbitski has shown
that an emulsion of the feces of fleas or any plague material when
placed upon the bitten part before the expiration of twenty-four hours
is sufficient to give the animal plague. After twenty-four hours the
animals did not develop plague, it being supposed that the slight
wound in the skin made by the biting apparatus had healed. It is
probable that this ejection of blood is purely accidental and does not
necessarily occur at the time of biting, but it is likely that the
insect had just previously had a full meal, which had been digested
and passed into the rectum. In the many biting experiments done by
McCoy and Mitzmain they report never having seen this ejection of
rectal contents taking place. It might also be stated that where they
used plague-infected fleas none of the animals developed plague after
being bitten.
ENUMERATION OF FLEAS THAT HAVE BEEN FOUND ON RATS.
Various writers have reported the following fleas taken off rats:
Family SARCOPSYLLIDÆ Taschenberg.
Genus DERMATOPHILUS.
1. _Dermatophilus cæcata_ Enderlein.—Seventeen specimens (females)
were found by Doctor Enderlein on the skin behind the ears of a
specimen of _Mus rattus_ from Saopaulo, Brazil.
Genus ECHIDNOPHAGA Olliff.
2. _Echidnophaga gallinacea_ Westwood.—Tiraboschi has found this flea
on the _Mus rattus_ in Italy.
3. _Echidnophaga rhynchopsylla_ Tiraboschi.—This flea has been taken
in Italy from _Mus rattus_ and _Mus alexandrinus_. It has been
described by Rothschild under the name of _Echidnophaga murina_.
Family PULICIDÆ Taschenberg.
Genus CERATOPHYLLUS Curtis.
4. _Ceratophyllus fasciatus_ Bosc.—This is the common rat flea of
Europe and the United States. It has also been found in Cape Town,
Australia, and is occasionally found on rats in India.
5. _Ceratophyllus londiniensis_ Rothschild.—This flea has been taken
off mice in England; off rats in Italy (_Ceratophyllus italicus_
Tiraboschi) and has been found once on _Mus rattus_ in San Francisco,
Cal.
6. _Ceratophyllus acutus_ Baker.—This is the common flea of the
California ground squirrel; and has been taken off _Mus norvegicus_ in
San Francisco, Cal.
7. _Ceratophyllus anisus_ Rothschild.—This flea has been described by
Rothschild from Yokohama, Japan, taken off _Felis_ sp. One specimen
was found in San Francisco, Cal., taken off _Mus norvegicus_.
8. _Ceratophyllus niger_ Fox.—This flea is commonly found in San
Francisco, Cal., in chicken yards and sparrows’ nests and has also
been found on rats, _Mus norvegicus_, and on man.
9. _Ceratophyllus consimilis_ Wagner.
10. _Ceratophyllus lagomys_ Wagner.
11. _Ceratophyllus mustelæ_ Wagner.
12. _Ceratophyllus penicilliger_ Grube.
These fleas have been taken off _Mus norvegicus_ in Europe.
Genus PULEX Linn.