Part 13

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.

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.

13. _Pulex irritans_ Linn.—This flea is widely distributed throughout the world, and while essentially the human flea has been found on many different species of animals and has frequently been encountered on rats. A very large number of specimens have been taken off rats in San Francisco, Cal.

Genus LŒMOPSYLLA Rothschild.

14. _Lœmopsylla cheopis_ Rothschild.—This is the common rat flea in tropical and subtropical countries. It has also been found in seaports of the temperate zone, where it has been brought by ship rats. Ninety-eight per cent of the rat fleas in India are of this species. It has been found in Australia, where it was described by Tidswell under the name of _Pulex pallidus_. In the Philippine Islands, where it was described by Hertzog as the _Pulex philippinensis_. It has been found in Brazil, where it was described by Baker as _Pulex brasiliensis_, and Tiraboschi has found it in Italy, where it has been described as the _Pulex murinus_. This flea has been frequently found on man in India.

Genus CTENOCEPHALUS Kolenati.

15. _Ctenocephalus canis_ Curtis.—This is the common dog flea found in many parts of the world and is frequently taken off rats.

16. _Ctenocephalus felis_ Bouché.—This is the common cat flea and is also a widely distributed species. Frequently taken off rats.

Genus CTENOPSYLLUS Kolenati.

17. _Ctenopsyllus musculi_ Dugés.—In England this flea is commonly found on the domestic mouse. It has a wide distribution and has been found on rats and mice in Europe, South Africa, India, Australia, Mexico, and other places, and has been taken off _Mus norvegicus_, _Mus rattus_, and _Mus musculus_ in San Francisco, Cal.

Genus NEOPSYLLA Wagner.

18. _Neopsylla bidentatiformis_ Wagner.—This flea has been taken off _Mus norvegicus_ in the Crimea.

Genus HOPLOPSYLLUS Baker.

19. _Hoplopsyllus anomalus_ Baker.—This is one of the common groundsquirrel fleas of California and has been found on _Mus norvegicus_ in San Francisco and Palo Alto, Cal. That these squirrel fleas are occasionally found on rats is interesting from the fact that plague has been demonstrated both in rats and the ground squirrel in California.

Genus HYSTRICHOPSYLLA Taschenberg.

20. _Hystrichopsylla tripectinata_ Tiraboschi.—Reported by Tiraboschi from _Mus musculus_ in Italy.

Genus CTENOPTHALMUS Kolenati.

21. _Ctenopthalmus agyrtes_ Heller.—Taken off _Mus Norvegicus_ in England.

_The results of the identification of 19,768 fleas in San Francisco and Oakland, Cal._

SAN FRANCISCO, 1908. Host: MUS NORVEGICUS. ───────────┬───────────┬───────────┬───────────┬───────────┬─────────── Month. │ C. │L. cheopis.│ P. │ Cten. │ Cten. │fasciatus. │ │ irritans. │ musculi. │ felis, │ │ │ │ │ Cten. │ │ │ │ │ canis. ───────────┼─────┬─────┼─────┬─────┼─────┬─────┼─────┬─────┼─────┬───── „ │Male.│ Fe- │Male.│ Fe- │Male.│ Fe- │Male.│ Fe- │Male.│ Fe- │ │male.│ │male.│ │male.│ │male.│ │male. ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── April to │1,343│2,510│ 485│ 837│ 31│ 76│ 78│ 211│ 16│ 31 July 31 │ │ │ │ │ │ │ │ │ │ August │ 489│ 883│ 145│ 228│ 156│ 206│ 27│ 90│ 17│ 22 September │ 543│1,180│ 655│ 930│ 339│ 387│ 33│ 109│ 46│ 119 October │ 254│ 435│ 509│ 652│ 59│ 64│ 9│ 45│ 6│ 18 November │ 129│ 252│ 256│ 288│ 52│ 69│ 20│ 54│ 6│ 6 ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── │2,758│5,260│2,050│2,935│ 637│ 802│ 167│ 509│ 91│ 196 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴───── Host: MUS RATTUS. ───────────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬───── │ 23│ 43│ 3│ 3│ 0│ 0│ 17│ 16│ 1│ 0 │ 4│ 7│ 1│ 0│ 9│ 16│ 3│ 3│ 0│ 0 ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── │ 27│ 50│ 4│ 3│ 9│ 16│ 20│ 19│ 1│ 0 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴───── Host: MUS MUSCULUS. ───────────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬───── │ 4│ 10│ 1│ 0│ 0│ 0│ 2│ 10│ 0│ 0 │ 11│ 10│ 1│ 6│ 4│ 4│ 0│ 3│ 1│ 1 ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── │ 15│ 20│ 2│ 6│ 4│ 4│ 2│ 13│ 1│ 1 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴───── OAKLAND, CAL., 1909. Host: MUS NORVEGICUS. ───────────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬───── February │ 135│ 304│ 166│ 178│ 1│ 1│ 229│ 506│ 1│ 3 March │ 253│ 456│ 167│ 215│ 2│ 5│ 125│ 243│ 1│ 2 April │ 227│ 479│ 105│ 129│ 0│ 1│ 62│ 143│ 0│ 1 ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── │ 615│1,239│ 438│ 522│ 3│ 7│ 416│ 892│ 2│ 6 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴───── Host: MUS ALEXANDRINUS. ───────────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬───── April │ 1│ 5│ 0│ 0│ 0│ 0│ 0│ 0│ 0│ 0 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────

This does not include a few other specimens of different species taken from _Mus rattus_ and _Mus norvegicus_, which have been included under the heading of “Enumeration of fleas which have been found on rats.”

SYNOPSIS OF FLEAS COMMONLY FOUND ON RATS.