Part 11
_muris_ Zeder: Echinorhynchus.—Stomach.
ARACHNOIDEA:
_coarctata_ Heyden: Myobia.—External. [See musculi Schrank.] _musculi_ Oudemans: Demodex.—Hair follicles. [See folliculorum musculi.] _musculi_ Schrank: Pediculus, Myobia.—External, head. _musculinus_ Galli-Valerio: Myocoptes.—External. _simplex_ Tyrrell: Psorergates.—External.
INSECTA:
_acanthopus_ Burmeister, 1838: Hoplopleura, Hæmatopinus.—External. _agyrtes_ Heller: Typhlopsylla.—External. _assimilis_ Taschenberg: Typhlopsylla.—External. _charlottensis_ Baker: Odontopsyllus.—External. _fasciatus_ Bosc: Ceratophyllus.—External. _italicus_ Tiraboschi: Ceratophyllus.—External. _londiniensis_ Roth.: Ceratophyllus.—External. _musculi_ Dugès: Ctenopsyllus, Ctenopsylla, Typhlopsylla.—External. _serratus_ Burm., 1838: Hæmatopinus.—External. _serraticeps_ Taschenberg: Ctenocephalus.—External. larva of a dipteron, gen. sp.? _taschenbergi_ Wagner: Ctenopsyllus.—External. _tripectinata_ Tiraboschi: Hystrichopsylla.—External. _walkeri_ Roth.: Ceratophyllus.—External.
MUS MUSCULUS ALBUS.—White Mice.
CESTODA:
_fasciolaris_ Rudolphi: Cysticercus.—Liver.
ARACHNOIDEA:
_crotali_ Humboldt (larva): Porocephalus.—Encysted in various organs. _proboscideum_: Pentastomum. [See crotali.]
MUS NAVALIS.
NEMATODA:
_labiodentata_ Linstow: Spiroptera.—Intestine.
MUS NORVEGICUS Erxl.—Norway Rat.
[See also _Mus decumanus_.]
PROTOZOA:
_lewisi_: Trypanosoma, Trypanozoon.—Blood.
INSECTA:
_bidentatiformis_ Wagner: Ctenophthalmus.—External. _brasiliensis_ Baker: Pulex.—External. _fasciatus_ Bosc: Ceratophyllus.—External. _italicus_ Tiraboschi: Ceratophyllus.—External. _murinus_ Tiraboschi: Pulex.—External. _musculi_ Dugès: Ctenopsyllus.—External.
MUS [NORVEGICUS] ALBUS.—White Rat.
PROTOZOA:
_muris_ Fantham: Piroplasma.—Blood. _perniciosum_ Miller: Hepatozoon.—Liver, blood.
CESTODA:
_fasciolaris_ Rudolphi: Cysticercus.—Liver.
NEMATODA:
_hepaticum_: Trichosoma.—Liver. _spiralis_ Owen, 1835: Trichinella.—Adult in intestine, larva in muscle.
ARACHNOIDEA:
_ensifera_ Poppe: Myobia.
MUS PUMILIS Dujardin.—Little Mouse.
CESTODA:
_murina_ Dujardin, 1845: Tænia, Hymenolepis.—Intestine. [See fraterna.]
MUS PYRRHORHINUS Neuwied.
[See also _Hesperomys pyrrhorhinus_.]
CESTODA:
_diminuta_: Tænia.—Intestine.
ARACHNOIDEA:
_crotali_ Humboldt: Porocephalus.—Encysted in various organs. _subcylindricum_: Pentastomum.—Liver.
MUS RAJAH.
CESTODA:
_blanchardi_ Parona: Davainea.
MUS RATTUS Linné.—German Hausratte.
PROTOZOA:
“amibes.” _intestinalis_ Lambl, 1859: Lamblia.—Intestine. [See duodenalis, muris.] _lewisi_ Saville-Kent, 1880: Trypanosoma.—Blood. species Siebold: Sarcocystis.—Muscles.
TREMATODA:
_spiculator_: Distomum.
CESTODA:
_cellulosæ_ Rudolphi: Cysticercus.—Peritoneum. _diminuta_ Rudolphi, 1819: Tænia, Hymenolepis.—Small intestine. _fasciolaris_ Rudolphi: Cysticercus.—Liver. _microstoma_ Dujardin: Tænia, Hymenolepis.—Small intestine. _minima_: Tænia. [See diminuta.] _murina_ Dujardin, 1845: Tænia, Hymenolepis.—Small intestine. [See fraterna.] _pusilla_ Gœze, 1782: Tænia, Catenotænia.—Small intestine. _ratti_ Rudolphi: Tænia.—Small intestine. _ratticola_ Linstow: Bothriocephalus.—Liver. species Eber: Tænia.—Intestine. _umbonata_ Molin: Tænia.—Intestine. _varesina_ Parona: Tænia.—[See diminuta.]
NEMATODA:
_annulosum_ Dujardin: Trichosoma, Calodium.—Intestine. _anulosum_: Trichosoma. [See annulosum.] _brauni_ Linstow: Spiroptera. _circularis_ Linstow: Physaloptera.—Stomach. _circumflexa_ Polonio: Trichina.—Encysted in peritoneum. _nodosus_ Rudolphi: Trichocephalus.—Cecum. _obvelata_ Bremser: Oxyuris.—Cecum. _oxyura_ Nitzsch, 1821: Ascaris. [See obvelata.] _ratti_ Diesing: Spiroptera.—Urinary bladder. _rhytipleuritis_ Deslongchamps: Filaria.—Stomach. species Gerstæcker: Spiroptera.—Wall of stomach and intestine. species Bakody: Spiroptera.—Encapsuled in wall of intestine, muscles. _spumosa_ Schneider: Heterakis.—Cecum, colon.
ACANTHOCEPHALA:
_moniliformis_ Bremser: Echinorhynchus, Gigantorhynchus.—Intestine.
ARACHNOIDEA:
_ægyptium_ Linné: Acarus, Ixodes, Hyalomma.—External. [See marginatum.] _agilis_ Koch: Lælaps.—External. [See echidninus, musculi.] _alepis_ Railliet & Lucet, 1893; Sarcoptes, Notoedres.—External, ears, genitalia. _echidninus_ Berlese: Lælaps.—External. [See agilis, musculi.] _marginatum_ Koch: Hyalomma.—External. [See ægyptium.] _muris_ Can., 1894: Notoedres.—External. [See alepis.] _musculi_ Mègnin: Hæmomyson.—External. [See agilis, echidninus.] _serratum_: Pentastomum.—Thoracic cavity.
INSECTA:
_brasiliensis_ Baker: Pulex.—External. [See cheopis.] _cæcata_ Enderlein: Dermatophilus, Rhynchoprion.—External. _cheopis_ Rothschild, 1903: Lœmopsylla.—External. [See brasiliensis, murinus, pallidus, philippinensis.] _fasciatus_: Ceratophyllus.—External. _gallinacea_ Westwood: Echidnophaga, Argopsylla.—External. _irritans_ Linné: Pulex.—External. _italicus_ Tiraboschi: Ceratophyllus. External. _londiniensis_ Rothschild: Ceratophyllus.—External. _mexicanus_ Baker: Ctenopsyllus.—External. _murinus_ Tirab.: Pulex.—External. [See cheopis, pallidus, brasiliensis, philippinensis.] _musculi_ Dugès: Ctenopysllus.—External. _pallidus_ Taschenberg: Pulex.—External. [See brasiliensis, cheopis, murinus, philippinensis.] _philippinensis_ Herzog: Pulex.—External. [See brasiliensis, cheopis, murinus, pallidus.] _rhynchopsylla_ Tiraboschi: Echidnophaga.—External.
MUS RATTUS ALEXANDRINUS.
[See also _Mus Alexandrinus_.]
INSECTA:
_brasiliensis_ Baker: Pulex.—External. _cæcata_ End: Dermatophilus.—External. _canis_ Curtis: Ctenocephalus.—External. _cheopis_ Roth.: Pulex.—External. _fasciatus_ Bosc: Ceratophyllus.—External. _felis_ Bouché: Ctenocephalus.—External. _gallinacea_ Westwood: Echidnophaga.—External. _irritans_ Linné: Pulex.—External. _londiniensis_ Roth.: Ceratophyllus.—External. _murinus_ Tiraboschi: Pulex.—External. _musculi_ Dugès: Ctenopsylla.—External. _philippinensis_ Herzog: Pulex.—External. _rhynchopsylla_ Tiraboschi: Echidnophaga, Argopsylla.—External.
MUS RUFESCENS Gray.
PROTOZOA:
_lewisi_ Saville-Kent, 1880: Trypanosoma, Trypanozoon.—Blood.
MUS SIPORANUS.
CESTODA:
_blanchardi_ Parona: Davainea.—Intestine.
MUS SURIFER.
NEMATODA:
_muricola_: Spiroptera.—Subcutaneous.
MUS SYLVATICUS Linné.—German Waldmaus.
TREMATODA:
_recurvum_ Dujardin, 1845: Distoma, Distomum, D. (Brachylaimus).—Intestine. _vitta_ Dujardin, 1845: Distoma, Distomum, D. (Brachylaimus).—Intestine.
CESTODA:
_muris sylvatici_ Rudolphi: Tænia.—Intestine. _pusilla_ Gœze: Tænia.—Intestine.
NEMATODA:
_cristatum_ Rudolphi: Ophiostomum, Rictularia.—Intestine. _lævis_ Dujardin: Strongylus, Metastrongylus.—Intestine. _minutus_ Dujardin: Strongylus, Metastrongylus.—Intestine. _muris sylvatici_ Dujardin: Trichosoma.—Intestine. _nodosus_ Rudolphi: Trichocephalus.—Intestine, cecum. _obtusa_ Rudolphi: Spiroptera. _obvelata_ Bremser: Oxyuris.—Cecum. _oxyura_ Nitzsch, 1821: Ascaris. [See obvelata.] _polygyrus_ Dujardin: Strongylus, Metastrongylus.—Intestine. _spirogyrus_ Leuckart: Strongylus.—Intestine. _stroma_ Linstow, 1884: Oxyuris.—Intestine. _tetraptera_ Nitzsch: Oxyuris.—Intestine.
ARACHNOIDEA:
_simplex_ Tyrell: Psorergates.—Skin.
INSECTA:
_agyrtes_ Heller: Ctenophthalmus, Typhlopsylla.—External. _assimilis_ Taschenberg: Typhlopsylla.—External. _fasciatus_ Bosc: Ceratophyllus.—External. _gallinæ_ Schrank: Ceratophyllus.—External. _italicus_ Tiraboschi: Ceratophyllus.—External. _londiniensis_ Rothschild: Ceratophyllus.—External. _musculi_ Dugès: Ctenopsyllus, Ctenopsylla.—External. _obtusiceps_ Ritsema: Hystrichopsylla.—External. _pentacanthus_ Rothschild: Neopsylla, Ctenophthalmus.—External. _poppei_ Wagner: Typhloceras, Typhlocerus.—External. _proxima_ Wagner: Typhlopsylla, Ctenopthalmus.—External. _talpæ_ Curtis: Hystrichopsylla.—External. _taschenbergi_ Wagner: Ctenopsylla.—External.
MUS SYLVESTRIS.
PROTOZOA:
_intestinalis_ Lambl, 1859: Lamblia.—Intestine. [See muris.]
MUS TECTORUM Sari.
CESTODA:
_fasciolaris_: Cysticercus.—Liver.
MUS VARIEGATUS.
CESTODA:
_muris variegati_ Janicki: Hymenolepis.—Intestine. _trapezoides_ Janicki: Davainea.—Intestine.
MUS VELUTINUS Balser, 1905.
INSECTA:
_dasyuri_ Skuse: Stephanocircus.—External. _hercules_ Roth.: Macropsylla.—External. _simpsoni_ Rothschild: Stephanocircus.—External. _simsoni_. [See simpsoni.]
MUS in the sense of “rats.”
The following parasites are reported either from “rats” or from “_Mus_” in the sense of “rats:”
PROTOZOA:
_brucei_: Trypanosoma, Trypanozoon.—Blood. _dimorphon_: Trypanosoma, Trypanozoon.—Blood. _equiperdum_: Trypanosoma, Trypanozoon.—Blood. _evansi_: Trypanosoma, Trypanozoon.—Blood. _evansii_: Trypanosoma. [See evansi.] _gambiense_: Trypanosoma, Trypanozoon.—Blood, artificial infection. _intestinale_ R. Blanchard, 1885: Megastoma.—Intestine. [See muris.] _muris_ Grassi: Amœba.
CESTODA:
_fasciolaris_: Cysticercus.
NEMATODA:
_hepaticum_ Railliet, 1889: Trichosoma.—Liver. _hepaticus_: Trichocephalus. species Davaine: Filaria.—Blood.
GORDIACEA:
_Gordius._ By error Cerruti & Camerano (1888b, 6) have interpreted a title by Leidy (1879) as meaning that he found _Gordius_ in a rat.
ARACHNOIDEA:
_sanguineus_ Latreille: Rhipicephalus.—External.
INSECTA:
_capitis_ Nitzsch: Pediculus.—External. _canis_ Curtis: Ctenocephalus.—External. _præcisus_: Hæmatopinus.—External.
MUS species.
Under various “_Mus_ sp.” entries, the following parasites are reported:
PROTOZOA:
_gambiense_: Trypanosoma.—Blood, artificial injection.
INSECTA:
_aganippes_ Roth.: Ctenopsylla.—External. _agyrtes_ Heller: Typhlopsylla.—External. _colossus_ Roth.: Pygiopsylla.—External. _ellobius_ Roth.: Ctenopsylla.—External. _hercules_ Roth.: Macropsylla.—External. _miacantha_: Polyplax.—Hair. _pinnatus_ Wagn.: Ceratophyllus.—External. _præcisus_ Neum., 1902: Hæmatopinus.—External.
WATER RAT.
[See also _Mus amphibius_.]
CESTODA:
_longicollis_: Cysticercus.—Axillary space.
INSECTA:
_spiniger_ Burm., 1838: Hæmatopinus.
MUS.
The following parasites are recorded under “_Mus._:”
PROTOZOA:
_falciformis_: Eimeria.—Intestine.
TREMATODA:
_migrans_: Dist.
CESTODA:
_blanchardi_ Parona: Davainea. _celebensis_ Janicki: Davainea. _gracilis_ Janicki: Davainea. _muris variegati_ Janicki: Hymenolepis. _nana_ Siebold: Hymenolepis. [See fraterna.] _polycalceola_ Janicki: Davainea. _relicta_ Zschokke: Hymenolepis. _trapezoides_ Janicki: Davainea.
NEMATODA:
_hepaticum_ Railliet: Trichosoma. _obvelata_ Bremser: Oxyuris.—Intestine.
ARACHNOIDEA:
_musculi_ Oudemans: Demodex.
INSECTA:
_cheopis_ Roth.: Lœmopsylla.—External. _felis_ Bouché: Ctenocephalus.
MUS.—A Field Mouse.
CESTODA:
_longicollis_: Cysticercus.—Thoracic cavity.
THE FLEA AND ITS RELATION TO PLAGUE.
By Passed Assistant Surgeon CARROLL FOX,
_United States Public Health and Marine-Hospital Service_.
THEORIES AS TO TRANSMISSION OF PLAGUE.
1. Direct contagion from man to man.
2. Through slight abrasions of the skin, mucous membranes of mouth, tonsils, nose, and conjunctiva receiving contaminated material.
3. Through the respiratory tract, from air contaminated with dried infectious sputum or dejecta. (Possibly the cause of primary pneumonic plague.)
4. Through the alimentary tract from food contaminated with saliva or excretions from plague patients, or dejecta or the feet of insects that have fed on plague material. In the case of rats, from eating the carcasses of infected rats.
5. Infected clothes, soil, or houses.
6. Through the bites of insects, especially the flea.
It has been noticed for many years that an epidemic of plague in man was associated with an epizootic of high mortality among rats, but it was not until Yersin discovered the _Bacillus pestis_ in 1894 that the disease in man and rats was shown to be identical. The first five theories are not satisfactory in explaining the epidemiology of plague, and in 1897 Simond advanced the theory that plague was carried by means of fleas. Hankin in 1898 also suggested an insect as an intermediate host. This theory has been developed by Ashburton Thompson, Gauthier and Raybaud, Liston, Verjbitski, and others, and finally by the last Indian Plague Commission, whose work makes a distinct advance in our knowledge of this subject. The reader is referred to the work of this commission for a review of the subject, which has been liberally used in the preparation of this paper.[AD]
Footnote AD:
Journal of Hygiene (Vol. VI, No. 4; Vol. VII, No. 3; Vol. VII, No. 6; Vol. VIII, No. 2).
INSECTS THAT HAVE BEEN SUSPECTED IN THE TRANSMISSION OF PLAGUE.
It is probable that all insects capable of sucking blood will take the _Bacillus pestis_ into their alimentary canal if they feed on a septicæmic plague animal. Ogata suggested that not only the flea but the mosquito also may be responsible for the transmission of plague. Yersin, Hankin, and Nuttall have each demonstrated the presence of _Bacillus pestis_ in the dejecta of flies and ants; and Nuttall and Verjbitski in the stomach and dejecta of the bedbug. Hertzog found the bacilli in the _Pediculus capitis_ taken from a child which died of plague, and McCoy[201] has found the organism in lice, _Hæmatopinus columbianus_, taken from a plague-infected squirrel. The plague bacilli have been frequently demonstrated in rat fleas taken from plague rats, and McCoy has shown its presence in the flea (_Ceratophyllus acutus_) of the California ground squirrel (_Citellus beecheyi_). The cockroach has also been thought to be instrumental in spreading the infection by contaminating food. The presence of bacilli in the stomach and dejecta of insects has not only been proven microscopically but by animal inoculation as well.
Assuming that the relation between rat plague and human plague has been proven without a doubt—that is, that an outbreak of human plague is associated with an infection in rats, or, in other words, that plague is primarily a disease of rats and secondarily a disease of man—the theory that it is conveyed through an intermediate parasitic host is the only one which will fulfill all the requirements, and after a study of their habits we are able to exclude all of the parasites but the flea as the active agent in its transmission.
Plague is rarely or never contracted either in rat or in man by eating contaminated food. Therefore those insects like flies and cockroaches, which are supposed to spread the infection by contaminating food with their dejecta, need not be considered.
The habits of the domestic mosquitoes are such that while they occasionally do bite animals they usually feed on the blood of man, and are not known to feed where there is much hair, as there is on the rat. This also applies to the bedbug. Verjbitski has shown experimentally that bedbugs would not feed on rats until the animals were shaved.
Pediculi are degenerate insects, their powers of locomotion being limited. Their eggs are laid on and are attached to the hair of the host. They are born, live, and die on the same host, and rarely pass from one animal to another of a different species. It can not be denied, however, that this parasite occasionally may be instrumental in spreading plague from rat to rat. The _Pediculus capitis_, if placed on a rat, will feed with avidity, but these insects are rarely found upon rats in nature.
We have no record of plague bacilli having been demonstrated in mites 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.