Chapter VI: Part 6

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.

A. WITHOUT A COMB OF SPINES ON THE PROTHORAX OR THE HEAD.
1. Two bristles on the gena, an ocular bristle placed
below the eye, an oral bristle placed just above
root of maxilla. Mesothorax not divided by an
internal incrassation. Claspers in male forming
prominent hump, claw like and covered by hairy
flap. An irregular row of about 10 teeth on inner _Pulex
side of hind coxa irritans_.

2. Two bristles on gena, an ocular placed in front of
and just above middle of eye, an oral bristle
placed just above root of maxilla. Mesothorax
divided by internal incrassation, claspers not
forming prominent hump, not claw like, not
covered by hairy flap. A regular row of about six _Lœmopsylla
teeth on inner side of hind coxa, cheopis_.

AA. WITH A COMB OF SPINES ON PROTHORAX BUT NOT ON HEAD.
3. Three bristles on lower genal row, upper genal row
represented by three or four small bristles
running along anterior margin of antennal groove.
Eye present, about five hairs on second joint of
antenna, not as long as third joint. Maxillary
palpi not as long as labial palpi. Labial palpi
reach to apex of fore coxa. Spines on posterior
tibia in pairs of about five groups. Head of male _Ceratophyllus
flattened on top, fasciatus_.

AAA. WITH A COMB OF SPINES ON THE PROTHORAX AND ON THE HEAD.
4. Eye present, seven spines on lower margin of gena. _Ctenocephalus
Spines on posterior border of tibia in pairs canis_ or
_felis_.

5. Eye absent, four spines on hind margin of gena.
Spines on posterior tibia single and in a close _Ctenopsyllus
set row musculi_.

DESCRIPTION OF PLATE II.
────────────────────────────────────────────┬──────────────────────
Fig. 1. Clasping organs of male. │P Process.
„ │F Finger.
„ │M Manubrium.
„ │IX St Ninth Sternite.
────────────────────────────────────────────┼──────────────────────
Fig. 2. Head of female. │
────────────────────────────────────────────┼──────────────────────
Fig. 3. Terminal abdominal segments, female.│8 T Eighth Tergite.
„ │8 St Eighth Sternite.
„ │10 T Tenth Tergite.
„ │10 St Tenth Sternite.
„ │Sp Spermatheca.
„ │Sty Stylet.
────────────────────────────────────────────┼──────────────────────
Fig. 4. Hind tibia. │

CERATOPHYLLUS FASCIATUS Bosc.

[Plate II.]

_Head._—Evenly and gently rounded in the female, flattened on top in
the male. Frontal notch distinct. Eye present, placed low down in
head. Gena acutely pointed posteriorly. Maxilla triangular. Maxillary
palpi not as long as the labial palpi. Labial palpi reach to apex of
anterior coxa, 5-jointed. Antennal groove in the male reaches to top
of head, in the female to within one-third. There are 3 bristles on
the lower genal row, the middle of which is the smallest, while the
upper genal row is represented by 3 small bristles, extending along
the edge of the antennal groove. In the male the lowermost bristle is
frequently paired. There are several fine hairs above the eye. The
occiput contains the normal row of apical bristles, the lowest of
which is the largest. There is one bristle back of the middle of the
antennal groove and a number of fine hairs along the posterior margin
of the antennal groove. The antenna is 3-jointed, the first joint
contains a row of about 5 very short fine hairs, while the second
joint contains about 5 not as long as the third joint.

PLATE II.

CERATOPHVLLUS FASCIATUS, BOSC.
]

_Thorax._—The pronotum has one row of about 10 bristles, and a
ctenidium composed of about 16 or 18 spines. The mesonotum has a
posterior row of about 10 long bristles and there is an anterior row
of more numerous smaller ones. The metanotum has also a posterior row
of about 10 large bristles, with an anterior row of more numerous
smaller ones, while still anterior to this there are 5 or 6 still
smaller bristles. The metathorax contains 8 or 10 bristles which are
small anteriorly, larger posteriorly. On the sternum of the metathorax
there are 2 large bristles, while the episternum has 3 smaller ones.
On the epimerum are 2 bristles placed anteriorly and 3 or 4
posteriorly, one of which is on the apical margin.

_Abdomen._—The first stigma is nearly in line with those of the other
abdominal segments. There are two rows of bristles on the abdominal
tergites, a posterior of about 12 or 14 and an anterior of smaller,
less numerous bristles. The antipygidial bristles in the female are 3
in number on each side, of which the middle is the longest, and the
inner one the smallest. The male has but 2 antipygidial bristles on
each side. The sternites from the third to the sixth have a single row
of about 10 bristles, while the seventh has about 12. The metanotum
has 2 teeth on each side, as have the first and second abdominal
tergites. The third and fourth abdominal tergites have 1 tooth on each
side.

_Legs._—The fore coxæ are normally clothed. The fore femur has on the
outer side 11 or 12 fine bristles irregularly disposed, while on the
mid femur there is a row of about 3 to 5 bristles on the inner
surface. The hind coxa has no patch of spines on the inner side, while
on the inner surface of the hind femur there is a row of about 5 to 7
bristles. The spines on the posterior tibia are in pairs of six
groups, while on the outer surface there is a row of about 7 bristles.
None of the apical bristles of the tarsi are as long as the next
succeeding joint. The fifth tarsal joints on all the legs have 5
lateral spines.

Length of joints of tarsi:

Mid tarsi (♂) 8 7 4½ 3 7
Hind tarsi 18 11 7 4 8
Mid tarsi (♀) 8 7 5 3½ 7
Hind tarsi 21 13 8 5 8

_Modified segments._—(♂). The manubrium of the claspers is straight
and narrow, while the process extends upward as a short, blunt cone,
where at the tip there are several fine hairs. The lower margin is
evenly and gently rounded. The finger is short, extending but a little
above the process. It is concave on its anterior surface and convex on
its posterior, and from the posterior margin there are 2 large and 2
small bristles alternating. Two long heavy bristles arise from the
process below the insertion of the finger. The ninth sternite is
broad, with a deep sinus in its posterior border. Its lateral surface
contains numerous fine hairs, these hairs being somewhat larger just
beneath the sinus. Along the dorsal border of the tenth sternite there
are 3 heavy bristles in line. At the tip of the tenth tergite there is
one heavy bristle. Besides these heavy bristles in this segment there
are numerous fine hairs.

(♀) The eighth tergite contains just anterior to the sensory plate
about 12 small hairs while just beneath the sensory plate there are 2
long bristles. Lower down there is a patch of about 6 bristles and on
the apical margin 4 to 6. The stylet is short, cylindrical, slightly
larger at the base than at the tip, where there is a long bristle. On
the under surface arises a fine hair. Substylar flap (tenth sternite)
has along its margin numerous hairs.

DESCRIPTION OF PLATE III.
────────────────────────────────────────────┬──────────────────────
Fig. 1. Clasping organs of male. │P Process.
„ │M Manubrium.
„ │F Finger.
„ │IX St Ninth Sternite.
────────────────────────────────────────────┼──────────────────────
Fig. 2. Head of female. │
────────────────────────────────────────────┼──────────────────────
Fig. 3. Terminal abdominal segments, female.│8 T Eighth Tergite.
„ │8 St Eighth Sternite.
„ │10 T Tenth Tergite.
„ │10 St Tenth Sternite.
„ │Sp Spermatheca.
────────────────────────────────────────────┼──────────────────────
Fig. 4. Hind coxa inner surface. │

LŒMOPSYLLA CHEOPIS Rothschild.

[Plate III.]

_Head._—Abruptly rounded. Flattened on top in ♂. Eye present. No
ctenidia on head. Antennal groove in the ♀ reaches to within one-third
of the top of the head. In ♂ reaches to top of head. Gena obtusely
pointed posteriorly. Maxilla triangular. Maxillary palpi are not as
long as labial palpi. Labial palpi reach to apex of fore coxæ,
4-jointed. Anterior edge of antennal groove overlapped by chitinous
flap. On posterior edge of antennal groove are a number of small
bristles, these being most distinct in the male. The first antennal
joint in the male contains 4 or 5 hairs at its outer edge, while
transversely there is a row of several fine hairs. The second joint
has a row of fine hairs not as long as the third joint. Divisions
marking separations of third joint most pronounced on dorsal edge. Two
bristles on gena. The oral bristle placed low down just above the base
of the maxilla; the ocular bristle in front and just above the middle
of the eye. Six bristles on the posterior margin of the occiput on
each side with 2 back of the antennal groove.

PLATE III.

LŒMOPSYLLA CHEOPIS, ROTHSCHILD.
]

_Thorax._—The pronotum is without a ctenidial comb, and has one row of
about 14 bristles. The mesonotum, the broadest of the three thoracic
nota, also has a single row of about 12 bristles. The metanotum has a
single row of about the same number. The mesosternite contains about 5
bristles. The pleura of the metathorax is normally divided. The
sternum contains 2 bristles, 1 anterior and 1 posterior. The
episternum contains 1 bristle, and the epimerum contains 2 rows of
bristles, an anterior row of 7 and an apical row of the same number.

_Abdomen._—The first abdominal tergite contains 2 rows of bristles, an
anterior and a posterior of about 6 bristles each, while the next 6
contains but a single row of about 14 bristles each, the lowest placed
just below the stigma. From the seventh tergite springs a single
antipygidial bristle. The sternites contain a single row of 8 or 10
bristles.

_Legs._—The fore coxa is normally clothed. The fore femur has on its
outer surface about 8 fine bristles. The mid femur has a single row of
about 6 bristles, while the hind femur has a row of the same number.
The hind coxa has on its inner surface a regular row of about 6 teeth.
The hind tibia has on its posterior border 5 groups of spines in
pairs, while on its outer surface there are about 8 small bristles in
a row. The apical bristle on the second tarsal joint of the hind leg
reaches to about the middle of the fifth tarsal article. The fifth
tarsal article on all of the legs has 4 lateral spines and a subapical
pair of hairs.

_Modified segments._—(♂) The manubrium of the claspers is short and
narrow. There are two free processes, the upper one, the finger, being
broadest and wider at the tip than at the base, its upper border being
more convex than the lower border and containing a number of bristles.
The ninth sternite is club-shaped, is nearly straight on its dorsal
margin, and the ventral margin contains a row of fine bristles from
base to apex.

(♀) No bristles in front of the sensory plate. Along its apical margin
externally there is a row of about 12 long bristles, and internally a
row of less numerous, shorter bristles. Laterally there is a more or
less regular row of about 8 bristles, and between this row and the
apical row 3 or 4 more.

DESCRIPTION OF PLATE IV.
────────────────────────────────────────────┬──────────────────────
Fig. 1. Clasping organs, male. │10 T Tenth Tergite.
„ │10 St Tenth Sternite.
„ │P Process.
„ │F Finger.
„ │M Manubrium.
„ │IX St Ninth Sternite.
────────────────────────────────────────────┼──────────────────────
Fig. 2. Head of female. │
Fig. 3. Last tarsal joint of hind leg. │
────────────────────────────────────────────┼──────────────────────
Fig. 4. Terminal abdominal segments, female.│8 T Eighth Tergite.
„ │8 St Eighth Sternite.
„ │10 T Tenth Tergite.
„ │10 St Tenth Sternite.
„ │Sp Spermatheca.
────────────────────────────────────────────┼──────────────────────
Fig. 5. Hind tibia. │

CTENOPSYLLUS MUSCULI Dugés.

[Plate IV.]

_Head._—The frons is prominent anteriorly, giving the head somewhat
the shape of a fez. There are 4 spines on the posterior border of the
gena. The antennal groove reaches to the top of the head. The
maxillary palpi are shorter than the labial palpi, which reach to
about two-thirds of the fore coxa and are 5-jointed. Maxilla
triangular. Eyes absent. At the most prominent part of the frons
anteriorly there are two short thick spines, while below these,
running along the anterior margin, there are 5 bristles. Above there
is an oblique row of 4 bristles, with 1 more placed near the top of
the antennal groove. Between this oblique row and lower bristles there
are numerous fine hairs. On the occiput there is a subapical row of
about 7 bristles on each side, while in front of this are 3 oblique
rows of bristles, the first containing 3, the second 4, and the third
5. On the posterior margin of the antennal groove there are several
small hairs. On the first joint of the antenna there are about 3
hairs, while on the second joint there are 4 or 5, the longest
somewhat longer than the third joint.

_Thorax._—The pronotum has an anterior row of about 10 bristles, and a
ctenidium of about 24 spines. The mesonotum contains about 4 rows of
bristles, more or less regularly disposed, each row consisting of
about 8 or 9 bristles. The metanotum has 2 rows of bristles, a
posterior row of about 10 bristles, and an anterior of the same
number, while there are several smaller bristles in front of this. The
mesothorax contains about 10 bristles. The episternum of the
metathorax has 2 bristles, and on the sternum there is 1 large one.
The epimerum has 2 rows of 4 bristles each, with 1 large one at the
apical margin.

PLATE IV.

CTENOPSYLLUS MUSCULI, DUGES.
]

_Abdomen._—The first abdominal tergite has 2 rows of 10 bristles each,
the posterior being comprised of the larger bristles. The next 6
tergites have 2 rows of bristles each, a posterior of large bristles,
about 12 in number, and an anterior of smaller bristles, also 12 in
number. On the apical edge of the metanotum there are 2 small teeth on
each side. The first abdominal tergite contains 3 such teeth while the
second and third have 1 each on each side. At the apex of the seventh
tergite in the female there are 4 antipygidial bristles, sometimes 5.
The male has but 3 antipygidial bristles. The abdominal sternites from
the third to the sixth have a single row of 6 bristles. The seventh
has a row of about 16 bristles.

_Legs._—The fore coxa has about 32 large bristles more or less
regularly disposed in 6 oblique rows. The hind coxa is without teeth
on the inner surface. The mid femur is without bristles on its lateral
surfaces. The hind femur is also without a row of bristles on its
lateral surfaces. The spines on the posterior border of the tibia are
single and in a close set row. The apical spines of the second tarsal
joint of the hind legs are shorter than the third joint. The last
tarsal joint on all the legs contains 4 lateral spines and a subbasal
pair situated between the first lateral pair.

_Modified segments._—(♀) Just beneath the pygidium is 1 long bristle.
On the eighth tergite there is a patch of hairs, 6 of which are on the
apical margin and about 5 or 7 anterior to these. The stylet is short,
almost as wide at the base as at the tip, where there is a long hair.
Posteriorly to this bristle there springs another one from the under
surface.

(♂) Manubrium of the claspers is narrow, curved at the tip. The finger
reaches to the level of the process, has a stout pedicle, is flat on
its anterior border, and is decidedly convex on its posterior border,
where there are 4 bristles. The shape of the ninth sternite is shown
in the figure.

DESCRIPTION OF PLATE V.
────────────────────────────────────────────┬──────────────────────
Fig. 1. Clasping organs of male │P Process.
„ │F Finger.
„ │M Manubrium.
„ │IX St Ninth Sternite.
────────────────────────────────────────────┼──────────────────────
Fig. 2. Head of female. │
────────────────────────────────────────────┼──────────────────────
Fig. 3. Terminal abdominal segments, females│8 T Eighth Tergite.
„ │8 St Eighth Sternite.
„ │10 T Tenth Tergite.
„ │10 St Tenth Sternite.
„ │Sty Stylet.
„ │Sp Spermatheca.
────────────────────────────────────────────┼──────────────────────
Fig. 4. Hind coxa, inner surface. │

PULEX IRRITANS Linnæus.

[Plate V.]

_Head._—Evenly and abruptly rounded in both sexes. Frontal notch
absent. Eye large. Maxillary palpi longer than the labial palpi.
Labial palpi reach to about half the length of the anterior coxa and
are 4-jointed. The mandibles are broad and markedly serrate. Maxillæ
triangular. Antennal groove short and wide, closed behind, thickened
on edges, and reaches to top of head in both sexes by chitinous
thickening. Second joint contains 8 or 9 fine hairs, shorter than the
third joint. Division of the third joint only to be seen on dorsal
surface. Two bristles on the gena, one placed low down just above the
maxilla, the other below the eye. From the lower margin of the gena
occasionally may be seen a small tooth. One bristle on the occiput
near the posterior lower angle. A few fine hairs on the posterior edge
of the antennal groove.

_Thorax._—The thoracic nota each contain a single row of about 10 or
12 bristles. There is no ctenidium on the pronotum. The mesosternite
is narrow and is not divided by an internal incrassation. The
episternum of the metathorax is large and contains about 2 or 3
bristles and is not quite separated from the sternum anteriorly. The
epimerum has an anterior row of about 7 or 8 bristles and an apical
row of about 6.

_Abdomen._—Each of the abdominal tergites, with the exception of the
first, has a single row of 8 or 10 bristles. The first has 2 rows of
about 4 each. The sternites from third to seventh have a single row of
about 6 bristles. There is one short antipygidial bristle on each
side.

_Legs._—The hind coxa has on its inner surface posteriorly a number of
fine hairs, while anteriorly there are 10 or 12 teeth in an irregular
line. The hind femur has on its inner surface a row of about 8 or 9
bristles. The spines on the posterior tibia are in pairs, and there
are about 7 bristles in a line on its outer lateral surface. The
apical bristle of the second tarsal joint of the hind leg reaches to
about the middle of the fifth joint. The last tarsal joints of all the
legs contain 4 lateral spines and a subapical pair, and between the
third and last lateral spine there is a hair.

_Modified segments._—(♀) The eighth tergite has no bristles above the
pygidium but has numerous short stout bristles laterally and on and
close to the apical margin. The stylet is short and stout and has at
its tip a long hair. The tenth sternite and tergite contain numerous
fine hairs, those on the sternite confined to the apical edge.

(♂) The male claspers are quite characteristic. The manubrium is large
and curved and points ventrally. The claspers have two processes, the
lower of which, with the finger, form together a kind of claw which is
covered by the other process forming a flap, quite hairy on its upper
margin. The ninth sternite is described very well by Rothschild[208]
as “boomerang” shaped. The eighth tergite has a small manubrium.

PLATE V.

PULEX IRRITANS, LINNÆUS.
]

DESCRIPTION OF PLATE VI.
────────────────────────────────────────────┬──────────────────────
Fig. 1. Clasping organs of male │F Finger.
„ │M Manubrium.
„ │IX St Ninth Sternite.
────────────────────────────────────────────┼──────────────────────
Fig. 2. Head of male. │
────────────────────────────────────────────┼──────────────────────
Fig. 3. Terminal abdominal segments, female │8 T Eighth Tergite.
„ │8 St Eighth Sternite.
────────────────────────────────────────────┼──────────────────────
Fig. 4. Hind coxa and femur, inner surface. │

CTENOCEPHALUS CANIS Curtis.

[Plate VI.]

_Head._—Strongly and evenly rounded in both sexes. Eye large. Maxilla
triangular. Maxillary palpi about as long as labial palpi. Labial
palpi reach to two-thirds of anterior coxæ, 4-jointed. Seven spines
along the lower margin of the gena. The posterior angle of the gena
ends in a small tooth. Occasionally this may be absent. Antennal
groove in the female reaches to within one-third of the top of head
and is prolonged upwards by a chitinous thickening and in the male
reaches almost to top of head. Two bristles on the gena, one placed
well toward the anterior lower angle and the other in front of the
eye. Usual number of bristles on posterior margin of the head, with 2
large ones back of the antennal groove. About 8 hairs on the second
joint of the antenna nearly as long as the third joint.

_Thorax._—A row of about 10 bristles on the pronotum, with a ctenidium
of about 14 to 16 spines. Two rows of bristles on the mesonotum, a
posterior of about 12, another of more numerous smaller bristles
placed well anteriorly. The metanotum contains a single row of about
10 or 12 bristles. The episternum of the metathorax has 3 or 4 stout
bristles, while the epimerum contains an anterior row of about 10
bristles and a posterior row of about 9.

_Abdomen._—The first abdominal tergite contains 2 rows of about 4
bristles each, while the other tergites to the seventh contain a
single row of from 12 to 16 bristles. The stigmata are large. There is
a single antipygidial bristle on each side. The sternites from third
to seventh have a single row of 4 bristles each.

_Legs._—The hind coxa has on its inner side a patch of from 6 to 12
spines, while the hind femur has a row of 10 or 12 bristles on its
inner surface. The spines on the posterior border of the hind tibia,
with the exception of the apical, are in pairs, while in the apical
group are about 3 stout bristles. The apical spine of the second joint
of the hind leg reaches to nearly the middle of the fifth joint. On
the fifth joint of all the legs there are 4 lateral spines and a
subapical pair, and between the third and fourth lateral spines there
is a hair.

_Modified segments._—(♀) The eighth tergite has no hairs back of the
stigma. The apical margin is rounded at the apex and contains 8 or 10
bristles. The stylet is short and wide and contains at its tip a long
and a short bristle.

(♂) The manubrium is short and narrow. The movable finger of the
clasper is short, thick, swollen at its middle, bluntly rounded at its
extremity, and contains on its upper border numerous hairs and a few
on its lower border.

Rothschild[209] has pointed out certain differences between the
_Ctenocephalus canis_ and _Ctenocephalus felis_. The differences are
that in the female of the felis the head is longer and more pointed.
This difference is not so pronounced in the male. Also certain
differences in the shape of the claspers and the number of bristles in
the episternum and epimerum of the metathorax and the hind femur,
those in the _C. canis_ being more numerous. Also that group of
bristles on the posterior border of the hind tibia between the fifth
pair and the apical bristles consists of two in the _Ctenocephalus
canis_, while there is but a single bristle with a small hair in the
_Ctenocephalus felis_.

REFERENCES.

Endnote 201:

1909, McCoy.—“Plague Bacilli in Ectoparasites of Squirrels.” Public
Health Reports, Vol. XXIV, No. 16.

Endnote 202:

1908, Schumann.—“A Disease of Rats Caused by Mites.” Centralblatt f.
Bact., Oct. 30th.

Endnote 203:

1909, McCoy and Mitzmain.—“An Experimental Investigation of the
Biting of Man by Fleas Taken from Rats and Squirrels.” Public Health
Reports, Vol. XXIV, No. 8.

Endnote 204:

1908, McCoy.—“A Report on Laboratory Work in Relation to the
Examination of Rats for Plague at San Francisco, California.” Public
Health Reports, Vol. XXIII, No. 30.

Endnote 205:

Wagner.—Aphanipterologische Studien aus dem zootomischen
laboratorium der Universität zu St. Petersburg.

Endnote 206:

1908, Mitzmain.—“How a Hungry Flea Feeds.” Entomological News,
December.

Endnote 207:

1908, Miller.—“Hepatazoon Perniciosum (N. G. N. SP.). A
Hæmogregarine Pathogenic for White Rats; With a Description of the
Sexual Cycle in the Intermediate Host; A Mite (Lælaps Echidninus).”
Bull. No. 46, Hyg. Lab. U. S. Pub. Health and Mar. Hosp. Serv.,
Wash.

Endnote 208:

1908, Jordan and Rothschild.—“Revision of Non-Combed Eyed
Siphonaptera.” Parasitology, Vol. I, No. 1.

Endnote 209:

1901, Rothschild.—“Notes on Pulex canis, Curtis, and Pulex felis,
Bouché.” Entomologist’s Record, Vol. XIII, No. 4.

1905, Rothschild.—“Some Further Notes on Pulex canis, Curtis, and
Pulex felis, Bouché.” Novitates Zoologicae, Vol. XII.

PLATE VI.

CTENOCEPHALUS CANIS, CURTIS.
]

RODENTS IN RELATION TO THE TRANSMISSION OF BUBONIC PLAGUE.

By Surgeon RUPERT BLUE,

_United States Public Health and Marine-Hospital Service_.

Man has associated the rat with bubonic plague since the dawn of
history. The monuments and coins of the earliest times yield abundant
evidence of this association. Æsculapius, the god of the healing art,
is represented by the Greeks with a rat at his feet. An early
scriptural reference may be found in the first Book of Samuel in the
fifth and sixth chapters. The historian records therein the occurrence
of a fatal epidemic of “emerods” in the land of the Philistines
coincident with an invasion of “mice.”

The inhabitants of southern China in recent times have learned to look
upon the finding of sick and dead rats in their homes as a harbinger
of evil, in fact, as a forerunner of that dread scourge—“wan-yick,” or
plague. In the villages and cities of the Kwantung and Kwangsi
provinces, as recorded by medical missionaries, epizootic plague
almost invariably precedes an outbreak among human beings. So well is
this fact known to the common people that many seek safety in flight,
feeling assured that in a short time “yang-tzu” or “wan-yick” will
claim a harvest of victims among those who remain.

Doctor Mahē, sanitary officer for the port of Constantinople, in 1889,
called attention to the fact that epidemics of plague were always
announced by a great mortality among rats and mice. In 1894 Yersin
reported the fatal epizootic among rats then prevailing in Canton and
Hongkong coincident with the outbreak of plague among the Chinese.
Recent researches have confirmed these observations and a great deal
has been added to the literature of plague, especially in relation to
its mode of transmission. Indeed, it should be said that wherever the
disease has prevailed in recent years the relation of rats to its
spread has been observed, and that since the discovery of the specific
bacillus by Yersin and Kitasato, in 1894, bacteriological
investigations have shown that there is no difference morphologically
or culturally between the bacilli of human and rat plague. Moreover,
the gross and microscopic lesions in the lymph nodes are practically
the same, and the _B. pestis_ recovered in both fulfills the
postulates of Koch.

Nothing was definitely known, however, of the mode of transmission of
the disease from rat to rat or from rat to man until the completion of
the experimental work of the Indian Plague Commission. Simond, Ogata,
Thompson, and Koch each expressed the belief that the infection was
transferred by the rat flea. Nuttall (1897) and Simond (1898)
demonstrated the presence of _B. pestis_ in the bodies of bugs
(_Cimex_) and fleas which had been taken from plague-sick rats, and
the latter observer, in the same year, succeeded in transmitting the
disease from rat to rat without contact.

The work of the Indian Plague Commission was undertaken (1905) with a
view to establishing the exact relationship between epizootics among
rats and epidemics among men, and included both field and laboratory
observations. The experiments of Gauthier and Raybaud (1903) and of
Simond were repeated on a larger scale and greatly improved in that
all rats and fleas used were first identified as to species. The
findings of the commission may be briefly summarized as follows: That
fleas and bugs taken from plague-sick rats contain _B. pestis_, and
that some of them remain alive in the bodies of the insects from five
to sixteen days; that plague is conveyed by the bites of fleas which
have previously fed on the blood of animals suffering with the
disease; that rat fleas bite man; that under experimental conditions
the infection is not transferred from rat to rat in the absence of
fleas.

A careful study of the findings of the workers in India justifies the
assumption that plague is a disease of the rodent primarily and
accidentally, and secondarily a disease of man. An analysis of the
epidemiological facts collected in San Francisco leads to the same
conclusion. As a result our practice with regard to suppressive
measures and quarantine procedure has undergone a radical change in
the last decade. If the infection is flea-borne from rat to man in the
majority of cases, then the extermination of the rat should be the
first principle upon which to base a campaign. In the former
contribution on the subject (1907) I stated that “if we destroy the
host there is no longer danger of infecting the parasite.” This basic
principle has been recognized and successfully applied in two
campaigns against plague in San Francisco. First in the outbreak in
Chinatown in 1903–4, and again in the larger epidemic of 1907.

The outbreak of 1907 began May 27, a little over a year after the
great fire and earthquake, but no cases were discovered between that
time and mid-August when the disease began to appear in various parts
of the city. The source of infection was, in all probability, a
recrudescence from a focus which was not destroyed in the campaign of
1903–4. There occurred 160 cases with 77 deaths, the last case
appearing January 30, 1908. The following table shows the incidence of
human plague:

─────────────────────┬───────┬───────
Year. │Cases. │Deaths.
─────────────────────┼───────┼───────
1907. │ │
May │ 1│ 1
August │ 13│ 6
September │ 56│ 25
October │ 34│ 25
November │ 41│ 12
December │ 13│ 7
1908. │ │
January │ 2│ 1
─────────────────────┼───────┼───────
Total │ 160│ 77
─────────────────────┴───────┴───────

EPIDEMIOLOGICAL OBSERVATIONS IN SAN FRANCISCO.

Abundant epidemiological data associating the rat[AE] with plague have
been collected in San Francisco. For the purpose of illustration a
detailed reference to a few cases will be made. Two small boys
(October, 1907) while playing in an unused cellar found the body of a
dead rat. The corpse was buried with unusual funeral honors. In
forty-eight hours both were ill with bubonic plague. A laborer finding
a sick rat on the wharf picked it up with the naked hand and threw it
into the bay. He was seized three days later with plague. Doctor C.
and family lived in a second-story flat over a grocery store in the
residence section. Being annoyed for some days by a foul odor the
doctor caused the wainscoting around the plumbing to be removed. One
or two rat cadavers were found in the hollow wall. In two or three
days the two members of the family who used the room sickened, one
dying on the fifth day of cervical bubonic plague. It is probable that
infected rat fleas were set free by the removal of the wainscoting.

Footnote AE:

_M. norvegicus_ and _M. rattus_.

Dead rats were frequently found in or near houses where plague had
occurred. Immediately upon the discovery of a case of plague trained
men were sent into the neighborhood and a thorough search made for
rats. This work consisted in the removal of defective wooden floors
and walls of insanitary buildings and other harboring places.
Extensive rat catacombs were frequently found in these operations. In
the yard of a house in which 4 cases had occurred 20 cadavers were
found under the board covering. In the walls of a Chinese restaurant
87 dead rats were uncovered.

Very little can be said of the relation of mice (_M. musculus_) to the
epidemic. While many thousands were trapped, only a few hundred were
examined microscopically and in these no infection was found. They are
nonmigratory in habit and for this reason are not considered of much
importance from an epizoological standpoint.

Transmission from man to man was observed in but a small percentage of
cases, 3 per cent to be exact. In these the probability of
transference by fleas (_P. irritans_) or by bugs (_Cimex_) must be
admitted. When more than one case occurred in a house a common source
of infection was indicated, such cases occurring simultaneously or
within from forty-eight to seventy-two hours after the first.
Deratization was the measure mainly relied upon. After an infected
house was rat proofed, and the harboring places in the block
destroyed, no further cases occurred.

The course of epizootic plague was not interrupted at any time by
climatic conditions, there being as many cases in proportion to the
rat population in the winter of 1908 as there were at the height of
the epidemic. The last case of human plague occurred January 30, 1908,
but the infection remained active among rats for eight months longer,
or until October 21, 1908. (See following table.)

─────────┬─────────┬─────────┬─────┬────────────┬────────┬─────────────
Month. │ Number │ Number │ Per │ Average │Rainfall│Character of
│examined.│infected.│cent.│temperature.│ in │ days.
│ │ │ │ │inches. │
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
1907. │ │ │ │ °F. │ │{Clear, 13.
September│ 1,002│ 27│ 2.69│ 60.6│ 0.11│{Part cloudy,
│ │ │ │ │ │15.
│ │ │ │ │ │{Cloudy, 2.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 10.
October │ 2,679│ 23│ .86│ 60.6│ 1.36│{Part cloudy,
│ │ │ │ │ │10.
│ │ │ │ │ │{Cloudy, 11.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 14.
November │ 3,954│ 36│ .88│ 57.8│ .04│{Part cloudy,
│ │ │ │ │ │13.
│ │ │ │ │ │{Cloudy, 3.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 6.
December │ 4,308│ 48│ 1.11│ 52.4│ 3.66│{Part cloudy,
│ │ │ │ │ │11.
│ │ │ │ │ │{Cloudy, 14.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
1908. │ │ │ │ │ │{Clear, 5.
January │ 6,622│ 70│ 1.05│ 50.8│ 4.88│{Part cloudy,
│ │ │ │ │ │11.
│ │ │ │ │ │{Cloudy, 15.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 11.
February │ 11,700│ 45│ .38│ 51.0│ 5.39│{Part cloudy,
│ │ │ │ │ │12.
│ │ │ │ │ │{Cloudy, 6.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 20.
March │ 19,263│ 52│ .26│ 54.8│ .90│{Part cloudy,
│ │ │ │ │ │10.
│ │ │ │ │ │{Cloudy, 1.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 17.
April │ 15,524│ 34│ .21│ 56.3│ .22│{Part cloudy,
│ │ │ │ │ │10.
│ │ │ │ │ │{Cloudy, 3.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 17.
May │ 11,311│ 20│ .13│ 55.4│ .76│{Part cloudy,
│ │ │ │ │ │12.
│ │ │ │ │ │{Cloudy, 2.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 16.
June │ 13,624│ 4│ 0.02│ 55.3│ 0.01│{Part cloudy,
│ │ │ │ │ │9.
│ │ │ │ │ │{Cloudy, 5.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 11.
July │ 11,204│ 2│ .017│ 57.4│ .02│{Part cloudy,
│ │ │ │ │ │17.
│ │ │ │ │ │{Cloudy, 3.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 11.
August │ 10,988│ 0│ .0│ 57.3│ .01│{Part cloudy,
│ │ │ │ │ │10.
│ │ │ │ │ │{Cloudy, 10.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 16.
September│ 15,902│ 0│ .0│ 59.3│ .29│{Part cloudy,
│ │ │ │ │ │9.
│ │ │ │ │ │{Cloudy, 5.
─────────┼─────────┼─────────┼─────┼────────────┼────────┼─────────────
│ │ │ │ │ │{Clear, 16.
October │ 10,178│ 2│ .019│ 58.8│ .061│{Part cloudy,
│ │ │ │ │ │7.
│ │ │ │ │ │{Cloudy, 8.
─────────┴─────────┴─────────┴─────┴────────────┴────────┴─────────────

The rats examined for September, 1907, were very largely collected
from the badly infected districts; the remaining months give a truer
picture of the extent of the epizootic in the entire rat population.

THEORIES AS TO THE CAUSE OF SEASONAL PREVALENCE.

The marked seasonal prevalence of plague in man in San Francisco may
be given as additional proof of the association of the rat with its
spread. In the cold, rainy season, from December to April, the
epidemic ceases while the epizootic is apparently not influenced. The
anomaly is accounted for when we remember that the rat and its
parasites are very susceptible to cold and rain. It is then that the
animal seeks a warm, comfortable place from which it does not venture
until driven thence by dire necessity. In other words, the association
of the rat with man is not so intimate in winter, while the reverse is
true of the relation of rat with rat. The rains, while interrupting
the overground migrations and domiciliary visits of rats, drive them
to overcrowded burrows and harboring places. Another factor should be
mentioned in this connection. Human fleas (_P. irritans_), and
probably rat fleas also, are markedly reduced in numbers at that
season of the year. We must conclude, therefore, that the seasonal
prevalence of plague in man is due to the effect of climatic
conditions upon the habits of rats and the life history of the insect
carriers of the bacilli.

An examination of the foregoing should convince everyone that all
former theories as to the prolonged viability of _B. pestis_ in
contaminated soil or in polluted streams, and of the periodical spread
of the infection therefrom, are no longer tenable. It may also be
stated that insanitary conditions, except in so far as they furnish
food and shelter to rats and other vermin, play no important rôle in
the continuance of plague. This general revision has also eliminated
overcrowding as an important factor. In the absence pneumonic cases,
and of suctorial insects, this _bête noire_ of the sanitarian may be
disregarded.

THE OCCURRENCE OF PLAGUE IN THE MARMOT OF ASIA AND THE GROUND SQUIRREL
OF CALIFORNIA.

Rudenko (1900) first pointed out the possibility of contagion by the
“Tarbagan,” a species of the _arctomyinæ_ found in Siberia. He
observed a connection in 1894 between this rodent and an outbreak of
plague in a Cossack family of Soktuewsk. According to Beliatsky and
Zabolotny, each having been an observer in the same field, the natives
of Siberia and Mongolia often acquire plague in this manner. Le Dantec
and other writers have called attention to the probable susceptibility
of the marmot (_Arctomys bobac_), a hibernating rodent of India and
China. The marmot of Thibet, in the opinion of this writer, is the
natural animal host and purveyor of the virus. The literature of the
subject presents no bacteriological evidence, however, of such a
relationship, and plague in the _arctomyinæ_ of Asia is merely an
hypothesis. There is positive evidence though of the susceptibility of
the tree squirrel (_Sciurinæ_) to plague infection. Dr. Alice Corthorn
(1898) reported the finding of a plague-infected squirrel in one of
the outbreaks in the Bombay Presidency.

PLAGUE INFECTION IN GROUND SQUIRRELS.[AF]

Footnote AF:

Genus Citellus, Oken; subgenus Otospermophilus. “California
Mammals,” Frank Stephens.

The demonstration of natural plague in the California ground squirrel
(_Otospermophilus beecheyi_) is perhaps the most important observation
of the antiplague work of the service in 1908. The existence of a
plague epizootic in Contra Costa County was suspected as early as the
summer of 1903, and efforts were made at that time to collect sick and
dead rodents for bacteriological examination. In August (1903) two
fatal cases of human infection occurred in widely separated sections
of the county. The investigation which followed failed to connect
either with a previous case of human plague, but showed an association
with ground squirrels. These deaths occurred during a fatal epizootic
among ground squirrels and suggested a connection which unfortunately
was not confirmed.

None of the circumstances were forgotten, however, and in the second
campaign, begun in September, 1907, in San Francisco, inspectors were
detailed to examine all persons dying in the area under suspicion. No
plague was reported that autumn and winter. Fatal cases occurred and
were reported by the inspectors in July, 1908, as follows: A boy (J.
F.) died July 15, near Concord, and a young woman (M. P.) died July
28, on a ranch 10 miles from Martinez. The two were not associated. An
investigation was ordered at once and a force of trappers was hurried
to the scene with instructions to collect squirrels from the ranches
in the vicinity. The first plague-infected squirrel was found August 5
on the ranch where the boy had died July 15. Of 425 squirrels
collected from August 1 to October 12, 4 showed the gross and
microscopic lesions of natural plague.

A lad (F. M.) sickened August 5, 1908, in Los Angeles, Cal., after
being bitten by a sick ground squirrel. A polyadenitis, which
afterwards proved to be plague, developed in a few days. A dead
squirrel was found nearby and pathological specimens taken from it
were sent to the United States Plague Laboratory in San Francisco.
McCoy recovered _B. pestis_ from the tissue of the animal. This was
the only case of plague reported in Los Angeles. In order to complete
the list of those who contracted plague in the country, two other
cases should be mentioned. F. S., a pregnant woman, died of
bubosepticæmic plague near Concord, Cal., February 29, 1904. The _B.
pestis_ was recovered in pure culture from the axillary glands. In
April, 1906, a school boy of east Oakland developed a multiple plague
adenitis. Investigation showed that he had shot and handled ground
squirrels in the country four or five days before his illness.

THE NATURAL HABITAT OF PLAGUE.

The location of the natural habitat of plague has concerned
sanitarians for many years. Not a few have settled upon India as the
endemic center, while others associate China with the epidemics which
have devastated Europe from remote times. Le Dantec, a recent writer,
suggests the “lofty mountains” between India, Thibet, and China as the
exact location, and selects the rodent (marmot) of that region as the
natural enzootic host.

A panzootic leaves in its wake enzootics of plague in various
countries which persist until the rodents upon which they thrive are
either exterminated or rendered immune. At varying intervals epidemics
spring from them and finally cease with the exhaustion or destruction
of the enzootic foci. Plague disappears in time from these temporary
abodes and retires to its original habitat in India or China.

Of serious import in this connection is the fact that all the
conditions necessary for the establishment of a permanent focus of
plague exist on the Pacific coast of the United States. The broad
valleys and lofty mountains of this region are rich in the
_arctomyinæ_, there being no less than 12 species in California alone.
In the high Sierras the marmot (_Marmota flaviventer_),[AG] a species
of the natural enzootic host of Le Dantec, is found in great numbers.
The ground squirrel infests the valleys and foothills in an unbroken
chain from Oregon to the Mexican border. Once planted in this ideal
soil, infection may never be uprooted or its growth and extension
controlled. Small outbreaks will occur here and there, and periodical
visitations of greater magnitude may be expected in cities where a
combination of epidemiological factors is permitted.

Footnote AG:

“California Mammals,” Frank Stephens.

The facts as set forth in this paper have caused grave apprehension in
the minds of those who have been at all conversant with the conditions
in the transbay counties since 1903. At that time the writer
recognized the probability of the establishment of a permanent focus
of plague in that locality, and subsequent discoveries have proven the
correctness of the assumption. This changes the aspect of the problem
from that of a local infection to one of national importance. Once
established in such a rural community, plague is dislodged with
difficulty and only after a campaign covering a considerable length of
time. Being a national problem it can be best solved by the Federal
Government.

REFERENCES.

The Croonian Lectures on Plague, W. J. Simpson; Journal of Hygiene,
Volume VI, No. 4; Volume VII, No. 6; Volume VIII, No. 2; Plague among
the Ground Squirrels of California, W. B. Wherry, Journal Infectious
Diseases, Volume V, No. 5; California Mammals, Frank Stephens.

RODENT EXTERMINATION.

By Passed Asst. Surg. WILLIAM COLBY RUCKER,

_United States Public Health and Marine-Hospital Service_.

It should be remembered that rodents are extremely wily creatures and
that any campaign against them is a contest between the wit of man on
the one hand and acute animal instinct on the other. The rat, by his
constant association with man, has become extremely wary, and is
frightened by anything in the least out of the ordinary. They will eat
the bread on which poison is spread so carefully that they will leave
behind the poison and take practically all the bread; they will open
traps by pressing down the pan, and they have been known to repeat
this operation several times within an hour, entering the trap, eating
the bait, and then liberating themselves. At other times they will
enter the trap and stand on the pan with their hind legs, eat the
cheese, then carefully turn around and back out. This, of course, is
not possible with snap traps, but they have been known to spring them
by causing pieces of wood to fall upon them, after which the bait
would be eaten. Rats are found wherever food exists in abundance or
where they can find suitable breeding and nesting places.

Rodent extermination is a problem, with difficulties arising from the
animal’s highly developed regard for self-preservation. In main, the
rat requires two conditions for life. He needs plentiful food and
places suitable for nesting and breeding. Eliminate either of these
elements and you drive away your rats. Yet the problem remains far
more difficult than shown in the simple terms of the above equation.
The fabulous speed at which rats multiply will baffle all but the most
determined and efficient efforts to exterminate them. Under normal
conditions each female bears 3 litters a year and each litter produces
10 young. Under conditions ideally favorable, it has been computed
that 1 pair of rats will in five years, providing all can live so
long, increase to 940,369,969,152. Such a result is, of course,
impossible in nature, for it means that every rat born of the original
pair survive five years; that every litter of 10 contains 5 males and
5 females; and that the ideally favorable conditions persist. On the
other hand, rodent existence is an unending struggle in which an
enormous percentage succumbs; the ratio of half males and half females
does not hold; and ordinary conditions of life are hardly even
favorable. Nevertheless, the above proves emphatically that no rat
eradication can be effective unless the breeding is curtailed. Any
campaign against rodents must aim (_a_) to slaughter the greatest
possible number of those already living and (_b_) to prevent the
possibility of further breeding.

The existing rats are best attacked by trapping, by poisoning, and by
their natural enemies. Traps and poisons alone have been found
insufficient to keep pace with the rat’s speed of multiplication. The
surest of the rat’s enemies are his natural ones, and once they have
been loosed upon him his chance of escape is reduced. The cat, dog,
skunk, and other rodent foes, given a fair chance, quickly drive out
rats. But these animals do not eradicate the pest. The rats will
probably migrate to some other shelter, returning when their natural
enemies have quieted down. Absolute extermination is reached only when
conditions make the continuation of species impossible for the rat.

The size and frequency of rodent litters decreases proportionately
with every cutting off of food supplies. Separate the rat from his
pabulum and he will not breed so freely nor so often as when he is
well fed. Destroy rat habitations and make it impossible for them to
find new nesting places, and breeding will virtually cease, since the
unsheltered progeny can no longer survive, and since the starving
parent rats are driven to cannibalism in the struggle for existence.

Campaigns against rodents must cover five directions: (1) Trapping,
(2) poisoning, (3) exposing them to natural enemies, (4) cutting off
food supply, and (5) destroying existing nests at the same time that
the making of new ones is prevented.

Parenthetically, it may be noted that while these principles apply
equally to the extermination of rats in cities and in country
districts, their application must vary according to the place.

TRAPPING.

The kind of traps to be used varies with the rodent to be captured and
the locality which it infests.

CAGE TRAPS.