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.
The large 19-inch French cage trap gives good results where rats are
plentiful. It should be made of stiff, heavy wire and well reenforced,
as a large, strong rat will force his head between the wires in a weak
trap and thus escape. Before setting, the lever on the trap should be
tested to see that it works properly. The trap should be placed on a
hard surface, with the rear end a little higher than the entrance, so
that the trap will close promptly. When setting the trap in the open
it should be fastened to a board on which about an inch of soft dirt
has been spread. Place the trap where the rat usually goes for food or
in a runway and disturb the surroundings as little as possible. It is
sometimes well to place the trap near where there is dripping water,
as the rats come there to drink. If the trap is set in hay or straw or
wood it should be covered (with the exception of the entrance) with
this material. When this is not possible it should be covered with a
piece of sacking or placed in a dark corner or beneath the floors.
When setting the traps in the sewer a dry place should be chosen.
The rat is more or less of an epicure, therefore the bait should be
changed at frequent intervals. Also he should be given food which he
is not in the habit of getting. For example: In a meat market
vegetables are the best bait, while in a location where vegetables are
plentiful fresh liver and fish heads, or a little grain, are best. The
following may be suggested as good bait to be used: Fish, fish heads,
raw meat, cheese, smoked fish, fresh liver, cooked corn beef, fried
bacon, pine nuts, apples, carrots, and corn. When trapping in chicken
yards a small chick or duckling is remarkably good. When a large
number of rats are caught in one trap, search for the female and leave
her alive in the trap, as she may call in the young or the males. The
bait should be fastened to the inner side of the top of the trap with
a piece of fine wire so that the first rat in can not force the bait
underneath the pan and thus prevent the entrance of other rats. A few
grains of barley should be scattered near the entrance of the trap and
a small piece of cheese or meat fastened to the pan with a piece of
wire. It is often well to touch the pan with a feather which has been
dipped in oil of anise or oil of rhodium. Before leaving the trap it
should be smoked with a piece of burning newspaper to kill the smell
of the human hands or the rats which have been in it. Do not handle
the trap after burning it out. When trapping in a neighborhood where
rats are known to exist the traps should not be moved for three or
four days unless they have rats in them, as it is well for the rats to
become accustomed to seeing them and thus careless about entering. It
is not wise to kill rats where they are caught, as the squealing may
frighten the other rats away.
SNAP TRAPS.
Snap or spring traps are best for use in houses and stores, with the
exception of fish and meat markets. Snap traps are best for use in
runways, beams, and shelves. It is sometimes well to disguise the trap
by covering its floor with a little sawdust or dirt. They should be
first tested to see that they work properly and that the staples are
secure. New traps should be smoked or stained to render them an
inconspicuous color.
The bait should consist of some firm material, such as fried bacon or
tough meat, and should be tied on so that the rat will be obliged to
pull on it and thus spring the trap. The trap should be placed in a
corner or close to the wall on a flat, hard surface, so that the rat
can not spring it with his tail or by walking on it.
BARREL TRAPS.
In warehouses and granaries large numbers of rats may frequently be
trapped by using a barrel or garbage can having a metal top which is
carefully balanced. Large pieces of strong cheese are placed in the
middle of the cover and a plank laid from the floor to the edge of the
barrel. The rat runs up the plank onto the smooth metallic lid which
tips and the rat is precipitated into the barrel.
In cities trapping is one of the most effective of the three methods
to slaughter rodents. The rat highways are easily discovered and in
them traps capture great numbers of the unwary. In the country one can
not so readily determine the rat highway. This difficulty diminishes
the effectiveness of trapping. To make up for what is thus lost
shooting has been resorted to with good results. In Honolulu, where a
vigorous campaign against rodents is being waged, a very large
proportion of the captured rats (_Mus rattus_ and _M. alexandrinus_)
have been shot from trees. In Contra Costa County, Cal., where ground
squirrels are being exterminated, it has been found that rodents
possess an instinctive suspicion of traps and that during the summer
months shooting is not only the most practical but also about the only
effective means of attacking them. Shotguns are the weapons to use. A
rifle requires the hunter to be a better shot than is ordinarily
obtainable for such work, and, furthermore, the danger from its longer
range and from ricocheting bullets menaces cattle and farm hands who
may be working in the vicinity. As to the shot and the powder charge
for shells hunters differ. It is a different problem for every
shotgun, depending upon the gun’s caliber and choke. The principle is
to put the greatest number of the largest shot the gun will carry into
the rodent body. Thus in 10 and 12 gauge guns shoot No. 8 shot and in
16-gauge guns shoot No. 9 shot, but this varies with each individual
gun. The use of soft lead or chilled shot seems a matter for personal
preference. The charge must be as much as the gun will carry. In the
country smokeless powder becomes a necessity during summer months
since black powder is liable to ignite the dry grass and stubble.
POISONING.
PLASTER FLOUR.
Plaster flour is prepared by mixing one part dry plaster of Paris with
two parts flour or meal. When this is taken in sufficient quantity by
the rodent it produces death by the formation of enteroliths, death
occurring in from four to eight days. This is a poison of uncertain
value and is recommended chiefly on account of its cheapness and small
danger to children and domestic animals. It can not be used in wet
weather, and judging from the small number of rats found dead with
plaster casts in the alimentary canal it is not believed to be very
efficacious.
PHOSPHORUS PASTE.
Phosphorus paste is prepared by mixing crude phosphorus in the
proportion of one-half to 10 per cent in a suitable base. The latter
may consist of cheese, sugar, and oil of anise mixed together and
heated to the consistency of sirup, the phosphorus being added after
the fire has been withdrawn and the mixture begun to cool. Other bases
are cheese, corn meal, and oil of rhodium; cheese, ground fish, or
meat and oil of valerian, glucose, and a small quantity of flour.
Glucose makes an exceptionally good base, as when properly mixed the
poison thus prepared is noninflammable even when heated. The liability
to spontaneous combustion of phosphorus mixtures eliminates their use
in hay, grain, or other warehouses or places where there is danger of
fire or the invalidation of insurance. It should not be forgotten that
phosphorus deteriorates very rapidly, especially when it is exposed to
the sun.
ARSENIC PASTE.
This consists of arsenious acid combined with a base of cheese, meal,
or macerated fish. It may be placed on raisins or prunes and is to be
recommended on account of its stability, the ease with which it is
handled, and the absence of danger from fire. It should, however, be
distributed with great care, every precaution being used to place it
where it is inaccessible to children and domestic animals.
BARIUM CARBONATE.
This has not proven an effective poison owing to the fact that it is
easily decomposed by the vegetable acids, especially lactic and oleic
acid found in cheese and oil. The poisonous effect is not greatly
altered by this change. A disagreeable metallic taste is produced and
the rats will not take it.
STRYCHNINE.
Strychnine is prepared as a poison by soaking wheat over night in
water and subsequently pouring off the excess fluid and placing the
wheat in a caldron containing hot glucose and strychnia sulphate, the
latter in the proportion of one-tenth of 1 per cent. After carefully
stirring so that each grain is thoroughly coated it is dried in
shallow iron pans over a slow fire with constant agitation of the
grain, or by exposure on sheets or canvas to the rays of the sun. This
mixture may be made much more efficient by the addition of cyanide of
potassium in the proportion one-half of 1 per cent. Poisoned grain has
not been found efficient in the destruction of rats, as its bitter
taste causes them to eat little or none of it. It is, however,
particularly efficient in poisoning squirrels, as it is taken readily
by them. The chief objections to its use are its cost, difficulties of
preparation, and liability to its being taken by chickens.
CARBON BISULPHIDE.
Carbon bisulphide is not a poison so much as an asphyxiant. As the
name indicates, it is a two-to-one mixture of sulphur and carbon. The
resulting liquid preparation should be kept in air-tight cans, since
it evaporates quickly. The principle of its efficacy against rodents
is that the fumes are heavier than air and, sinking into a rat or
squirrel hole, drive out the necessary oxygen. To use bisulphide
saturate a small pad of some absorbent cotton, jute, wool, or flannel
material with the liquid, thrust this into the rodents’ burrow, and
carefully stop all apertures through which the fumes might escape.
Animal life of every sort in that burrow is quickly asphyxiated. In
buildings the use of carbon bisulphide is greatly hampered by the
difficulty to confine the gas by stopping all cracks and other
openings. Also the odors of decomposition in animals so killed stand
against its use anywhere but in the country.
Against rats and squirrels in country places carbon bisulphide has
proved one of the best of all weapons. Where it kills, it kills whole
families at a time, not one by one, as must ever be the case with
other poisons and with traps or shotguns. Not only does it kill the
rodent but it also destroys the rodent’s fleas and vermin, which is
most important. A dead infected rat is still a menace, since its fleas
may inoculate other rats and human beings with the infection. Destroy
the fleas and that greatest danger is removed. The recent campaigns
against rodents in the United States have been waged because rats and
squirrels were infected with bubonic plague; hence the added value of
carbon bisulphide. Unfortunately, though this asphyxiant proved so
effective in the work against squirrels in Contra Costa County, Cal.,
it proved to be well-nigh useless during the summer season when dry
heat checks the adobe and makes the ground generally porous.
Nevertheless, the value of carbon bisulphide, especially for sanitary
purposes, can not be easily overestimated for work in the country
during the fall, winter, and spring seasons.
Fumigants in general are effective. They possess no marked or peculiar
advantages as special weapons against rodents. Their use is limited
chiefly to warehouses, elevators, and ship holds. They are deadly to
rats in the same way that they are fatal to every sort of life. Many
difficulties and some dangers stand in the way of their use. It is
safe to advise that no one unacquainted with their action should ever
employ fumigants.
NATURAL ENEMIES.
The war upon rats carried on in San Francisco has proved the great
value of cats and dogs as natural enemies of the rat. That city now
has a law requiring all structures of 800 or less square feet and
outside certain limits to be raised high enough above the ground to
allow access to cats and dogs. (All other buildings in the city must
be rat proof.) An index to the worth of rodent foes in their
extermination points from what happened during the great London
plague. At that time the disease was supposed to be air carried; any
furry material might hold and spread infection. The magistrates
decreed that all cats and dogs should be killed to prevent plague from
lodging and traveling in their hair. Rats were thus free to live and
breed unmolested, and live and breed they did until the plague killed
them off; then and then only did the disease cease its ravages among
human beings.
DOGS.
Slight training will make excellent ratters of Fox, Irish, or Scotch
terriers. Fox terriers have proved especially valuable as retrievers
when shooting squirrels, which in one case out of five will escape to
their burrows unless sharply retrieved.
CATS.
Cats are little less valuable than dogs against rats, while they are
useless against squirrels. The ordinary cat is too well fed to attack
large rats and goes, almost solely, after mice.
All other animals naturally preying upon rodents class with wild
life—weazels, ferrets, badgers, skunks, and minks. These can be used
only in country places, where, however, their raiding of chicken coops
tends to counterbalance their value as ratters. The skunk alone is an
infrequent slayer of fowl, whereas he harvests innumerable farm pests
from worms to crickets. Yet an insurmountable prejudice against skunks
stands in the way of realizing his full usefulness against rodents. In
addition, various hawks and most owls kill off rats. Since rats come
out chiefly in the nighttime, owls have the better chance to be
serviceable in their destruction. The efficiency of these birds in
rural districts quite equals that of a dog against rats, while besides
dogs we have not as yet found a safe natural foe of ground squirrels.
But all such attacks upon rats fail of absolute eradication. One must
make it impossible for them to find sustenance and must destroy not
only all existing rat houses, but also all chance of their digging or
finding new ones.
CUTTING OFF THE RAT’S FOOD SUPPLY.
This is important not alone for its effect in a campaign upon rodents,
but equally because it necessitates sanitary care and cleanliness in
handling foodstuffs intended for humans and garbage coming therefrom.
Abattoirs and places where cattle and hogs are fattened perhaps
furnish the greatest number of rats. Stables, food-supply stores,
groceries, meat, fish and vegetable markets, restaurants, bakeries,
and the various places where food is prepared for human consumption
are usually infested. In each of the places the barriers vary
according to the nature of the premises. Rat-proof receptacles for the
foodstuffs must be installed wherever practical. In San Francisco an
ordinance requires every stable to have metal lined feed bins. Markets
and places where eatables are constantly being shifted about must be
properly screened against rats. Screening should be of heavy wire and
sufficient fineness, not larger than halfinch mesh. In all places the
food has to be raised such a height above floorings as to be beyond
the rat’s reach. This applies also to corn and grain cribs in the
country. Yet, no matter how carefully the bulk of the food may be kept
from rats, negligence in handling it, in spilling or scattering small
amounts upon floors or the ground, will nullify every precaution.
No less painstaking must be the disposition of garbage. Ordinance now
requires that all premises in San Francisco be provided with “sanitary
garbage cans.” Preferably these should be of zinc or galvanized iron
and fitted with tight covers. Under no circumstances should the cover
be allowed to remain off its can. Garbage is to be placed in a can
without delay and care must prevent the dropping of it upon the
ground. Rats once served communities as scavengers; wherever the
scavenger work is laxly done, rats are welcomed. Finally, garbage must
never be allowed to accumulate and should be removed daily, not less
often than every other day.
Special conditions, closely related to the next topic, are encountered
in large warehouses and grain sheds. Places where large quantities of
food may be stored for a length of time should be constructed of
reenforced concrete to be rat proof. Then, again, where vessels are
changing cargoes, rat-proof compounds should be erected for the
temporary storage of freights. The water fronts of seaports are
invariably rat ridden; and in San Francisco a compound for freight
held in transit was found invaluable. No effort can be spared in
keeping rats from their food if their extermination is to be
accomplished.
With regard to ground squirrels, the use of poisoned wheat very
properly enters here. With the changing season ground squirrels change
their habitat and food. During early spring these rodents come down
from their winter dwellings in the hills and seek burrows in meadow
lands and cultivated spots. Months have passed since the squirrel
tasted wheat; his fickle appetite betrays him. From the first spring
months until harvest time one can kill thousands of ground squirrels
by tempting them with poisoned wheat. But so soon as harvest time
comes, they seek new growing green stuffs to eat and thereafter, on
through winter, poisoned wheat is ineffective.
BUILDING THE RAT OUT OF EXISTENCE.