Issued October 1938
Revised February 1946 E-454
United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine
COMBATING THE AMERICAN DOG TICK, CARRIER OF ROCKY MOUNTAIN
SPOTTED FEVER IN THE CENTRAL AND EASTERN STATES
By F. C. Bishopp, assistant chief of Bureau, and Carroll N. Smith
and Harry K. Gouck, Division of Insects Affecting Man and Animals
The American dog tick or common wood tick (Dermacentor variabilis
(Say)) of the Mississippi Valley and the region eastward carries the
dread human disease known as Rocky Mountain spotted fever and is capable
of carrying rabbit fever (tularemia) and of inflicting annoying bites.
It occasionally causes a type of paralysis in man or dogs and is extremely
troublesome by its attachment in great numbers to dogs. The annoyance
from this tick's habit of crawling on people and the fear of its bite
tend to drive vacationists away from otherwise attractive outing places.
It does not infest houses, although a few ticks may be carried into
houses from time to time on clothing or by dogs. If a house is contin-
uously tick-infested, and especially if the ticks are found in considerable
numbers, a different species is probably concerned, namely, the brown dog
tick (Rhipicephalus sanguineus (Latr.)). i/
With its proved record of disease transmission, obviously the bite
of this tick should be avoided and efforts should be put forth to control
the pest. Rocky Mountain spotted fever does not occur among the ticks in
every locality, and only a small percentage of them are infected. In
most instances, therefore, the attachment of a tick will cause no serious
consequences. However, this disease appears here and there in unexpected
places, and when a person does happen to be bitten by an infected tick
the results may be serious, as the mortality in the East usually runs from
20 to 25 percent.
Stages and Development of the Tick
The American dog tick and related species pass through four stages--
the egg, the seed tick or larva, the nymph, and the adult. The brown
eggs are laid in a dense mass in some protected place on the ground. The
eggs hatch in 1 to 2 months, depending on the temperature, producing tiny
6-legged seed ticks. Under experimental conditions the seed ticks may
live as long as 18 months without food, but in nature their longevity
seldom exceeds 12 months. When opportunity offers, they attach to small
I_/ For information on the control of this species see
Bishopp, F.C., The brown dog tick, with suggestions for its control.
U. S. Bur. Ent. and Plant Quar. E-292, 3 pp., illus. (rev.) 1939.
wild rodents, such as meadow mice. They fill themselves with blood in
2 to 13 days, the majority requiring about 4 days, then drop and hide in
some protected place. After 6 days to 8 months, depending on the tempera-
ture, they molt their skins. The resulting nymphs are larger than the
larvae and have 8 legs. They also attach to small animals and engorge in
3 to 11 days. If a host animal is not at hand they may live for as long
as 19 months, but in nature they are seldom active for more than a year.
In 17 days to 10 months, depending on the temperature, the engorged nymphs
molt to adult males and females. These are the flat, brownish, active
creatures, known as wood ticks, which attach to larger animals including
dogs and man. The males have a network of white lines on their backs
(fig. 1). The females have a hard shield marked with white on the fore
part of the back; the rear part is softer and reddish, stretches,. and
becomes grayish as blood is ingested (fig. 2). The adult ticks may live
nearly 3 years without food under experimental conditions, but in nature
only a few are active, for two reasons--engorgement of the females requires
from 5 to 13 days, and mpting takes place during this time. In from 3 to
58 days, depending on the temperature, after the engorged females drop
from the host, egg laying begins. An engorged female may lay from 3,000
to 6,500 eggs, after which she dies.
Wood ticks are most active in spring and in the early part of the
summer. There are relatively few ticks abroad after Aupust 1. In the
South their activity is not so sharply restricted seasonally.
Where the American Dog Tick is Found
The American dog tick occurs here and there over the country from
eastern Montana, Kansas, and Texas, eastward to the coast, and also in
parts of western California. It is most commonly found in parts of
Minnesota, Wisconsin, Iowa, and Texas, and along the Atlantic coast from
Cape Cod to Florida. It is most abundant in damp areas that are covered
with underbrush and tall grass and weeds.
The young ticks (seed ticks and nymphs) engorge upon small wild ani-
mals, particularly meadow mice, but pine mice, white-footed mice, cotton
rats, rice rats, and sometimes rabbits and squirrels are also satisfactory
hosts. The adults prefer dogs and various kinds of wild animals, such
as raccoons, opossums, and coyotes. They also infest horses and, to a
lesser extent, cattle, hogs, and other domestic and wild animals. Man,
although frequently bitten, is not a preferred host.
Methods of Control
Methods of control applicable to many of the conditions under which
this tick occurs have not been developed. Several things may be done,
however, to reduce the number of wood ticks, especially around habitations.
(1) Since the dog is the principal host utn which the adult ticks
feed, an effort should be made to prevent the ticks from engorging
thereon. Each female tick engorged on a dog is capable ofproducing
from 3,000 to 6,500 young. Therefore, dogs should be kept avay from
tick-infested areas, the ticks hand-picked daily, or better, a derris
dip applied to the dogs regularly every 4 days. If but few ticks are
present and hand picking is employed, it is best to use tweezers or
forceps rather thah to pick them off with the fingers. If the fingers
are used, the hands should immediately be carefully washed to reduce
the danger of acquiring infection from the ticks. Rubbing the eyes or
nose while the hands may be contaminated should be avoided.
Rotenone-bearing materials, such Rs derris and cube, are effective
in killing the tick on dogs when applied as a dip or dust. The dip is
made by mixing 2 ounces of fine derris or cube powder, 1 ounce of neutral
soap, and 1 gallon of tepid water. The powder should contain at least
3 percent of rotenone. The dog may be put in a tub containing the dip,
or it may be applied thoroughly with a brush, sponge, or dipper. The
dip is allowed to dry on; if necessary, the surplus liquid may be removed
with a towel. The dip can be kept for at least a week without deteriora-
tion. If the dip cannot be used conveniently, derris powder may be applied
to the skin on all infested parts. To prevent ticks from engorging, the
powder must be applied every other day. The persistence of either the dip
or the dust depends on the type of dog, and the interval between treatments
can be determined best by experience. The powder or dip should not be
permitted to get into the eyes.
(2) Near habitations, parks, etc., rodents, especially meadow mice
and rats should be destroyed by trapping or poisoning. Poisoning operations
should be carried on only under the direction of competent authorities.
(3) Underbrush, grass, and weeds around dwellings, schools, parks,
and other places frequented by the public should be kept closely cut.
This practice tends to drive out rodents and removes protection favorable
to the ticks.
(4) Close grazing of brushy and weedy areas with sheep will do much
to reduce tick abundance. If sheep with fine wool are used, relatively
few ticks become engorged on them and many ticks are killed by the wool
(5) Clearing and burning over vacant overgrown lots in towns and
cities reduces the tick hazard.
(6) Many of the ticks in an infested area can be killed by applying
spray or dust to the ground and vegetation. The most satisfactory material
to use for this purpose is DDT, as it provides a high initial kill and a
lasting residual action. It may be applied as a spray or as a dust,
whichever is more convenient. By either method from 2 to 3 pounds of DDT
per acre should be applied. A convenient dust to use is one containing
10 percent of DDT in pyrophyllite. At this concentration from 20 to 30
pounds of dust per acre would be used, which is sufficient to assure
Sprays containing DDT in various formulations with different solvents
and emulsifiers have been found satisfactory and it seems probable that
any water-miscible preparation will be effective. The following prepara-
tions when used in 100 gallons of water per acre have given excellent
results: (1) 2 1/2 pounds of technical DDT dissolved in 12 1/2 pints of
soluble pine oil; (2) 2 1/2 pounds of technical DDT dissolved in 6 1/4
pints of xylene and 1 1/4 pints of Triton X-1O0 (an aralkyl polyether
alcohol). The amount of spray needed to secure adequate coverage varies
with the type of vegetation. If 100 gallons per acre is not convenient,
the quantity of water used may be increased or decreased, but the amount
of DDT used should remain between 2 and 3 pounds per acre. At this rate
DDT will not injure vegetation, and most of the solvents and emulsifiers
used in water-miscible commercial preparations will also be harmless,
but no assurance can be given that this will be true in every case. At
this dosage an initial reduction in tick abundance of better than 90
percent may be expected, and the treated area should remain practically
free of ticks for a month or more.
CAUTION: DDT is poisonous if taken by mouth, and to a lesser extent
by contact. Oil solutions of DDT, particularly the more concentrated ones,
should not be allowed to remain on the skin.
Methods of Avoiding Tick Attack and Infection
The reduction of the number of ticks in a given area by the methods
described lessens the danger of attack, but the following precautions
against being litten, or otherwise infected, should be taken when one is
in a tick-infested areas
(1) Wear high-top shoes over the bottoms of the trousers, or, if
such shoes are not at hand, wear the socks over the bottoms of the
trousers. Most ticks crawl up from the ground or low vegetation, and
this arrangement keeps them from crawling beneath the clothing.
(2) Train yourself to feel the ticks when they start to crawl on
the neck or body, and remove then.
(3) Occasionally glance over the clothing, especially when two
people are together, to detect the presence of ticks before they reach
the neck or get into the clothing. Light-colored clothing aids in this.
(4) After walking in ticky areas examine the outer clothing and
look underneath the collar before entering an automobile or house.
(5) If ticks are abundant, it is well to remove all clothing upon
coming in from the field. Drop the garments loosely into a large metal
container, such as a 25-pound lard can, pour 2 teaspoonfuls of carbon
tetrachloride on top of the clothing or in a saucer set on the clothing,
and put the lid on over a sheet of wrapping paper or o seal it with
adhesive tape. This will kill all ticks in 8 hours. Carbon disulfide
may be used in the same way but, being inflammable, it is more dangerous.
If this fumigation cannot be carried out, examine the clothing carefully
for ticks, including folds and seams.
(6) Never sleep in clothing worn during the day, as ticks sometimes
ide in the folds and attach at night.
(7) Do not lay field clothing on the bed.
(8) Examine the body thoroughly upon arising, again at noon, and
upon retiring. Individuals should exaiaine each other, and a long mirror
is an aid. Pay particular attention to the hairy parts, especially the
back of the head. Ticks often lie very close to the skin when first
attached ard are easily overlooked. Combing the hair upward with a
moderately fine-toothed comb helps reveal the presence of ticks.
(9) If a tick is ound attached, remove it a; once by grasping it
with the fingers or eeers and pulling steadily here is no danger
of the mouth parts of this species being broken ff in re ouwl. Ticks
should not be left attached any longer than can e helped 2%Ifected
ticks are not likely to reduce the disease unless they a been
allowed to feed for several hours, usually 6 to 8o
(I0) Cauterize the point of attachment with a slti o carbolic
acid or of silver nitrate. This can be done best by dipp;g the roint of
a round wooden toothpick in the solution and then illirg t lhtly
into the skin at the exact point of attachment. Iodine iay b us if
the other materials are not at hand.
(11) The blood from crushed ticks may be a source f infeeion
if it gets into the eye or into a skin abrasion. Therefore, the use of
insecticides on dogs is advised, rather than hand picks. If the ticks
are picked, remove them carefully with small forceps and drop thu at
once into kerosene or boiling water. Do not rub the eyes Wash the
hands thoroughly when the job is finished.
(12) Do not allow ticks that have been removed frn a to
escape, because they may attach to man, and the infected s a
capable of conveying the disease very soon after att
(13) If Rocky Mountain spotted fever is e ed
physical exertion should be avoided, a competent pys 'on-
sulted, and prompt hospitalization is desirable. The n period of
spotted fever in man is 2 to 12 days.
(14) A preventive vaccine has been developed by the United States
Public Health Service, but the production is limited and the material is
being used in special cases only.
Digitized by the Internet Archive
Figure l.--The male American dog tick as seen
from above. About 13 times natural size.
Figure 2.--The female American dog tick as seen
from above. About 13 times natural size. After
attachment to an animal for a few days the body
of the female becomes greatly distended with
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