[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 13 The Arthropods
TABLE 13-1
Hard and Soft Adult Ticks:
Typical Features at a
Glance
Characteristic
Hard
Ticks
Soft
Ticks
Fused spherical body (head,
X X
thorax, abdomen together)
Four pairs of legs X X
Visible capitulum on dorsal side X
Capitulum on ventral side
X
Scutum
X
ranges from 1 to 2 years, depending on the hatch
season. Ticks can also pass many microorganisms
to their offspring, resulting in a renewable source
of the infectious agent. The tick eggs develop and
hatch on the ground. Motile larvae emerge from
the eggs and migrate to sites such as blades of
grass and twigs. The larvae eagerly jump onto the
first viable host that passes by. Once on the host,
the larvae feed via a blood meal for a few days,
fall off the host and back to the ground, and molt
into nymphs. These eight-legged nymphs once
again migrate to potential host-passing sites and
wait for another host. After attaching to a second
host, the nymphs repeat the same process that
they underwent in their larval stage. After falling
onto the ground the second time, the nymphs
molt and transform into adult ticks. It is interesting
to note here that a tick completes a blood meal
by making a cut into the host epidermis using a
toothed structure near their mouth called a hypstome.
The blood obtained during this process
does not clot due to the presence of an anticoagulant
in the tick’s salivary gland. As the tick feeds,
its body expands. Hard ticks—in particular,
members of the Ixodidae family—only feed once
as adults. Adult soft ticks, however, feed repeatedly.
Following mating, eggs are deposited on the
ground and the cycle repeats itself.
Epidemiology and Geography
Ticks are found throughout the world, including
the United States and Mexico. In addition to select
parasites, hard ticks are responsible for transmitting
bacterial, viral, and rickettsial diseases. Hard
ticks such as Ixodes spp. (deer ticks) have a wide
geographic range covering most of North
America. These ticks are the primary vector for
Borrelia burgdorferi, the cause of Lyme disease,
and Babesia spp. (see Chapter 6), both of which
are noted for being found in New England. Similarly,
Dermacentor spp. (dog ticks) can be found
from the eastern United States to the Rocky
Mountain range. Dermacentor ticks are associated
with a number of rickettsial diseases, including
Rocky Mountain spotted fever. Although
concentrated in the southern states east of the
Rocky Mountains, Amblyomma americanum
(lone star tick) can also be found in the Mid-
Atlantic states. Amblyomma ticks are the vector
for human ehrlichiosis. Soft ticks belong to the
genus Ornithodorus are primarily responsible for
transmitting Borrelia spp., which causes relapsing
fever. Different species of Ornithodorus are found
in different geographic ranges within the United
States and Canada. The Borrelia spp. that these
ticks transmit are usually given the same species
name as the tick; for example, Ornithodoros
hermsi is found primarily in the northwestern
United States and Canada and transmits Borrelia
hermsii, whereas Ornithodoros turicata is the
primary soft tick species found in the southwestern
and midwestern states.
Clinical Symptoms
Patients infected with ticks often exhibit skin
reactions to the bite site, including inflammatory
infiltration of tissues, edema, local hyperemia,
and hemorrhage. Additional potentially severe
tissue reactions and secondary infections may
occur when the mouth parts of a tick remain in the
skin after attempting to remove the entire tick.
Tick paralysis may occur when the salivary secretions
of certain tick species (Dermacentor) are
introduced into the host. A toxemia results and, if
the tick is not readily removed, death may result.
Treatment
The recommended therapy for tick infestation
consists of removal of the tick. This may be
accomplished by placing a few drops of ether or