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Ectoparasites 219

In addition, they are a member of the class Arachnida
Source: Centers for Disease Control and Prevention (CDC) of infectious organisms that are termed arboviruses
because they possess eight legs and are not insects, which
have six legs. Ticks are capable vectors for a number

because they are transmitted by arthropods. Some of
these diseases include Rocky Mountain spotted fever,
caused by Rickettsia rickettsii; Lyme disease, caused by
Borrelia burgdorferi, Tularemia (rabbit fever), caused
by Francisella tularensis; and human granulocytic

organism.
These organisms are bloodsucking arachnid para-
sites of the order Acarida. Unlike the mites of an associ-
FIGURE 10-6 Patient’s hand reveals a scabies ehrlichiosis (HGE), caused by the Ehrlichia chaffeensis
infestation of the mite species Sarcoptes scabiei var. ated group, adult ticks are relatively large and are easily
hominis seen by the naked eye and act as vectors for certain bac-
teria and viruses known to cause disease in humans.
They are the only venomous creature that hunts down
Source: Centers for Disease Control and Prevention (CDC) than actively seeking them. There are two kinds of ticks
humans in some species of ticks. Fortunately, most in-
sects and arachnids try to avoid human contact rather

simply called “hard” and “soft” and each carry different
diseases. Hard ticks, of the family Ixodidae, do not bur-
row into the skin. But soft ticks, of the family Argasidae,
that primarily infest birds but sometimes will affect hu-
mans, do burrow into the skin and must be removed with

imbedded when the body is pulled away (Figure 10-8),
resulting in infections in the bite area, and transmission
of disease organisms.
FIGURE 10-7 Chiggers (larvae of mites from the great care, for otherwise their heads will possibly remain
genus Trombicula) can cause intense itching and The life cycle for ticks is relatively simple when
reddish welts on skin compared with those of the endoparasites, as they exist
in stages that do not remotely resemble the adult form
rashes may be found where tight clothing is near (Figure 10-9). The nymphal stage is of particular inter-
the skin such as collars and cuffs of shirts and socks est because usually there is only a single nymphal stage
(Figure 10-7). Irritation of the skin appears to occur through which a tick passes; however, some ticks pass
due to an allergic reaction to the saliva of the insect through multiple nymphal transitions during the nymphal
rather than to the mechanics of the bite and these stage prior to becoming a mature adult organism.
organisms may also act as vectors for a number of Dog ticks may bring slow paralysis, similar to
infectious diseases. Currently these mites are not that of the Guillain-Barre Syndrome, and may be fatal
known to transmit any infectious diseases in the to children. Interestingly, both the nymphal and adult
United States. However, in some Asian countries, stages of most species of ticks are capable of infecting a
they are vectors of scrub typhus, which is caused by host organism with a variety of disease agents. Nymphal
a rickettsial (bacteria-like) organism. forms of ticks are more difficult to discover on the body,
although they are perfectly capable as vectors for many
Ticks infectious diseases that may result in acute or chronic
disease with lasting effects in humans (Figure 10-10).
Ticks belong to the phylum Arthropoda, meaning that Fortunately ticks must remain embedded for at
they move by use of jointed legs (arthro) and feet (poda). least 18 hours or more in order to transmit the various