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246 CHAPTER 11

75 percent of the cases of tularemia. The other TICK IDENTIFICATION
25 percent of cases are spread over those called
glandular, oropharyngeal, pneumonic, oculoglandular, It may be important to determine the identity of a tick
and typhoidal. The incubation period is widely variable when assessing the potential for various infections. The
and ranges from only a few to 14 days, with most human following information and images may be useful when
cases appearing within 5 days of exposure. Humans performing a body search and in finding foraging or
characteristically experience skin lesions while infected embedded ticks in order to determine the potential risks.
wild and domestic animals do not. The victims may
experience a moderately elevated temperature, but in Dermacentor variabilis: The American dog tick is
some cases the body temperature rises to a high degree found throughout the eastern United States and in
in severe illnesses. When fever is moderate to very Colorado and the Pacific Northwest (Figure 11-22).
high levels the tularemia bacillus can often be isolated It occurs in some areas on a cyclical basis by becom-
from blood cultures during this stage of the disease ing more abundant in certain habitats during alternate
(Figure 11-21). Face and eyes may redden and become years. Adults are characterized by creamy-gray mark-
inflamed and the inflammation spreads to the lymph ings on the scutum and by short mouthparts. Larvae
nodes which enlarge and may suppurate (form pus that and nymphs feed on small mammals, especially meadow
mimics bubonic plague). mice which inhabit vegetation where ticks are often
Lymph node involvement usually occurs during found, and are seldom seen by people. Adults appear
a high fever and death occurs in less than 1 percent of in April and reach peak numbers in June but are sel-
cases if therapy is initiated quickly following diagnosis of dom seen after early July. They feed on a wide range
an infection. The drug of choice is the antibiotic called of mammals that includes deer and other wild animals,
streptomycin although tularemia may also be treated livestock, pets, and humans. A female may deposit more
with gentamycin for 10 days or a tetracycline-type anti- than 6000 eggs before she dies.
biotic such as doxycycline for 2 to 3 weeks. The more The American dog tick is the most common tick
toxic chloramphenicol or fluoroquinolones may be pre- found on humans and dogs in many areas of the coun-
scribed and administered as well. The vaccine produced try. Engorgement by adult females has been associated
by attenuated (dead or damaged) live vaccine is available with tick paralysis. This species is the primary vector
to vaccinate at-risk individuals but its use is restricted of spotted fever and also can be a vector of the diseases
to these groups who may be occupationally exposed to tularemia and anaplasmosis. Evidence to date does not
F. tularensis. support involvement in the transmission of Lyme disease
by the dog tick but researchers continue to pursue this
possibility.

Source: Centers for Disease Control and Prevention (CDC) Source: Centers for Disease Control and Prevention (CDC)

FIGURE 11-21 Francisella tularensis, colonies FIGURE 11-22 Female Dermacentor variabilis,
grown on Chocolate Agar, 72 hours American dog tick