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Organisms Borne by Ticks and Other Vectors 231

Source: Centers for Disease Control and Prevention (CDC)

FIGURE 11-5 Dorsal view of both a female and male wood tick, Dermacentor andersoni; smaller female’s scutum when
compared to the male’s larger scutum

dog ticks are important in North America: Dermacentor The Ixodes pacificus tick, found in the Pacific
variabilis, commonly called the American dog tick or the Northwest region of the United States, is responsible for
eastern wood tick and the Rhipicephalus sanguineus, or the majority of Rocky Mountain spotted fever cases in that
the brown dog tick, which is seldom implicated in trans- area, along with the previously mentioned Dermacentor
mitting diseases to humans. species. Livestock are the primary adult hosts of this
As mentioned previously, ticks are divided into species of tick, whereas in the eastern United States, is
two general groups called hard and soft ticks. Hard ticks the deer. Immature stages of the Ixodes scapularis ticks
are named due to a hard, shiny scutum that resembles a prefer the deer as their primary host but may spend their
shield on their backs, also known as the dorsal region. juvenile stage of life on small mammals and birds before
The male tick is smaller than the female and the scutum they are able to infect a deer. The female adult tick is able
covers the majority of the dorsal surface of the male. to lay more than 2000 eggs, but must have a period of
The female’s scutum is not as large, an anatomical fea- several days of ingesting blood before being able to lay
ture that equips her to expand in size as she feeds and her eggs (Figure 11-6). And to again emphasize a point,
becomes engorged with the host’s blood. Common ex- the North American Ixodes are hard ticks due to the
amples of hard tick species in the United States and over
North America that are vectors of various diseases are
the genera Amblyomma, Ixodes, Dermacentor, Anocentor,
Haemaphysalis, Hyalomma, Rhicephalus, and Boophilus.
The first three representatives of these genera are those
most prominent in causing human diseases in the United
States. Examples of soft ticks species that are not as
medically important as hard ticks include the genera Source: Centers for Disease Control and Prevention (CDC)
Arga, Alectrobius, Alyeonasus, Cario, Ornithodoros, and
Otobius. The following figure of a female wood tick,
Dermacentor andersoni, is a known vector of Rickettsia
rickettsii, the causative agent for Rocky Mountain spot-
ted fever (Figure 11-5). D. andersoni is sometimes con-
sidered the Rocky Mountain tick and is responsible for FIGURE 11-6 Upon laying eggs, this female Ixodes
a portion of the tick-borne diseases contracted in, or at hard tick will die, whereas the soft tick female may lay
least attributed to, the Rocky Mountain region. many batches