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Organisms Borne by Ticks and Other Vectors 229

to both hairy and smooth skin. There the tick becomes (RMSF), ehrlichiosis, Southern Tick–associated Rash
firmly anchored and feeds on the blood of the host. Illness (STARI), or tularemia, as well as some others
This practice provides an excellent mechanism through that are not as common. Certain species of ticks are
which ticks are quite proficient at transmitting parasites more likely to transmit these diseases based on the spe-
and other organisms into the bodies of their hosts while cies found in certain geographic locations. Commonly
they are feeding on the host’s blood. encountered species by geographic location are listed
Ticks do not have wings and are unable to jump as throughout this section. For example, white-tailed deer,
they have somewhat short legs that are not adapted for common throughout the southern United States, are
jumping, so physical contact is the only method of infesta- frequently infested by “seed ticks,” which are small
tion. Ticks often fall onto their hosts from vegetation when forms of the deer tick called Ixodes scapularis, a spe-
the potential host walks under the tick’s plant haven but cies that is widely distributed along the eastern sea-
some actually stalk the host on foot. They become aware board of the United States. Deer tick populations in the
of their prey due to heat and carbon dioxide emitted from eastern United States are dependent almost entirely on
their victims, and they become most active in the spring the white-tailed deer herd as the number of ticks in a
when the temperature rises and the days are longer. Ticks given area roughly correlates with the size of the deer
often take several days to complete a blood meal and the population.
attachment must last for roughly a day in order to trans- The life cycle of a tick is quite simple and under-
mit the infective organisms to their hosts. goes similar stages in development as those of other
ectoparasites. The nymphal stage as a smaller version
of the adult stage, is rather diverse; however, for most
TICKS AND DISEASES COMMONLY there is often only one nymphal stage through which a
TRANSMITTED BY THESE tick passes. However, some species of ticks pass through
VECTORS several metamorphic changes during the nymphal stage
before becoming a mature adult tick (Figure 11-2). And,
Fortunately not all tick bites result in the contraction dependent upon the species of the tick and the organ-
of an infectious disease. Some of the more common ill- isms with which the tick is infected, different stage(s) of
nesses that can be contracted from a tick bite include the tick’s life cycle may be required for transmission of
Lyme disease, babeosis, Rocky Mountain spotted fever a tick-borne pathogen. If these criteria as to species and

Source: Centers for Disease Control and Prevention (CDC)

FIGURE 11-2 Morphologic features associated with the four life cycle stages a tick experiences during its passage to adulthood