Many zoonoses are spread by ticks including Lyme disease, Rocky Mountain spotted fever, ehrlichiosis, tularemia, tick typhus,and babesiosis. Mosquitoes transmit the arboviral encephalitides (see Chapter 101), dengue fever, malaria, and yellow fever. Preventive measures to avoid tick-borne and mosquito-borne diseases include using insect repellants that contain DEET (N,N-diethyl-m-toluamide) on skin and insect repellants that contain permethrin on clothing; avoiding tick-infested habitats(thick scrub oak, briar, poison ivy sites); avoiding excretory products of wild animals; wearing appropriate protective clothing(closed-toe shoes, long pants, etc.); and promptly removing ticks.
Ticks are best removed using blunt forceps or tweezers to grasp the tick as close to the skin as possible to pull the tick steadily outward. Squeezing, twisting, or crushing the tick should be avoided because the tick’s bloated abdomen can act like a syringe if squeezed. Transmission of infection seems to bemost efficient after 30 to 36 hours of nymphal tick attachment forhuman granulocytic ehrlichiosis, after 36 to 48 hours for B. burgdorferi, and after 56 to 60 hours for Babesia. Prophylactic antimicrobial therapy after a tick bite or exposure is not recommended.
Lyme disease is a tick-borne infection caused by the spirochete, B. burgdorferi. The vector in the eastern and midwestern United States is Ixodes scapularis, the black-legged tick that is commonly known as the deer tick. The vector on the Pacific Coast is Ixodes pacificus, the western black-legged tick. Ticks usually become infected by feeding on the white-footed mouse (Peromyscus leucopus), which is a natural reservoir for B. burgdorferi. The larvae are dormant over winter and emerge the following spring in the nymphal stage, the stage of the tick that is most likely to transmit the infection to humans.
Over 20,000 cases are reported annually in the United States, with 96% of cases reported from New England, the eastern parts of the Middle Atlantic states, and the upper Midwest
Figure 122-1 The geographic distribution of 24,364 confirmed cases of Lyme disease in the United States in 2011. The risk for acquiring Lyme disease varies by the distribution of Ixodes scapularis and Ixodes pacificas, the proportion of infected ticks for each species at each stage of the tick’s life cycle, and the presence of grassy or wooded locations favored by white-tailed deer. (From Centers for Disease Control and Prevention: Lyme Disease—United States, 2011, available from the CDC website: http://www.cdc.gov/lyme/stats/ maps/map2011.html (Fig. 122-1). In Europe most cases occur in Scandinavian countries and central Europe. Because exposure to ticks is more common in warm months, Lyme disease is noted predominantly in summer. The incidence is highest among children 5 to 10 years old, at almost twice the incidence among adolescents and adults.
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