Page 215 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com in the United States. The major bacterial diseases transmit- mebooksfree.com
PART II Clinical Bacteriology
204
Lyme disease is the most common vector-borne disease
ted by ticks in the United States are Lyme disease, Rocky
Mountain spotted fever, ehrlichiosis, anaplasmosis, relaps-
ing fever, and tularemia. Ixodes scapularis ticks transmit
three diseases: two bacterial diseases, Lyme disease and
human granulocytic ehrlichiosis, and the protozoan dis-
ease, babesiosis. Coinfection with B. burgdorferi and Babe-
sia occurs, especially in endemic areas such as Massachusetts
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and other northeastern states.
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mebooksfree.com mebooksfree.com mebooksfree.com Pathogenesis is associated with spread of the organism mebooksfree.com
Pathogenesis

from the bite site through the surrounding skin followed by
dissemination via the blood (bacteremia) to various organs,
especially the heart, joints, and central nervous system. No
FIGURE 24–6
exotoxins, enzymes, or other important virulence factors
Ixodes scapularis—”blacklegged” tick. Engorged
have been identified.
female tick after feeding. (Source: Dr. Gary Alpert, Centers for Disease Control
Note that the organism must adapt to two markedly dif-
and Prevention.)
ferent hosts, the tick and the mammal (either mice or
humans). It does so by changing its outer surface protein
(OSP). These OSPs vary antigenically within humans.
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Multiple episodes of Lyme disease are due to reinfection,
rather than relapse caused by reactivation of the organism.
There is no evidence for a latent stage of B. burgdorferi.

The clinical findings have been divided into three stages;
however, this is a progressive disease, and the stages are not
discrete. In stage 1 (early localized stage), the most com-
mon finding is erythema chronicum migrans (also called
erythema migrans), an expanding, erythematous, macular
rash that often has a “target” or “bull’s eye” appearance
(Figure 24–8).
The rash appears between 3 and 30 days after the tick
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bite. Both the tick bite and the rash are painless and
nonpruritic.

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mebooksfree.com mebooksfree.com mebooksfree.com FIGURE 24–8 Erythema chronicum migrans rash of Lyme dis- mebooksfree.com

FIGURE 24–7
Ixodes tick on a blade of grass questing for a
ease. Note oval-shaped expanding erythematous macular “bull’s eye”
host, such as a deer or human. (Source: Drs. Amanda Loftis, Will Reeves, and
rash of primary Lyme disease. (Used with permission from Vijay K. Sikand, MD.)
Chris Paddock, Centers for Disease Control and Prevention.)

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