34-11: Lyme Disease (Lyme Borreliosis)

GENERAL CONSIDERATIONS

This illness, named after the town of Old Lyme, Connecticut, is the most common tick-borne disease in the United States and Europe and is caused by genospecies of the spirochete B burgdorferi. In the United States, the causative genospecies is B burgdorferi senu strictu, whereas in Europe and Asia B garinii and B afzelii predominate. Most US cases are reported from the mid-Atlantic, northeastern, and north central regions of the country. The true incidence of Lyme disease is not known for a number of reasons: (1) serologic tests are not standardized (see below); (2) clinical manifestations are nonspecific; and (3) even with reliable testing, serology is insensitive in early disease.

The tick vector of Lyme disease varies geographically and is Ixodes scapularis in the northeastern, north central, and mid-Atlantic regions of the United States; Ixodes pacificus on the West Coast; Ixodes ricinus in Europe; and Ixodes persulcatus in Asia. The disease also occurs in Australia. Mice and deer make up the major animal reservoir of B burgdorferi, but other rodents and birds may also be infected. Domestic animals such as dogs, cattle, and horses can also develop clinical illness, usually manifested as arthritis.

Ticks feed once during each of their three stages of life. Larval ticks feed in late summer, nymphs in the following spring and early summer, and adults during the fall. In the northeastern United States and the mid-Atlantic states, the preferred host for the nymphs and larvae is the white-footed mouse (the black-striped mouse in Europe). This animal is tolerant of infection—a fact that is critical in maintaining infection, since the mouse can remain spirochetemic and transmit the agent to the larvae the following spring after being infected by the nymphal form in early summer. Adult ticks prefer the white-tailed deer as host. Although only 20–25% of nymphs harbor spirochetes compared with 50–65% of adults, most infections occur in the spring and summer (when nymphs are active), and fewer cases occur in the cooler months (October to April), when adults feed. This is probably due to the greater abundance of nymphs; greater human outdoor activity in spring and summer, when nymphs feed; and the fact that adult ticks are larger, easier to detect by the human host, and thus can be removed before disease is transmitted. Less than 1% of larvae are infected with spirochetes, and transmission of the disease through contact with larvae is unlikely. In the western United States, the I pacificus nymph prefers to feed on lizards, which are not susceptible to infection. It is only the few nymph and larval ticks that feed on wood rats, which can be infected, that are capable of transmitting disease.

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