Mode of Transmission

Certain species of ticks in the genus Ixodes feed on field mice in their larval stage. Mice, who serve as the reservoir for B. burgdorferi, are asymptomati-cally infected and carry a high number of these spirochetes in their bloodstream. The white-footed mouse (Peromyscus leucopus) is a key reservoir species for B. burgdorferi in the United States. After feeding on an infected mouse, the newly infected Ixodes larva will harbor B. burgdorferi for the remainder of its life. Larval ticks develop into nymphs following this blood meal from a mouse. Immature ticks typically require two to four days of attachment to the host to complete a blood meal. The nymph must take another blood meal prior to molting into an adult, and this feature of the tick life cycle places persons at risk. The peak feeding time for nymphs is from May through late summer, when human outdoor activity is at a peak (70,71).

Adult ticks will take a final blood meal, usually in late fall or winter, prior to laying eggs. White-tailed deer are the preferred hosts for adult ticks, but serve as poor reservoirs for B. burgdorferi. White-tailed deer thus serve to maintain the population of ticks, and not that of B. burgdorferi. Ixodes scapularis are not found in geographic regions where deer are absent, and tick abundance appears to be directly related to deer abundance. B. burgdor-feri is transmitted trans-stadially from larvae to nymph to adult, so even adult tick bites pose a risk to people. Importantly, a tick must be attached to its human host for at least 24 hours for Borrelia transmission to occur (64,71,72,73).

Lyme disease is acquired from the bite of an infected tick. Ixodes scapularis is the dominant vector of B. burgdorferi. Although B. burgdorferi has been detected in other blood-feeding arthropods such as the American dog tick (Dermacentor variabilis), mosquitoes, fleas, and tabanid flies (deer flies, horse flies), the presence of the spirochete in these arthropods is transient, and they are unlikely to transmit the spirochete to new hosts. There is no evidence to support person-to-person transmission. Transplacental transmission has been reported, but it appears that adverse birth outcomes are rare. Transmission of B. burgdorferi by the transfusion of blood obtained from a spiro-chetemic donor has never been reported (74-78).

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