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In most cases, transmission of the spirochete Bb is by the Ixodes scapularis tick (I.scapularis), which is commonly known as the deer or mouse tick, or more correctly, the black legged tick. The Ixodes tick is very small in comparison to the more common dog tick. To understand the transmission of the Bb spirochete, one must review the life cycle of the I. scapularis or “black legged” tick.
The adult Ixodes tick lays eggs in the spring. The larvae develop in a month and by summer are ready to feed on mice, birds, rabbits and deer (many believe only the white footed mouse and perhaps other rodents are intermediate hosts and that deer and other putative hosts merely provide temporary lodging and source of feeding). It is during this time that larva feeding on Bb infected mice acquire the Bb spirochete. In fall and winter the larva becomes dormant. In spring the larva molt into a nymph form, which feed on deer, mice, and rabbits and, as an incidental or accidental host, humans. It is during this spring and summer season (primarily May through September) that the infected nymph form of the tick transmits the spirochete to humans, as well as continuing the future spread of the spirochete by transmitting it to the white footed mouse. By the fall, the nymph has transformed to an adult tick. Although adult ticks carry Bb, they seldom transmit the disease to humans because the adult forms are active during the fall and winter, a time when humans have a more limited outdoor exposure and wear more clothing when outdoors because of the elements. This is also a time in which the ticks are larger (see below illustration) and therefore more visible, a particularly germane point when one considers that the tick form may require up to 48-72 hours in order to transmit the Bb spirochete (see photo below).