Ticks – the vectors of B. burgdorferi The bacteria are maintained in an enzootic cycle involving hard-bodied ticks belonging to the Ixodes ricinus species complex and a wide range of reservoir vertebrate hosts. The global distribution of Ixodes species is shown in Figure 3. In the eastern United States the vector is primarily Ixodes scapularis and in the western US it is I. pacificus. I. ricinus and I. persulcatus are the vectors in Europe and Eurasia, respectively. These ticks have a 2-year life cycle (Figure 4). Adult ticks feed and mate on large animals, especially white-tailed deer, in the autumn and early spring. However, white-tailed deer are not considered reservoirs of B. burgdorferi because they do not support a sufficiently high level of spirochetes in their blood to infect ticks. Nevertheless, deer are important in tick reproduction and serve to increase tick numbers in an area and spread ticks into new areas. Female ticks then drop off the animals and lay eggs on the ground. By summer, the eggs hatch into larvae, which feed on mice and other small mammals and birds through to early autumn; then they become inactive until the following spring when they molt into nymphs. Nymphs feed on small rodents and other small mammals and birds during the late spring and summer and molt into adults in the autumn, completing the 2-year life cycle. Larvae and nymphs typically become infected with borreliae when they feed on infected small animals, particularly the white-footed mouse. The tick remains infected with the borreliae as it matures from larva to nymph or from nymph to adult. Infected nymphs and adult ticks then bite and transmit the bacteria to other small rodents, other animals, or humans in the course of their normal feeding behavior. The ticks are slow feeders,
Figure 3. The global distribution of Ixodes spp. ticks able to transmit the agent of Lyme disease, Borrelia burgdorferi. Modified from http://geo.arc. nasa.gov/sge/health/sensor/disease/ lyme.html
requiring several days to complete a blood meal. Transfer of the borreliae from the infected tick to a vertebrate host probably does not occur unless the tick has been attached to the body for 36 hours or so.