Lyme borreliosis in eastern North America is caused by the spirochete Borrelia burgdorferi (s.s.) and is maintained in a transmission cycle consisting of its principal host, the white-footed mouse (Peromyscus leucopus), and its principal vector, the black-legged tick (Ixodes scapularis).  In simple terms, the B. burgdorferi transmission cycle is maintained by the infection of hosts in early summer by I. scapularis nymphs infected the previous year. Those hosts then serve as sources of infection to I. scapularis larvae, which emerge later in the summer.  In addition to P. leucopus, a variety of avian and mammalian species are known to be capable of infecting I. scapularis ticks with B. burgdorferi, although the contributions of these species to Lyme disease transmission dynamics are unclear.  In the United States, the B. burgdorferi range is expanding from two major foci, one in southern New England, and one in the upper Midwest.  Birds have been postulated to drive the expansion of B. burgdorferi, although this mechanism has not been thoroughly investigated.  We sampled migrating songbrids at 14 sites distributed throughout the eastern half of the United States to determine if birds are likely to acquire B. burgdorferi-infected I. scapularis nymphs and to infect and disperse I. scapularis larvae.

Results/Conclusions

We collected a total of 938 I. scapularis from 339 birds representing 46 species.  We found that larval I. scapularis abundance was significantly higher on birds sampled in the Northeast (1.64 larvae per bird) than in the Midwest (1.28 larvae per bird), although birds in the Midwest were more likely to be contemporaneously infested with larvae and nymphs.  Furthermore, I. scapularis nymphs emerge later in the Midwest than in the Northeast, but larvae emerge earlier.  As a result of this phenomenon, the temporal lag in peak occurrence of nymphs and larvae on birds in the Midwest (3 days) was substantially shorter than in the Northeast (43 days).  Because circulating B. burgdorferi infection in birds has been shown to reach its maximum within 28 days of inoculation, it is more likely that B. burgdorferi infections in birds will be transmitted to larvae in the Midwest than in the Northeast.  This work leads us to predict that bird-mediated northward expansion of B. burgdorferi populations is likely to occur in the Midwest, but not in the Northeast.