Ixodes scapularis ticks have the capacity to harbor numerous pathogens and viruses; the pathogen of most concern for humans is Borrelia burgdorferi, the causative agent for Lyme disease. Different host species may act as diverse niches where strains can evolve; such strain-host specialization may influence B. burgdorferi genomic diversity and vary the capacity to infect, disseminate, and persist within hosts. Some strains may be specialized for mammalian or avian hosts which may alter the circulation of these strains in the vector population and influence human and wildlife exposure to specialized strains. White-footed mice (Peromyscus leucopus) were used as the natural mammalian host and uninfected wild caught American robins (Turdus migratorius) were used as an avian representative host. To determine B. burgdorferi strain specialism or generalism, I. scapularis nymphs infected with one of three B. burgdorferi strains (B31, 297, cN-40, or clean control nymphs) were placed on mice (n=20) or robins (n=20) and allowed to feed to repletion. These strains were chosen because they represent allelic variations in spirochete proteins which promote immune evasion in certain hosts. Tissue and blood samples were collected every seven days and xenodiagnostic larvae were placed on hosts every 7-14 days for 64 days post infection.
Results/Conclusions
Evidence for strain-host specialization was observed as the cN-40 strain persisted significantly longer in robins (~70% infection for 64 dpi; P<0.05) compared to mice (<20% infection) while B31 and 297 persisted significantly longer in mice (~80% or 70% infection for 64 dpi, respectively; P<0.05) compared to robins (~50% or 30% infection, respectively), as evidenced by the infection in the xenodiagnostic larvae recovered from each host. Additionally, cN-40 was not detectable via quantitative PCR in mouse blood or tissues past day 14 while the B31 and 297 strains were not detectable in bird blood after day 21. In mice, B31 and 297, but not cN-40, were detected in various tissues at day 64 (skin, heart, knee joints, and bladder). In robins, cN-40 was detected in skin, heart, and brain tissues, but B31 and 297 were only minimally detectable in skin tissue. These results suggest that there is a difference in the ability of B. burgdorferi strains to selectively escape clearance of different host immune responses. This information is critical for understanding how pathogens are maintained and enable the spread of Lyme disease which can help inform public health risks and potentially produce vaccines against this infectious disease.