Novel relapsing fever spirochete in bat tick.

To the Editor: Tick-borne relapsing fever in western North America is a zoonosis caused by spirochetes in the genus Borrelia that are transmitted by argasid ticks of the genus Ornithodoros (). Human disease occurs in many focal areas and is associated with infections of Borrelia hermsii, B. turicatae, and possibly B. parkeri (,). Although the ecologic parameters that maintain B. hermsii and B. turicatae differ, human infections usually occur in rustic cabins (B. hermsii) and caves (B. turicata) inhabited by ticks and their terrestrial vertebrate hosts (). Recently, Gill et al. () provided evidence that the argasid bat tick, Carios kelleyi, feeds upon humans. Subsequently, Loftis et al. () used PCR analysis and DNA sequencing to detect in C. kelleyi an unidentified Borrelia species that was closely related to B. turicatae and B. parkeri. We report the partial molecular characterization of another novel tick-borne relapsing fever spirochete in C. kelleyi, which expands our knowledge for this group of pathogenic spirochetes and their potential vertebrate hosts and tick vectors.

C. kelleyi were collected August 18, 2005, from a house in Jones County, Iowa, built in 1857. Bats had been excluded from the attic since 1992. Nine months before ticks were collected, bats were prevented from roosting under the eaves. DNA was extracted from 31 nymphal C. kelleyi, as described previously (). For each tick, regions of the glpQ, flaB, and 16S rRNA genes were amplified and sequenced as described (,,). Sequences were assembled by using the SeqMan program in the Lasergene software package (DNASTAR, Madison, WI, USA).

Fourteen (45.1%) of 31 ticks were positive by PCR for >1 of the genes tested. Partial DNA sequences were determined from tick no. 16, for which amplicons for all 3 genes were obtained. The partial flaB sequence had 4 bases different from the 300-base sequence (98.66% identity) reported previously (GenBank accession no. AY763104) for another Borrelia sp. found in C. kelleyi (). We constructed a 1,992-bp concatenated sequence that contained 1,273 bp of the 16S rRNA, 351 bp of flaB, and 368 bp of glpQ. This concatenated sequence was aligned with homologous, trimmed DNA sequences of the same length obtained from representative full-length sequences determined previously for B. hermsii, B. turicatae, and B. parkeri (,) (Figure). This C. kelleyi spirochete was more closely related to B. turicatae and B. parkeri than to B. hermsii but was clearly distinct from all 3 species (DNA sequence identities of 98.89%, 98.75%, and 95.98% to B. turicatae, B. parkeri, and B. hermsii, respectively).

Figure

Phylogram comparing the novel spirochete in the bat tick Carios kelleyi with Borrelia parkeri, B. turicatae, and B. hermsii based on the concatenated partial 16S rRNA-flaB-glpQ DNA sequences in the Carios spirochete (1,992 bp total) (produced with ClustalV software from DNASTAR [Madison, WI, USA]). Scale bar represents the number of base substitutions per 100 aligned bases. GenBank accession numbers for the C. kelleyi spirochete sequences used to construct the tree are EF688575, EF688576, and EF688577. Spiro, spirochete.

A glpQ amplicon from another nymphal tick (no. 3) was equenced (GenBank accession no. EF688578) and was unique in the database; it was also considerably different from the glpQ sequence determined from tick 16, with 325 of 368 bases matching (88.3% identity). The Borrelia glpQ sequence from tick 3 had 85.1%–89.1% identity compared with glpQ sequences from B. hermsii, B. turicatae, and B. parkeri. This finding suggests the presence of at least 2 relapsing fever group spirochetes in C. kelleyi that await further characterization.

We found a novel Borrelia in bat ticks that is closely related to, but distinct from, the other known species of tick-borne relapsing fever spirochetes in North America. The human health implications of the new relapsing fever group spirochete are not yet known. The willingness of C. kelleyi to feed on humans and the fact that infection with bacteria closely related to true relapsing fever spirochetes occurs in these ticks suggest that human habitation near bats and their associated tick colonies could pose a public health risk. Growth in laboratory animals or culture could help isolate these novel organisms for further studies to establish the distribution and public health implications of this newly identified Borrelia sp.