Volume 20, Number 3—March 2014
Tick-borne Pathogens in Northwestern California, USA
To the Editor: In northwestern California, USA, the western black-legged tick, Ixodes pacificus, is a known vector of Borrelia burgdorferi, the spirochete that causes Lyme disease. B. miyamotoi, which is more closely related to spirochetes that cause relapsing fever, has also been detected in 2 locations in California (1,2) and has recently been implicated as a human pathogen in the northeastern United States (3,4). Other studies may have unintentionally included B. miyamotoi infections among measures of B. burgdorferi if the diagnostics were for spirochetes (e.g., direct fluorescent antibody tests or dark-field microscopy) or genetically targeted for Borrelia spp. (5).
To investigate Borrelia spp. ecology in California, we collected adult I. pacificus ticks by dragging a 1-m2 white flannel blanket along vegetation and/or leaf litter in 12 recreational areas in the San Francisco Bay area during January–May 2012 (Table). Habitat varied from chaparral and grassland to coastal live oak woodland. Ticks were pooled for examination by quantitative PCR (qPCR) for the presence of Borrelia spp. We interpreted the prevalence of Borrelia spp. from positive pools as the minimum infection prevalence (i.e., assuming 1 positive tick/positive pool). DNA was extracted from ticks by using the DNeasy Blood and Tissue Kit (QIAGEN, Valencia, CA, USA) according to the manufacturer’s protocols and then stored at −20°C until use. DNA was analyzed by qPCR, with use of primer and fluorescent hybridization probes previously developed to differentiate Borrelia spp. spirochetes (5). To identify the Borrelia spp. genotype, we attempted to sequence the 16S–23S (rrs-rrlA) intergenic spacer of each sample positive by qPCR (8). The nested PCR product was further purified by using the QIAquick Kit (QIAGEN) and then sequenced (Environmental Genetics and Genomics Laboratory, Northern Arizona University, Flagstaff, AZ, USA; www.enggen.nau.edu/dna.html) by using capillary Sanger sequencing on an ABI 3730 sequencer (Life Technologies, Grand Island, NY, USA). BLAST (http://blast.ncbi.nlm.nih.gov/Blast.cgi) was used to compare each sequence to other Borrelia spp. sequences available from GenBank.
From a total of 1,180 adult ticks, we found 43 samples positive for Borrelia spp., resulting in a minimum infection prevalence of 3.6% (Table). We obtained intergenic spacer sequence data for 27 of the positive samples; 6 samples were B. burgdorferi sensu stricto, 7 were B. burgdorferi sensu lato (both on the basis of alignments of 816 bp), and 14 were B. miyamotoi (on the basis of alignments of 503 bp). The B. miyamotoi sequences for our samples from California and those for isolates from the eastern United States (9) and Japan (8) formed a monophyletic clade that was oriented as a sister clade to the 3 Borrelia spp. that cause tick-borne relapsing fever in the United States (B. hermsii, B. turicatae, and B. parkeri).
We found borreliae-infected adult I. pacificus ticks at all 12 sites from which tick sample sizes exceeded 30. When the presence of B. burgdorferi sensu stricto or B. burgdorferi sensu lato was detected (4/12 sites each), prevalence was 0.6%–2.2% and 0.7%–2.5%, respectively. B. miyamotoi was detected at 7/12 sites, and prevalence ranged from 0.7% to 7.5%. A previous survey of B. burgdorferi in nearby Santa Cruz County recreational areas reported an infection prevalence of ≈6% among adult I. pacificus ticks (6); the study did not, however, differentiate between Borrelia spp. and therefore may have included B. miyamotoi among its prevalence measures (5). In our study, B. burgdorferi was found more frequently in woodland habitats, but it was also detected in a grassland–chaparral habitat several hundred meters from the nearest woodland. We did not detect B. bissettii, a species recently implicated as a human pathogen in Mendocino County, California (10). The high level of habitat variation in northwestern California presents a varied risk for Borrelia-associated tick-borne disease in humans because of diverse variations in vertebrate reservoir ecology, tick abundance, and human exposure to ticks. This variation emphasizes the need to understand the local epidemiology and ecology of a disease.
In adult I. pacificus ticks in the San Francisco Bay area, B. miyamotoi is as abundant as its congener B. burgdorferi. Human disease caused by B. miyamotoi infection has not been reported in California, and transmission efficiency of B. miyamotoi by I. pacificus ticks is unknown. However, it is possible that B. miyamotoi infections in ticks and humans have not been accurately diagnosed. We advocate for increased scrutiny of the eco-epidemiology of B. miyamotoi in human, tick, and possible vertebrate host populations in northwestern California.
We thank the students of the Stanford University Conservation Medicine class of 2012, Denise Bonilla and the California Department of Public Health Vector-Borne Disease Section, and local community members of Portola Valley and Woodside for assistance in collecting ticks; the Midpeninsular Regional Open Space District, San Mateo County Parks, Jasper Ridge Biological Preserve, City of Palo Alto, and California State Parks for permission to collect ticks; and Eric Lambin and Mike Teglas.
This work was made possible by the generosity of the Bay Area Lyme Foundation and Richard Hoffman.
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