Prevalence of Borrelia burgdorferi and diversity of its outer surface protein C (ospC) alleles in blacklegged ticks (Ixodes scapularis) in Delaware

Characterizing the diversity of genes associated with virulence and
transmission of a pathogen across the pathogen’s distribution can inform
our understanding of host infection risk. Borrelia burgdorferi is a
vector-borne bacterium that causes Lyme disease in humans and is common in
the United States. The outer surface protein C (ospC) gene of B.
burgdorferi exhibits substantial genetic variation across the pathogen’s
distribution and plays a critical role in virulence and transmission in
vertebrate hosts. In fact, B. burgdorferi infections that disseminate
across host tissues in humans are associated with only a subset of ospC
alleles. Delaware has a high incidence of Lyme disease, but the diversity
of ospC in B. burgdorferi in the state has not been evaluated. We used PCR
to amplify ospC in B. burgdorferi-infected blacklegged ticks (Ixodes
scapularis) in sites statewide and used short-read sequencing to identify
ospC alleles. B. burgdorferi prevalence in blacklegged ticks varied across
sites, but not significantly so. We identified 15 previously characterized
ospC alleles accounting for nearly all of the expected diversity of
alleles across the sites as estimated using the Chao1 index. Nearly 40% of
sequenced infections (23/58) had more than one ospC allele present
suggesting mixed strain infections and the relative frequencies of alleles
in single infections were positively correlated with their relative
frequencies in mixed infections. Turnover of ospC alleles was positively
related to distance between sites with closer sites having more similar
allele compositions than more distant sites. This suggests a degree of B.
burgdorferi dispersal limitation or habitat specialization. OspC alleles
known to cause disseminated infections in humans were found at the highest
frequencies across sites, corresponding to Delaware’s high incidence of
Lyme disease.