| Summary: | Ticks are obligatory hematophagous ectoparasites that transmit pathogens among various vertebrates, including humans. The microbial and viral communities of ticks, including pathogenic microorganisms, are known to be highly diverse. However, the factors driving this diversity are not well understood. The tropical horse tick, <i>Dermacentor nitens</i>, is distributed throughout the Americas and it is recognized as a natural vector of <i>Babesia caballi</i> and <i>Theileria equi</i>, the causal agents of equine piroplasmosis. In this study, we characterized the bacterial and viral communities associated with partially fed <i>Dermacentor nitens</i> females collected using a passive survey on horses from field sites representing three distinct geographical areas in the country of Colombia (Bolivar, Antioquia, and Cordoba). RNA-seq and sequencing of the V3 and V4 hypervariable regions of the 16S rRNA gene were performed using the Illumina-Miseq platform (Illumina, San Diego, CA, USA). A total of 356 operational taxonomic units (OTUs) were identified, in which the presumed endosymbiont, Francisellaceae/<i>Francisella</i> spp., was predominantly found. Nine contigs corresponding to six different viruses were identified in three viral families: Chuviridae, Rhabdoviridae, and Flaviviridae. Differences in the relative abundance of the microbial composition among the geographical regions were found to be independent of the presence of <i>Francisella-</i>like endosymbiont (FLE). The most prevalent bacteria found in each region were <i>Corynebacterium</i> in Bolivar, <i>Staphylococcus</i> in Antioquia, and <i>Pseudomonas</i> in Cordoba. <i>Rickettsia</i>-like endosymbionts, mainly recognized as the etiological agent of rickettsioses in Colombia, were detected in the Cordoba samples. Metatranscriptomics revealed 13 contigs containing FLE genes, suggesting a trend of regional differences. These findings suggest regional distinctions among the ticks and their bacterial compositions.
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