Molecular detection and characterization of Anaplasma phagocytophilum and Neoehrlichia mikurensis in wild rodents and their ectoparasites in South Korea

Abstract

Wild rodents act as crucial reservoir hosts for various tick-borne pathogens, such as Anaplasma phagocytophilum and Neoehrlichia mikurensis, which are responsible for the emergence of zoonotic diseases in humans. While tick-borne pathogens have been examined in various animal species, the genetic diversities present in wild rodents and their ectoparasites remain poorly understood. This study examined the prevalence and genetic characteristics of A. phagocytophilum and N. mikurensis in wild rodents, mites, and ticks from South Korea. PCR amplification and sequencing of the 16S rRNA, msp4, and groEL genes were performed to genotype A. phagocytophilum (16S rRNA and msp4) and N. mikurensis (16S rRNA and groEL). A. phagocytophilum was identified in 25.8 % of rodents and in ixodid ticks collected from rodents, with a minimum infection rate (MIR) of 2.8 %. A. phagocytophilum was detected in mites (MIR: 0.4 %) from rodents, indicating their potential role in pathogen circulation. Of the 461 wild rodents included in this study, five (1.1 %) tested positive for N. mikurensis. Furthermore, one positive pool was identified in Ixodes nipponensis nymphs (MIR: 0.1 %), representing the first documented occurrence of N. mikurensis in ticks in South Korea. Phylogenetic analysis indicated that the A. phagocytophilum sequences obtained in this study cluster with sequences from South Korea and China associated with rodents or I. nipponensis, while remaining distinct from those of European origin. The N. mikurensis sequences clustered with East Asian strains, forming two distinct groups separate from European lineages. These findings corroborate the hypothesis that wild rodents and their ectoparasites play a role in the natural maintenance and transmission of A. phagocytophilum and N. mikurensis in South Korea. Given the growing acknowledgment of these pathogens as emerging threats to human health, continued surveillance and molecular characterization are essential to understand their regional distribution and implications for public health.

Keywords: Anaplasma phagocytophilum; Genetic characteristics; Neoehrlichia mikurensis; Prevalence; South Korea.

Graphical abstract

Fig. 1

Map illustrating the southwestern region of the Korean Peninsula where wild small rodents were collected from April 2023 to March 2025. Three sampling sites, designated as G1, G2, and G3, are located primarily within the Gwangju metropolitan area (depicted in high magnification).

Fig. 2

Monthly distribution of ticks collected from wild rodents from April 2023 to March 2025. The ticks were categorized into larval and nymphal stages; no adult specimens were identified. The identified tick species include I. nipponensis, H. longicornis, H. flava, and A. testudinarium. Peak tick populations were recorded in September–October 2023 and August 2024, attributed to a significant increase in larval numbers during the summer months. Abbreviations: A. testudinarium, Amblyomma testudinarium; I. nipponensis, Ixodes nipponensis; H. flava, Haemaphysalis flava; H. longicornis, Haemaphysalis longicornis.

Fig. 3

Monthly prevalence of A. phagocytophilum in wild rodents and ticks. The line graphs show the detection rates of A. phagocytophilum in wild rodents (solid blue line) and ticks (dotted red line) from April 2023 to March 2025. Rodents consistently demonstrated a higher prevalence than ticks, with seasonal increases observed in spring and autumn each year. The prevalence in rodents reached approximately 60 % in September 2024 and March 2025. Abbreviation: A. phagocytophilum, Anaplasma phagocytophilum.

Fig. 4

Phylogenetic trees of A. phagocytophilum derived from 16S rRNA (825 bp) and msp4 (360 bp) gene sequences. The 16S rRNA phylogenetic tree (A) was generated using the Kimura 2-parameter model with a gamma distribution (K2+G), and the msp4 phylogenetic tree (B) was constructed using the Tamura 3-parameter model with a Has invariant sites (T92+I). Partial sequences of A. phagocytophilum identified in this study have been deposited in GenBank, as indicated by the accession numbers and colored markers. Reference strains used for comparison are indicated in brackets. The scale bar represents the number of nucleotide substitutions per site. Abbreviation: A. phagocytophilum, Anaplasma phagocytophilum.

Fig. 5

Phylogenetic trees of N. mikurensis derived from 16S rRNA (467 bp) and groEL (346 bp) gene sequences. The 16S rRNA phylogenetic tree (A) was generated using the Kimura 2-parameter model with a gamma distribution (K2+G), and the groEL phylogenetic tree (B) was constructed using the Tamura 3-parameter model with a Has invariant sites (T92+I). Partial sequences of N. mikurensis identified in this study have been deposited in GenBank, along with accession numbers and colored symbols. Orange dots represent sequences of N. mikurensis identified in I. nipponensis (pooled) samples, whereas blue dots denote five sequences from wild rodents (A. agrarius). Reference strains used for comparison are indicated in brackets. The scale bar represents the number of nucleotide substitutions per site. Abbreviations: A. agrarius, Apodemus agrarius; N. mikurensis, Neoehrlichia mikurensis; I. nipponensis, Ixodes nipponensis.