Long-term surveillance reveals weak coupling between chigger burden on Apodemus agrarius and Orientia tsutsugamushi pool positivity

Abstract

Long-term vector surveillance often reports ectoparasite burden and pooled pathogen detection together, but their quantitative relationship is rarely tested. We analysed rodent-associated chigger surveillance conducted from 2018 to 2025 across five habitat strata in Yesan-si, Republic of Korea. During the eight-year surveillance period, 152 small mammals were captured and examined, comprising 123 Apodemus agrarius , 26 shrews ( Crocidura spp.), two Micromys minutus , and one Craseomys regulus . Chiggers were recovered from 135 of 152 hosts (88.8%): 120 of 123 A. agrarius , 12 of 26 Crocidura spp., both M. minutus , and the single C. regulus . A total of 20,239 chiggers were recovered, of which 12,763 were assigned to molecular testing. These chiggers were distributed into 502 host-specific pools containing no more than 30 chiggers, with no pool combining chiggers from different hosts; 58 pools were positive for Orientia tsutsugamushi . Inferential analyses were restricted to A. agrarius because it accounted for most examined hosts and recovered chiggers and was the only host taxon with sufficient host-level replication and pool observations for stable hierarchical modelling. Among the 120 chigger-infested A. agrarius that generated molecular pools, 50 of 468 pools were positive (10.7%). Chigger counts were strongly overdispersed. In a hierarchical binomial mixed model, standardized log host chigger burden showed little association with pool positivity (odds ratio 0.998; 95% credible interval 0.672–1.483), whereas between-year heterogeneity was substantial. Pool positivity was temporally heterogeneous, with the strongest signal in 2023. In the A. agrarius analytic cohort, this increase involved multiple hosts (29/45 positive pools; 13/17 hosts with ≥ 1 positive pool), rather than a single sparse host record. Because this operational surveillance prioritized O. tsutsugamushi screening of pooled chiggers rather than species-resolved mite identification, habitat terms were treated as sampling-stratum adjustments and were not interpreted as species-specific vector effects. These findings indicate that temporal pathogen-detection dynamics in local surveillance cannot be inferred from host chigger burden alone.