Vertical transmission drives the persistence of Rickettsia parkeri and “Candidatus Rickettsia andeanae” in Amblyomma maculatum sensu lato populations in a semi-arid Madrean riparian–grassland system

Rickettsia parkeri is a tick‑borne pathogen associated with Amblyomma spp. ticks in the Western Hemisphere. Vertebrate exposure to R. parkeri has been documented primarily among larger hosts of adult stage ticks, whereas the frequency and significance of exposure among hosts of immature life stages have been comparatively less well characterized. In southern Arizona, R. parkeri co‑occurs with the closely related spotted fever group rickettsial species “ Candidatus Rickettsia andeanae,” allowing the evaluation of transmission pathways and interactions between sympatric rickettsiae. Rodent sera ( n = 320) collected during a 14‑month longitudinal field study were screened for antibodies reactive to R. parkeri by indirect immunofluorescence assay. Molecular screening for Rickettsia DNA was performed on 1941 immature stage Amblyomma maculatum sensu lato ticks, 1378 rodent ear biopsies, and 211 rodent whole blood specimens. Immature and adult ticks were assayed for R. parkeri and “ Ca . R. andeanae” DNA, and prevalence ratios were calculated to assess a potential role of co‑feeding transmission. To evaluate vertical transmission, field‑collected adult females were fed on heifers, and post‑oviposition carcasses and F1 larvae were tested for Rickettsia . Despite frequent exposure to R. parkeri (63% seroreactivity), R. parkeri DNA was rarely detected in rodent ear biopsies (3.1%) and was absent from whole blood, indicating infrequent and transient presence in tissues at tick feeding sites and no detectable rickettsemia. In contrast, transovarial transmission was highly efficient: filial infection rates were 100% in clutches from either R. parkeri or “ Ca. R. andeanae ”-positive females with high maternal rickettsial loads. Co-infections in immature ticks were significantly fewer than expected, consistent with competitive exclusion between these rickettsial species. In this system, R. parkeri is maintained predominantly through efficient vertical transmission within A. maculatum s.l. populations, with limited and likely inefficient vertebrate‑mediated horizontal amplification, largely attributable to co‑feeding transmission. “ Ca. R. andeanae” appears to be maintained almost exclusively through vertical transmission. Collectively, these results show that tick populations themselves constitute the primary biological reservoir for these rickettsiae and that competitive interactions within ticks act as a limiting influence on realized rickettsial prevalence where multiple species co‑occur.