We evaluated the presence of Ehrlichia spp. in unfed capybara ticks, Amblyomma dubitatum, and explored its association with capybaras density, ticks density and environmental variables. We observed that in the Iberá wetlands ecoregion A. dubitatum is infected by "Candidatus Ehrlichia hydrochoerus” and in a lesser extent with an Ehrlichia species closely related to Ehrlichia chaffeensis. The frequency of "Ca. Ehrlichia hydrochoerus" presence in A. dubitatum was not associated with vector abundance, but the probability of finding "Ca. Ehrlichia hydrochoerus"-infected ticks increased when the density of capybaras was low two months before. We hypothesize that when the density of capybaras decreases, A. dubitatum immature stages may seek out alternative hosts one of which could exhibit high realized reservoir competence for "Ca. Ehrlichia hydrochoerus", leading to an increased prevalence of this ehrlichiae in questing A. dubitatum. High minimum temperatures and high cumulative rainfall in the time period previous to tick collection (15 to 60 days) were positively correlated with the prevalence of "Ca. Ehrlichia hydrochoerus" infection in A. dubitatum. Our results suggest that a combination of factors (both biological and abiotic) could raise the risk of human exposure to tick-borne Ehrlichia in the Iberá wetlands ecoregion.

We screened lung tissue from 770 sigmodontine rodents sampled in the Delta and Paraná Islands ecoregion for Borrelia spp. and obtained sequences clustered within the Borrelia burgdorferi sensu lato (s.l.) complex. Seventeen individuals were PCR-positive (prevalence 2.2%), distributed mainly in Akodon azarae (n = 11) and across five additional species. Using GLMMs that accounted for site and trapping session, infection probability was positively associated with individual body length and with ecological proxies of prior host and vector abundance: Ak. azarae abundance two sessions earlier (lag S₋₂) and Ixodes loricatus nymph abundance in the previous session (lag S₋₁) were the strongest predictors. These results point to transmission dynamics driven by recent host and immature-tick abundance rather than by climatic descriptors. Presence of cattle was associated with lower Borrelia prevalence in grazed grids (0.5% vs. 4.8% in Ak. azarae), and a univariable model restricted to Ak. azarae detected a significant negative association with cattle; however, this effect was not statistically significant in multivariable models that included host size and lagged abundance metrics. Our findings indicate localized endemic circulation of B. burgdorferi s.l. linked to host/vector demography, and highlight the need for integrated vector and reservoir monitoring to resolve mechanistic pathways and potential implications for animal and public health.

In 2018, we detected a novel Ehrlichia strain infecting Amblyomma neumanni ticks in Argentina. The novel strain is phylogenetically related to the ruminant pathogen E. ruminantium and represents a potential risk for veterinary and public health because A. neumanni ticks parasitize domestic and wild ruminants and bite humans.