Ecological interactions driving population dynamics of two tick-borne pathogens, Borrelia burgdorferi and Babesia microti .

() and () are vector-borne zoonotic pathogens commonly found co-circulating in and populations. The restricted distribution and lower prevalence of has been historically attributed to lower host-to-tick transmission efficiency and limited host ranges. We hypothesized that prevalence patterns are driven by coinfection dynamics and vertical transmission. We use a multi-year, multiple location, longitudinal dataset with mathematical modelling to elucidate coinfection dynamics between and in natural populations of , the most competent reservoir host for both pathogens in the eastern USA. Our analyses indicate that, in the absence of vertical transmission, is viable at lower tick numbers than However, with vertical transmission, is viable at lower tick numbers than . Vertical transmission has a particularly strong effect on prevalence early in the active season while coinfection has an increasing role during the nymphal peak. Our analyses indicate that coinfection processes, such as facilitation of infection by , have relatively little influence on the persistence of either parasite. We suggest future work examines the sensitivity of vertical transmission and other key processes to local environmental conditions to inform surveillance and control of tick-borne pathogens.