The role of large ungulate grazers on Ixodes ricinus and tick-borne pathogens in the New Forest - a case study for future rewilded landscapes

Large ungulate grazers can manage habitats via conservation grazing, a practice using livestock to control vegetation growth, which has many ecological benefits but has the potential to provide additional hosts for ticks and consequently have an impact on tick-borne disease risk. Cattle and sheep are suspected to be transmission hosts for several tick-transmitted pathogens, so the presence of livestock could increase disease hazard. However, some ungulate species do not transmit other pathogens such as Borrelia burgdorferi sensu lato (s.l.), so conservation grazing could reduce prevalence of these pathogens, and thus environmental disease hazard, by diverting ticks from feeding on transmission hosts. To better understand these dynamics, we used a paired experiment in the New Forest in southern England. Questing ticks were collected at 20 sites between 2021 and 2023. Ten sites were inside "inclosures" (New Forest term for fenced woodlands to exclude livestock) and the remaining ten were not fenced, which permitted livestock grazing. Grazing led to significantly shorter ground vegetation and fewer questing Ixodes ricinus nymphs. We tested 2974 nymphs for multiple pathogens and determined there were no significant differences in nymphal infection prevalence or density of infected nymphs for B. burgdorferi s.l. and Anaplasma phagocytophilum between sites. However, we found that the density of infected nymphs for Borrelia garinii and Borrelia valaisiana was lower where there was grazing. In this study, we show that conservation grazing by ponies and cattle could lower tick density, probably by affecting the vegetation understory, and could potentially lower disease hazard for some genospecies of B. burgdorferi s.l. but not A. phagocytophilum.