Effect of woody plant encroachment by eastern redcedar on prevalence of tick-borne pathogens and the role of birds in carrying ticks

Woody plant encroachment (encroachment), the increase in cover, abundance, and biomass of trees and shrubs, is occurring throughout the world’s grassland and savanna ecosystems. In the U.S. Great Plains region, eastern redcedar (ERC), a native tree with invasive properties, is reflective of these worldwide trends, rapidly encroaching into grasslands regionwide. The spread of ERC, which occurs mainly due to fire suppression, affects entire ecosystems, including vectors, bird communities, and the ecology of vector-borne disease. In the ERC system, there are several unknown factors regarding how vectors, pathogens and hosts come together in a suitable environment (i.e. nidus of infection). To address this knowledge gap, we collected ticks from vegetation and tested ticks for infection of pathogens, including Rickettsia, Ehrlichia, and Borrelia, at several sites representing different stages of ERC encroachment across western Oklahoma USA from spring to fall of 2022 to 2024. Additionally, from May to October of 2023, we mist-netted birds, sampled larval and nymphal ticks from them, and tested these ticks for the same three pathogen groups. Overall, we found that there was similar pathogen prevalence across all stages of ERC encroachment, including grassland sites not yet experiencing encroachment. The only exception was Ehrlichia, which appeared to have higher levels of infection in ticks when ERC first encroaches compared to older stages, indicating that wildlife hosts may interact with differing stages of ERC in ways that increase tick-borne disease risk for humans. In relation to avian hosts, we found that birds in ERC-encroached areas have a prevalence of tick infestation that is nearly five times higher than the national average. Unexpectedly, we found Ixodes scapularis larvae and nymphs on birds, an association previously unknown in the southern Great Plains region. We hypothesize that birds are likely moving ticks, which are infected with multiple microorganisms and pathogens of concern to humans, into and between areas experiencing ERC encroachment. Based on our collective results, we build on the conclusion that ERC encroachment at all stages requires immediate action to protect humans and wildlife, and intervention may be especially important at earlier encroachment stages when management is more feasible.