The deliberate release of a non-native species amplifies zoonotic disease risk via spillback

Spillback – where non-native species increase the prevalence of native pathogens – is an important mechanism by which non-natives species may contribute to the emergence of zoonoses. However, spillback is rarely directly demonstrated because it is difficult to disentangle from confounding factors which correlate with non-native species abundance and native pathogen prevalence. Here, we capitalise on 25 independent, quasi-experimental releases of non-native pheasants (Phasianus colchicus) to compare vector abundance and native pathogen prevalence between sites with similar local conditions but different non-native densities. Questing adult (but not nymph) Ixodes ricinus were more abundant in woods where pheasants are released compared to control woods, and Borrelia sp. (the causative agent of Lyme disease) prevalence in questing nymphs and adults was 2.5 times higher, with a particularly strong effect on Borrelia garinii. This work provides direct evidence that non-native species can amplify zoonotic pathogens via spillback in an ecologically meaningful context.