Rewilding-style grazing could reduce tick density and hazard of tick-borne disease

Rewilding, which looks to regenerate degraded ecosystems through restoration of natural processes, is recognised as one approach for tackling biodiversity loss. A common facet of rewilding is the use of conservation grazing, where livestock control vegetation growth in fields and forests. While this disturbance is aimed at increasing biodiversity - for example through controlling invasive species - it can have undesired effects such as reducing tree regeneration and increasing bare ground. It has also been linked to the spread of certain ticks in the south of England, with implications for the risk of tick-borne diseases.

Tick-borne diseases are on the rise in Europe, with Lyme borreliosis, also known as Lyme disease, the most common. Other than public awareness campaigns, there are few measures currently available to reduce the risk of contracting such diseases. Although a vaccine is available against tick-borne encephalitis - the second most common tick-transmitted disease in Europe - uptake is low.

In Europe, the most common tick species of public and veterinary health importance is the sheep or deer tick, Ixodes ricinus, which mostly feed on deer and other mammals including squirrels, hedgehogs, hares, and domestic livestock (sheep and cattle). These ticks are vectors for a variety of pathogens, which different mammals may or may not transmit. For instance, Borrelia burgdorferi is a complex of bacteria, some of which can be passed on to ticks from infected birds or small mammals but are not transmitted by deer or cattle. Some studies have shown that high densities of deer can lead to a decrease in B. burgdorferi prevalence in ticks, by diverting ticks from feeding on transmission hosts.

With rewilding now being promoted and taken up by land managers, researchers in southern England's New Forest - a UK hotspot for Lyme disease - set out to explore how conservation grazing could affect the density of infected ticks. This 60,000-hectare National Park has been home to free-roaming ponies, donkeys, cattle and pigs since the 11th century, and some areas (about 8,300 ha) have been fenced since the 17th century. These fenced "inclosures" provide an opportunity to test hypotheses on the role of large domestic ungulates on tick-borne disease hazard. While geographically limited, the study could have relevance for other parts of Europe.

The scientists used a paired experiment to compare woodlands with and without grazing from ponies and cattle. They carried out three surveys for nymphs of the sheep tick on a total of 20 sites. Ten of the sites excluded the grazers and 10 allowed them, although wild deer could enter them all. To sample ticks, they dragged a cloth over 10 metre transects, repeated 20 times. The surveys were all on dry June days between 2021 and 2023, and at the time of the sampling, the scientists also recorded data on vegetation type and height. Following the field sampling, they used PCR testing to screen for the tick-borne pathogens which cause Lyme disease and anaplasmosis (Borrelia burgdorferi and Anaplasma phagocytophilum, respectively).

The researchers found that grazed areas had significantly lower numbers of I. ricinus nymphs compared to ungrazed areas. The researchers suggested that this could be because the abundance of small mammals, which host tick larvae, is reduced in short vegetation. In addition, taller vegetation provides a favourable microclimate for ticks.

The researchers tested nearly 3,000 tick nymphs for pathogens. They found B. burgdorferi in just over 7% of ticks from outside inclosures (grazed areas), and just over 5% of those from within inclosures (ungrazed areas). These figures were 2.5% and 3.7%, respectively, for infection with A. phagocytophilum.

The researchers note that these differences are not statistically significant, although they had hypothesised that incidence would be higher in inclosures, since non-transmitting livestock in grazed areas would divert ticks from transmission hosts. The finding could be due to deer taking refuge inside inclosures to avoid competition with livestock and resting in more dense vegetation - thereby acting as 'dead-end hosts', diverting ticks from feeding on transmission hosts like rodents and birds. In this way, livestock in grazed areas and deer within fenced areas may be acting to keep pathogen levels roughly similar.

They did observe a difference between the prevalence of the B. burgdorferi genospecies B. garinii and B. valasiana, however, which was lower where ponies and cattle grazed. This may be because they are mainly transmitted by birds, say the researchers, and the reduced vegetation height caused by grazing made these areas lower-quality bird habitat. Previous work in the UK found that these were the two most prevalent genospecies.

The study shows that conservation grazing by ponies and cattle could lower tick density, probably by affecting vegetation height, and potentially reduce the related disease hazard.

The findings could feed into European policy measures, shifting from existing surveillance and data sharing towards linking nature restoration with health outcomes. Rewilding currently falls under the EU's Nature Restoration Regulation, but there could be scope to support grazing practices in forests regardless of whether they are part of a rewilding scheme. Conservation grazing practices can be funded under the Common Agricultural Policy.

The researchers noted that further work might consider the effects of grazing pressure on other mammals, better quantify numbers of livestock and deer, and consider the effects of livestock treatment against parasites.

Reference:

Gandy, S.L., Brown, F.V., Jones, N.J., Biddlecombe, S.M., Kirby, G., Johnston, C.J., Hansford, K.M., Vaux, A.G., Apaa, T.T., Johnson, N. and Medlock, J.M., 2025. The role of large ungulate grazers on Ixodes ricinus and tick-borne pathogens in the New Forest-a case study for future rewilded landscapes. Ticks and Tick-borne Diseases, 16(5), p.102541.

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"Science for Environment Policy": European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.