Dr Rufus Sage head of lowland game research and Dr Andrew Hoodless director GWCT research
In the late 1990s GWCT worked with researchers at the University of Oxford to investigate the potential for pheasants to harbour Ixodes ricinus ticks and to contract and transmit from and to those ticks, various strains of Borrelia spp. bacteria which cause Lyme disease in humans. This work concluded that pheasants host mainly nymphal ticks (intermediate life stage) during February-May and August-October and are hosts for Lyme borreliosis bacteria and hence potentially play an important role in the maintenance of the disease (Kurtenbach et al. 1998a).
Pheasants carry similar numbers of ticks to grey squirrels but are relatively minor hosts compared to wood mice (for larval ticks) and roe deer (for adults) (Hoodless et al., 1998). Given the almost ubiquitous distribution of small mammals and deer in lowland woods, the addition of pheasants was thought likely to maintain or increase the abundance of ticks in locations where they are already present and to shift the prevalence of Borrelia genospecies towards the bird-specific strains Borrelia garinii and Borrelia valaisiana (Kurtenbach et al., 1998b). In published GWCT research reviews, the issue has been flagged as a potential negative effect of releasing (e.g. Sage et al. 2020, Hoodless & Sage 2021) and for many years GWCT has highlighted the potential for increased tick numbers and increased exposure of people to Borrelia bacteria in and around pheasant release sites in woodlands.
A new study (Michels et al. 2025, see link below) undertaken primarily by researchers from the University of Exeter, adds some useful detail around the extent to which risk of exposure to Borrelia bacteria is increased in these places. In particular, ticks in pheasant release woodland areas in south-west England, where the study was conducted, were 2.5 x more likely to have Borrelia infections than ticks in non-game-release woodland areas. This was true for both adult ticks and for nymphs.
In turn, the paper shows that adult ticks had about twice the infection rate of nymphs. The paper suggests adult ticks tended to be marginally more abundant in pheasant woods than woods without pheasant releases. However, humans are not normally bitten by adult ticks, so this reduces the actual risk to humans in all woodlands but especially pheasant release areas of woodland. While the study collected many more tick nymphs than adults in all woods, importantly it did not find more nymphs at pheasant release sites than other woodland areas. This means that tick abundance does not lead to an increase in the potential for a human to acquire Borrelia in a pheasant release wood compared to other woods.
Because individual ticks are more likely to have the Borrelia bacteria, we are left with an overall 2.5 fold increase in the risk of a person walking through a woodland area near to a pheasant release pen acquiring Borrelia and hence Lyme disease. Looking at actual proportions in the new paper, if you get bitten by a tick nymph in SW England, there is on average a 6.5% chance that the tick is carrying the Borrelia bacteria if you are near to a pheasant release pen, and a 2.5 % chance of that in another wood. If you don’t remove that tick, there is a chance of then developing Lyme disease which varies dependant on a number of factors not explored here.
The paper concludes that pinning down the mechanism by which pheasant release amplifies Borrelia prevalence in ticks would help define measures that reduce the risk of ticks transmitting Borrelia to humans. The paper shows high variation in the infection rate of tick nymphs between the woodland sites so this suggests other factors are involved. Hosts for ticks include a variety of birds and mammals. A key part of the ecology of ticks is their ability to leave one animal host and to then jump off the end of a piece of grass or the leaf of a bush onto another one. The potential for ticks to do this is related to vegetation structure and density near to the ground and some woodlands or grasslands are more likely than others to enhance tick populations and to enhance the probability of individual ticks acquiring Borrelia.
We know that game-managed areas of woodlands have different vegetation than otherwise similar woodland areas (in particular more understorey and low cover provided for pheasants) so it is likely that this is a factor. The new paper explored this to some extent and found no effect. However, we suggest this is still likely to be part of the story and it would be useful to undertake a more detailed investigation of a range of vegetation types and structures in relation to Borrelia prevalence. If aspects of vegetation around pheasant release sites can be identified as contributing factors, then this is something that can be addressed through management.
We continue to urge people to take a range of actions that will help reduce the risk of developing Lyme disease following a walk in the countryside and a tick bite. It is possible to get some general guidance. Information on the regional Lyme disease distribution can be found at Common animal-associated infections (England): first quarter 2023 - GOV.UK, including information on the relative risk in different parts of the country. Some regions have virtually no ticks or Borrelia whereas in the south-west of England (where the new study was conducted) ticks are relatively common.
After walking through woodlands or grasslands, anywhere where there is natural ground vegetation, have a look for ticks on your legs etc. soon afterwards. A tick can be safely removed and if done reasonably quickly, this can prevent Borrelia transmission and hence the possibility of Lyme disease Tick removal: Tick awareness A5 leaflet for printing. It is worth buying and always carrying fine tweezers or a credit-card sized tick remover to help with this : Tick Removal Tool | Tick Remover Card | Lifesystems. And as the research done so far suggests, be especially vigilant when walking in woodland areas near to pheasant release pens.
The new paper is free to download and can be read here.
Michels, E., Hansford, K., Perkins, S.E., McDonald, R.A., Medlock, J.M. and Tschirren, B., 2025. The Release of Non‐Native Gamebirds Is Associated With Amplified Zoonotic Disease Risk. Ecology Letters, 28(4), p.e70115.
Other References
Hoodless, A.N., Kurtenbach, K., Nuttall, P.A. and Randolph, S.E. (1998) The role of pheasants as hosts for ticks and Lyme disease spirochaetes in southern England. Game & Wildlife 15, 477–489.
Hoodless, A. & Sage, R. (2021) Climate and management effects on tick-game animal dynamics. Climate, Ticks, and Disease. (ed. P. Nuttall). CAB International 2021. Pp.132-138. DOI: 10.1079/9781789249637.0019.
Kurtenbach, K., Carey, D., Hoodless, A.N., Nuttall, P.A. and Randolph, S.E. (1998a) Competence of pheasants as reservoirs for Lyme disease spirochetes. Journal of Medical Entomology 35, 77–81.
Kurtenbach, K., Peacey, M., Rijpkema, S.G.T., Hoodless, A.N., Nuttall, P.A. and Randolph, S.E. (1998b) Differential transmission of the genospecies of Borrelia burgdorferi sensu lato by gamebirds and small rodents in England. Applied and Environmental Microbiology 64, 1169–1174.
Sage, R.B., Hoodless, A.N., Woodburn, M.I.A., Draycott, R.A.H., Madden, J.R. and Sotherton, N.W. (2020) Summary review and synthesis: effects on habitats and wildlife of the release and management of pheasants and red-legged partridges on UK lowland shoots. Wildlife Biology 2020, wlb.00766.