Lyme disease, caused by the bacterium Borrelia burgdorferi and transmitted primarily by ticks of the genus Ixodes, is one of the most prevalent zoonoses in the Northern Hemisphere. Its incidence has increased in regions such as Europe, North America, and parts of Asia due to factors such as climate change, urbanization, and increased outdoor activities.
In this context, the red fox (Vulpes vulpes) emerges as a key ecological actor in limiting this disease, acting as a natural predator of rodents that serve as primary reservoirs for the bacteria.
I will attempt to explain the fox's role in controlling Lyme disease, analyzing recent scientific evidence and addressing the implications of its conservation for public health.
Lyme Disease: Epidemiology and Transmission
Lyme disease is a multisystem bacterial infection that affects the skin, joints, nervous system, and, in severe cases, the heart. It is primarily transmitted by the bite of Ixodes ticks, such as Ixodes scapularis in North America and Ixodes ricinus in Europe.
Rodents, especially the white-footed mouse (Peromyscus leucopus) in North America, are the main reservoirs of Borrelia burgdorferi, as ticks acquire the bacteria by feeding on these animals during their larval or nymphal stages. The disease presents symptoms such as erythema migrans, fever, fatigue, and, in later stages, neurological and joint complications. In Spain, the incidence has increased, especially in northern regions, due to increased human contact with tick habitats.
Globally, it is estimated that between 240,000 and 440,000 cases are reported annually in the United States, while the prevalence is also significant in Europe, especially in forested and rural areas. The number of reported cases in Europe has been increasing since the early 1990s, as has their geographic distribution. Tick hosts (rodents) are expanding their range to increasingly higher altitudes and latitudes, suggesting that Lyme disease will continue to be a major health problem in the coming decades.
Some authors have estimated the incidence of the disease at 56.31 cases per 100,000 inhabitants in Western Europe.
Factors such as rising temperatures, which favor the expansion of tick populations, and the increase in outdoor recreational activities have increased the risk of infection.
The Ecological Role of the Red Fox as a Predator
The red fox (Vulpes vulpes) is an opportunistic carnivore whose diet largely includes small rodents, such as mice and rabbits, which constitute more than 75% of its diet in many regions. These rodents are the primary hosts of ticks in their early life stages, making them key vectors in the transmission of Borrelia burgdorferi.
These bacteria are naturally found in small rodents. Ticks bite these small mammals for the first molt of their life cycle, and then bite a larger mammal, such as humans or large ungulates (deer, fallow deer, or roe deer), for the second or third molt. Foxes often prey on small mammals.
By reducing rodent populations, foxes indirectly reduce the density of infective ticks in an ecosystem, acting as a "limiting vector" for Lyme disease.
In Europe, we can find fox population densities ranging from 0.08 foxes/km² in the lowest density areas to 3-4 foxes/km² in areas with abundant natural resources. In urban areas, densities can soar to 30 foxes/km².
In the Ebro Valley in Aragon, densities have been estimated between 0.8 foxes/km² in dryland areas and 2.5 foxes/km² in irrigated areas. In the Doñana Reserve, densities of 1.4-1.7 foxes/km² were estimated. In the Serra da Malcata (Portugal), fox abundance ranges between 0.74 and 0.91 foxes/km². In the Baza Mountains, the abundance of juvenile foxes was 0.9 individuals/km² and that of adults was 1.6 individuals/km².
Recent research on the impact of the Red Fox
Research has delved deeper into the relationship between predators and Lyme disease. A study published in 2019 in the Netherlands showed that ecosystems with stable populations and a greater presence of predators such as the red fox have lower rates of Borrelia burgdorferi infection compared to areas where these predators are scarce or have been eliminated due to hunting.
Specifically, and surprisingly, they found that small mammal populations remained stable, but the number of ticks on their bodies was much lower if there was an optimal density of predators in the ecosystem, and vice versa. The percentage of infective ticks in the environment, that is, those carrying the bacteria that cause Lyme disease, also decreased.
The trophic dynamics of reducing and controlling rodent reservoir populations by interrupting the enzootic cycle of the bacteria limits the likelihood of infected ticks reaching humans or domestic animals. This ecological role is particularly relevant in areas where biodiversity is more intact, as the presence of natural predators helps maintain rodent populations in balance.
This finding suggests that conserving predators like foxes could be an effective strategy to mitigate the risk of zoonotic diseases.
In North America, similar studies have explored the role of predators in controlling rodent reservoirs. For example, a paper published in Frontiers in Public Health (2014) highlights that biodiversity, including the presence of predators such as foxes, coyotes, and birds of prey, can reduce the prevalence of Lyme disease by decreasing the density of competent hosts for the bacteria. However, these studies also point out that habitat fragmentation and predator hunting can increase rodent populations, exacerbating the risk of transmission.
Other publications highlight the importance of protecting foxes to prevent zoonotic diseases, which has led multiple associations to advocate against indiscriminate fox hunting, arguing that their elimination could have negative consequences for public health. These discussions reflect a growing awareness of the ecological role of predators in disease mitigation.
Controversies and challenges in fox conservation
Despite its ecological importance, the red fox faces significant threats, including hunting and negative perceptions in some rural communities.
In many regions, hunters argue that the red fox can be a problem for agricultural activities and for the repopulation of game species, such as hares or partridges, which has led to organized hunts. However, other groups and some scientists maintain that there is no conclusive data demonstrating that fox overpopulation has a significant impact on agriculture, and that their control should be based on rigorous technical studies rather than traditional hunting practices.
Another challenge is the perception of foxes as carriers of diseases such as rabies or mange. It is true that, in the case of the red fox, their presence poses a risk in the event of a rabies outbreak because they are also asymptomatic carriers in many cases, but with the vaccination of a sufficient number of foxes and minimal safety measures, rabies has been eradicated from Europe for decades.
Since 2013, only six cases have been reported, all of them contracted outside Europe.
As for fox hunting, it does not appear to be reducing their populations, as has been seen in Saarland and Luxembourg. Luxembourg stopped hunting its foxes in 2015, while the German region of Saarland (of similar size and fox population density) did not. The population in both places remained stable over time; the Saarland foxes produced more offspring but dispersed more.
Furthermore, in Luxembourg, data show that the prevalence of the so-called fox tapeworm (Echinococcus multilocularis), which can potentially affect humans (if the meat is consumed or if contact with contaminated feces occurs), has decreased from 40% in 2014 to 25% in 2017.
Across Germany, it has been found that, despite the increase in the fox population, rabies cases remained at zero and mange is below 5%.
In countries like Spain, where sylvatic rabies is not present, these concerns are less relevant; mange is treatable and less likely to be transmitted from wild foxes than from pets. Therefore, the benefits of foxes as rodent predators far outweigh the potential risks, provided health monitoring and control strategies are implemented.
Implications for Public Health and Conservation
Therefore, red fox conservation not only has ecological benefits but also direct implications for public health. Protecting fox populations can be a cost-effective and sustainable strategy to reduce the incidence of Lyme disease, especially in endemic areas.
This implies the need for conservation policies that balance the demands of rural communities with the protection of biodiversity. For example, developing management plans for hunting reserves that include technical studies on fox populations could help inform decisions about their management.
Furthermore, health education is crucial for preventing Lyme disease. Measures such as avoiding tick-infested areas, using repellents, and performing body checks after outdoor activities are recommended by the Centers for Disease Control and other health authorities. However, these preventative measures could be complemented with ecological strategies, such as preserving natural predators, to reduce long-term risk.
The best policy for maintaining the biodiversity of our ecosystems and the policy for maintaining public health are likely much more closely aligned than some had thought.
Some Conclusions
Studies show that the red fox plays an essential role as a limiting vector of Lyme disease by controlling rodent populations that act as reservoirs for Borrelia burgdorferi.
Most zoonotic pathogens are harbored by wild animals that occupy low trophic levels. The removal of their main predators and the resulting restructuring of their communities can increase the risk of zoonotic diseases. Predation on reservoir hosts plays a key role in the suppression/reduction of these diseases.
A paradigmatic case of disease emergence/increase driven by changes occurring in wildlife communities is Lyme disease.
Scientific evidence, supported by studies in Europe and North America, suggests that the presence of foxes in ecosystems reduces the incidence of this zoonosis, once again highlighting the importance of biodiversity in public health. In this sense, foxes play a crucial ecological role as "natural regulators" of the Lyme disease transmission cycle. Their presence helps maintain balance in ecosystems and can be an indirect but effective tool in the prevention of zoonotic diseases.
The likelihood of transmission of zoonoses affecting humans is closely linked to the presence of small animals. If their population densities skyrocket because we fail to properly maintain ecosystems or eliminate potential predators, we are shooting ourselves in the foot.
A healthy ecosystem naturally regulates ticks, but when we disrupt their balance, they multiply. Living soil, natural predators, conscious and sustainable livestock management, and biological control are necessary.
Much of the tick lifecycle occurs in the soil and among leaf litter, where they hatch, molt, and wait to attach themselves to a host. Down there, they are controlled by spiders, beetles, mites, ants, and other pests. But this natural army is disappearing due to pesticides, herbicides, and antiparasitics like ivermectin, which are wiping out their natural predators in this "terrestrial" phase.
Many beetles not only recycle nutrients, they also feed on tick eggs and larvae. When they disappear, ticks gain ground.
Next comes the "aerial" phase, when they cling to a rodent to undergo the first molt of their life cycle, then bite a larger mammal for the second or third molt, such as humans, roe deer, or deer. Foxes typically prey on small mammals. If we remove foxes (and their limiting capacity), rodent populations increase, and with them, the density of infective ticks in the ecosystem.
Changes in predator abundance can have cascading effects on the risk of tick-borne diseases. The emergence of these effects as a result of predator activity requires consideration and, where appropriate, protection of predatory species such as the red fox.
However, fox conservation faces many challenges related to human activities and traditional cultural perceptions. In many parts of Europe, this species is persecuted, underscoring the need for evidence-based policies and public education campaigns. By integrating predator conservation with health prevention strategies, it will be possible to sustainably mitigate the impact of some zoonoses, such as Lyme disease, benefiting both ecosystems and human communities.
Some bibliographic and media references
Agencia SINC. (2025). Ticks and Lyme disease, a growing plague in Spain.
CDC. (2018). Frequently asked questions about Lyme disease.
Chautan, M., Pontier, D., & Artois, M. (2000). Role of rabies in recent demographic changes in red fox (Vulpes vulpes) populations in Europe. Mammalia, 64(4), 391–410.
European Centre for Disease Prevention & Control (2017). Annual Epidemiological Reports. Stockholm: ECDC.
Elsevier. (2019). Diseases caused by Borrelia.
Gruber, V. (2019). Success for fox hunting ban in Luxembourg. European Widerness Society.
Hoffmann, M. & Sillero-Zubiri, C. (2016). Vulpes vulpes. The IUCN Red List of Threatened Species. http://dx.doi.org/10.2305/IUCN.UK.20161.RLTS.T23062A46190249.en.
Hofmeester, T. R., Jansen, P. A., Wijnen, H. J., Coipan, E. C., Fonville, M., Prins, H. H., ... & van Wieren, S. E. (2017). Cascading effects of predator activity on tick-borne disease risk. Proceedings of the Royal Society B: Biological Sciences, 284(1859), 20170453.
La Voz de Galicia. (2019). The fox becomes necessary as a limiting vector of Lyme disease.
Levi, T., Kilpatrick, A. M., Mangel, M., & Wilmers, C. C. (2012). Deer, predators, and the emergence of Lyme disease. Proceedings of the National Academy of Sciences, 109(27), 10942–10947.
López-Martín, J. M. (2017). Fox – Vulpes vulpes. In: Virtual Encyclopedia of Spanish Vertebrates. Salvador, A., Barja, I. (Eds.). National Museum of Natural Sciences, Madrid. http://www.vertebradosibericos.org
Miranda, I. (2019). Not even fox overpopulation saves hunting from criticism from animal rights activists. ABC.
Molinero, A.G. (2019). Fox hunting: An environmental and epidemiological error. Gupta Chimera Archives.
NIH MedlinePlus Magazine. (2021). On the front lines against Lyme disease.
Romani, L., Steer, A. C., Whitfeld, M. J., & Kaldor, J. M. (2015). Prevalence of scabies and impetigo worldwide: a systematic review. The Lancet Infectious Diseases, 15(8), 960–967.
Steere, A. C., Strle, F., Wormser, G. P., Hu, L. T., Branda, J. A., Hovius, J. W., ... & Mead, P. S. (2016). Lyme borreliosis. Nature Reviews Disease Primers, 2, 16090.
Sykes, R. A., & Makiello, P. (2016). An estimate of Lyme borreliosis incidence in Western Europe. Journal of Public Health, 39(1), 74-81.