A recent study has detected the presence of closantel, an anti-worm drug commonly used by farmers on livestock, in Scottish red foxes (Vulpes vulpes). This discovery raises concerns about the contamination of wildlife and the potential for parasites to develop drug resistance, which could have significant environmental and ecological consequences.
Veterinary Residues in Foxes: A Growing Concern
The study, published as part of Issue 608, highlights the risk of red foxes ingesting closantel through various environmental routes. The drug, used as a deworming agent in livestock, was found in 18 out of 118 fox liver samples collected between 2014 and 2019. Concentrations of the drug in these samples ranged from 6.5 micrograms per kilogram (µgkg−1) to 1,383 µgkg−1, with most readings falling between 20 and 100 µgkg−1.
The researchers suggest that foxes are likely being exposed to closantel by consuming treated livestock, faeces from herbivores given the drug, or contaminated rodents and water. While the immediate impact on the foxes themselves is unclear, the study raises the concern that low-level exposure to such veterinary drugs could lead to the development of drug-resistant parasites in wildlife.
Environmental Contamination and Drug Resistance
Veterinary residues in the environment pose a significant threat to wildlife. While birds of prey have traditionally been used to monitor environmental contamination, the researchers in this study argue that foxes could serve as valuable sentinel species. Red foxes, with their varied diet and widespread presence across diverse habitats, are frequently exposed to a range of environmental pollutants. Additionally, because foxes are often culled for pest control, their carcasses are more readily available for sampling.
This study’s findings add to growing concerns about the contamination of the environment by veterinary drugs. The European Commission has already identified pharmaceutical residues in water and soil as an emerging environmental issue. Moreover, the UN’s Strategic Approach to International Chemical Management (SAICM) acknowledges the deep interconnectedness of animal, environmental, and human health, endorsing a "One Health" strategy that calls for greater attention to these global health challenges.
Monitoring for Veterinary Drug Residues
Monitoring wildlife for the presence of veterinary drugs such as closantel is crucial for understanding contamination pathways and mitigating their impacts. While foxes may benefit in the short term from ingesting drugs that help to treat parasites, prolonged or low-level exposure to these substances could have the unintended consequence of fostering drug-resistant parasites. Although parasite resistance to closantel is not yet widely documented, it has been recorded in some cases of Barber’s pole worm (Haemonchus contortus) and liver fluke (Fasciola hepatica).
The study’s authors recommend further investigation into the contamination pathways of closantel, as well as its broader effects on wildlife and ecosystems. They also suggest that foxes could play a critical role in future monitoring efforts to assess the risk posed by veterinary drugs to both wildlife and the environment.
A Call for Responsible Action
This study highlights the pressing issue of veterinary residues in the environment and the potential dangers they pose to wildlife. As top predators, foxes are particularly vulnerable to environmental pollutants, and their contamination with closantel—a veterinary anti-worm drug—underscores the broader problem of pharmaceutical pollution.
The environmental impact of veterinary drugs is becoming an increasingly concerning issue. As noted by veterinary surgeon Dr. Peter Caldwell and echoed in various studies, dewormers and other treatments can unintentionally harm ecosystems, affecting everything from green spaces to water sources. In the case of foxes, exposure to closantel raises alarms not only about the health of these animals but also about the wider risk of parasites developing drug resistance. The study suggests that monitoring red foxes and other wildlife for residues could be a key tool in understanding and addressing this emerging environmental threat.
Impact on Wildlife and Ecosystems: Both Castro’s discussion of flea treatments and this study on foxes emphasize the profound effects veterinary drugs can have on non-target species. While Castro warns of damage to invertebrates, which disrupts food chains involving fish and birds, the fox study focuses on the direct ingestion of closantel by foxes and its potential to drive drug-resistant parasites. In both cases, the unintended consequences are clear: widespread environmental contamination with pharmaceuticals is endangering animal populations and ecosystems.
The Role of Biodiversity in Mitigation: Planting for diversity plays an essential role in countering environmental contamination. Diverse plant species can help decontaminate soils by breaking down pollutants, improving soil health, and filtering harmful substances. Additionally, this diversity provides critical habitats and natural food sources for wildlife, including foxes and their prey. By promoting a range of plant life, not only does the quality of the soil improve, but foxes may also have access to medicinal plants that aid in their self-care. Many animals instinctively use certain plants to treat ailments, and a varied ecosystem can offer foxes access to plants that help manage internal parasites or other health issues. Furthermore, supporting biodiversity also strengthens prey populations, creating a more balanced ecosystem that ultimately reduces foxes' reliance on scavenging from contaminated livestock sources.
The Need for Responsible Use: Veterinarians and wildlife caregivers have a critical role to play in mitigating these risks. Castro advocates for cautious, targeted use of dewormers and flea treatments, suggesting alternative approaches such as worm egg counts and seasonal adjustments to reduce the environmental footprint of veterinary medicines. Similarly, the fox study serves as an implicit caution to those who interact with wildlife, urging more careful consideration of how the drugs used on domestic and farm animals may end up impacting wild species.
Broader Environmental Consequences: Pharmaceutical residues are increasingly recognised as a global environmental issue. Both the European Commission and the UN’s Strategic Approach to International Chemical Management (SAICM) have acknowledged the need to address the pollution of water, soil, and wildlife habitats by veterinary drugs. By continuing to monitor wildlife like foxes, we can gain vital insights into the routes of contamination and take proactive measures to protect not only animals but entire ecosystems.
For those who care for or feed foxes, it is crucial to understand the risks posed by exposing these animals to substances that may have unintended harmful effects. Red foxes play a vital role in maintaining the balance of our ecosystems, and protecting them from environmental contamination is a responsibility we all share. By actively monitoring and regulating the use of veterinary medicines and encouraging biodiversity through planting for diversity, we can help reduce the threat of drug-resistant parasites and ensure the health of wildlife and their habitats for future generations.
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