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News article7 September 2023Directorate-General for Environment

Anti-worm drug used by farmers on livestock detected in Scottish red foxes, Vulpes vulpes

Issue 608: Red foxes are ingesting closantel, a veterinary drug. A study suggests that it is useful to monitor red foxes for veterinary residues in the environment as they are at risk of developing drug-resistant parasites.

Anti-worm drug used by farmers on livestock detected in Scottish red foxes, Vulpes vulpes
Photo by Grayson Photography UK , Shutterstock

Contamination of the environment by veterinary medicines is a concern because of its impact on wild animals and because of the possible development of drug-resistant parasites. At present there is a lack of information on the residues of veterinary drugs in wildlife, as only birds of prey are used to monitor levels of environmental contamination.

Wildlife found dead in the UK are usually collected and examined for sources of poisoning, such as pesticides. Birds of prey, due to their position in the food chain and conservation importance, are frequently the subject of these examinations. However, collecting poisoned animals introduces a significant sampling bias, and the true extent of a toxic pollutant in the environment may not be known. Additional monitoring of species that are abundant in the environment, sampled using a less biased approach, could provide useful additional information about environmental chemical exposure.

The researchers of this study chose to focus on foxes as the sentinel species for monitoring veterinary drug residues, as they are culled for pest control, making carcasses readily available. In addition, the researchers suggest that the varied diet of the European red fox, a top predator occupying a diverse range of habitats, exposes them to many toxic substances.

Environmental pollution by pharmaceuticals, including veterinary medicines, is an emerging global threat, which is being considered as a policy issue under the UN’s Strategic Approach to International Chemical Management (SAICM). The European Commission also views the pollution of waters and soils with pharmaceutical residues as an emerging environmental problem, and endorses a ‘One Health’ strategy to address global health challenges, acknowledging the deep interconnectedness of animal, environmental and human health.

Monitoring wildlife for environmental veterinary medicine residues enables contamination routes to be investigated to mitigate impacts and reduce the risk of parasites developing drug resistance.

The researchers took tissue samples from 118 livers of foxes shot for pest control purposes, between 2014 and 2019. The liver samples were analysed for the presence of 18 veterinary medicines, including 16 anthelmintic – anti-worm – agents and two metabolites used on farm livestock. closantel, an anthelmintic drug, was detected in 18 of the fox samples tested, with concentrations ranging from 6.5 micrograms per kilogram (μgkg−1) to 1 383 μgkg−1.  However, most samples containing the drug ranged from 20 < 100 μgkg−1.

The researchers highlight that the frequency (over 15%) and level of closantel drug residue found in the liver samples raises concerns about the method of the foxes’ exposure, as well as the impact on wild animals and the environment. A number of potential routes for foxes to ingest the drug were discussed in the study, including eating treated livestock, eating the faeces of treated herbivores, eating contaminated rodents, or drinking water containing the drug or the urine of a treated animal.

The researchers note that the levels of drugs they found in Scottish foxes varied greatly, which could be related to the time since ingestion, but also to the amount of the drug initially ingested. Some foxes may benefit from the drug treating parasites within them, suggest the researchers; however, an exposure to low levels of the drug could lead to parasites becoming resistant to closantel.

Parasite resistance to closantel, the researchers note, is not currently widely recorded, but has been detected in Barber’s pole worm (Haemonchus contortus) and liver fluke (Fasciola hepatica). The fact that foxes are being contaminated may also indicate that other species are being exposed to this drug.

The study concludes that the contamination pathway of closantel and its impact on foxes, other wildlife and the environment warrants further investigation. Meanwhile, foxes may be considered for use in future monitoring to improve risk assessment of veterinary drugs on wildlife.

Further Information:

  1. The EU Veterinary medicines Directive 2001/82/EC requires monitoring of veterinary residues in live animals and animal products that are food items. However, environmental risk assessments (ERA) presently don’t consider metabolites of veterinary drugs, and the persistence, bioaccumulation and toxicity (PBT) test may not be triggered for all new products, meaning that longer term environmental effects could be missed1. The high-profile case of veterinary drug diclofenac potentially poisoning rare European vultures and eagles if they consume treated cow carcasses, flagged the risk that veterinary drug residues can pose to wildlife in Europe.


  1. See: Lyons, G. (2014) Pharmaceuticals in the environment: A growing threat to our tap water and wildlife. A CHEM Trust report. Available from: [Accessed June 14 2023].


Giergiel, M., Campbell, S., Giela, A., Sharp, E., Casali, F., Śniegocki, T., Sell, B., and Jedziniak, P. (2023). Residues of an anthelmintic veterinary drug (closantel) detected in red foxes (Vulpes vulpes) in Scotland. Ecotoxicology and Environmental Safety, 253: 114651. Available from:

To cite this article/service:

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.


Publication date
7 September 2023
Directorate-General for Environment

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