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Scientists comment on a House of Lords environment and climate change committee report on pet parasite medications.
Prof Oliver Jones, Professor of Chemistry, RMIT University, said:
“This is an interesting and balanced report. The evidence that medications designed to treat things like fleas and ticks in our pets get into the environment is very strong. The consequences of their presence are less clear. While the risk of harm to humans and pets is low, it is certainly possible that these compounds may have unintended consequences on the environment by killing aquatic invertebrates and other species. The key word, however, is “may”. The evidence at this stage is largely circumstantial. Just because something can cause harm does not automatically mean that it does so. As noted in the report, there are many types of contaminants in rivers that might affect things that live there, so how do we know which is worst, or where to focus our efforts? This requires careful thought.
“Prophylactic treatment of pets for fleas and ticks is not a bad thing in itself. If you don’t treat an animal until after it is infected, you are potentially causing suffering that could be avoided. Treating pets early also stops their owners from getting infected. What this report shows is that this practice is not without risk. The committee’s overall suggestions of getting more data on the amount of these compounds that are used and avoiding unnecessary treatments are sensible. The unenviable task regulators have is to try to balance the risks of not treating pets for parasites early enough against the potential negative effects on the environment of doing so.”
Prof Guy Woodward, Professor of Ecology, Imperial College London said:
“We know from many laboratory studies that these chemicals can induce negative ecotoxicological impacts on a range of biological processes affecting growth, development and reproductive in many species, and invertebrates, which form the core of many freshwater food webs, are particularly sensitive.
“Imidacloprid and fipronil have already been banned from outdoor agricultural use in the EU and UK, since 2018 and 2017, respectively, based largely on the potential environmental threat they pose to pollinators, such as honeybees. They have also been correlated with large-scale declines of European farmland bird populations. Concerns now are being raised about the same chemicals being used in pet parasite medicines (PPMs) leaking into the environment and their potential to disrupt freshwater ecosystems – through both direct toxic effects on individual organisms and indirectly, by altering interactions within the food web (e.g., eroding food resources for fishes).
“Some of our recent modelling work has shown that down-the-drain “wash off” of pet parasiticides is likely the major route of contamination in freshwaters, and the concentrations we see in the wild overlap with those of lab studies that show clear ecotoxicological effects on a range of freshwater species.
“These chemicals are very effective at killing invertebrates at even very low doses – that is what they are designed to do, after all – but problems can arise when they leak out into the wider environment. Plenty of ecotoxicological lab studies have demonstrated a wide range of impacts on model test organisms, and correlational field data have also associated them with declines in ecologically important species, such as dragonflies and mayflies, with suggestions of knock-on consequences for other species they interact with in the food web, such as fishes.
“Lab studies have shown that these chemicals can have negative impacts on growth, reproduction and survival of individual organisms across a range of species and at concentrations that we are seeing in the field.”
Prof Leon Barron, Professor of Analytical & Environmental Sciences, Imperial College London, said:
“We’ve tested thousands of water samples and aquatic organisms across most UK waterways and coastal areas and these products come up consistently as posing the worst risk to aquatic life.
“As a cat owner, who used spot-on products myself until 2022, I couldn’t initially reconcile this tiny drop on the back of our cat’s neck at home with the concentrations we were seeing in the environment. The fact is that the vast majority of this is coming from household wastewater. After you use it, it washes off the pet, contaminates your skin, clothes, upholstery, and floors. When these are washed, it ends up down the drain and out into the environment.
“Wastewater treatment is ineffective at removing some of them, so it isn’t just the raw sewage spills we’re talking about, it’s also in the water that’s been treated.
“While upgrading water treatment technology can be a solution, a more immediate solution is reducing how we use them. Talk to your vet and see if your animal is at risk of ticks and fleas and whether you actually need them. There’s got to be a better balance, which maintains health of our pets while also protecting the environment.”
Prof Hany Elsheikha, Chair of Interdisciplinary Parasitology, University of Nottingham, said:
“The message that parasiticides should be used more judiciously has become increasingly prominent in veterinary education and continuing professional development. However, raising awareness alone does not necessarily translate into changes in practice, and the extent to which these discussions improve clinical decision-making remains to be seen.
“Every decision to treat a companion animal for parasites involves a trade-off. Treat too little and parasites may compromise animal welfare, public health, and disease control. Treat too frequently and concerns arise over environmental contamination, unnecessary chemical exposure, and the long-term preservation of treatment efficacy.
“Responsible parasite stewardship does not end with choosing the appropriate product; it also depends on ensuring that the right product is administered correctly, for the right patient, at the right time, and in a manner that minimises unnecessary environmental exposure.”
Professor Tim Nuttall, Personal Chair in Veterinary Dermatology at the University of Edinburgh’s Royal (Dick) School of Veterinary Studies, said:
“I broadly agree with the moves to restrict the general sale of antiparasitic medications (along with other drugs that have consequences beyond the treated animals in terms of their impact on resistance and ecosystems). Treatments bought without advice may be used incorrectly, meaning that the treatment may not be effective and/or select for resistance. In addition, many animals do not need broad-spectrum ectoparasite treatment all year round.
“We have been teaching vet students to use targeted therapy based on risk and need since 2019. Briefly:
Vets and owners should be confident that treatment is required either to treat an existing infestation or where the benefits from prevention outweigh risks to the patient and environment from treatment.
Treatment should be targeted to the risk, avoiding unnecessarily broad-spectrum use.
Treatment should be feasible for the owners to apply, safe for the treated animal and in-contacts, and reflect life-style (for example avoiding water soluble topical treatments in animals that swim or are bathed).
Treatment should be given when needed – blanket all year round treatment should be avoided where possible.
“Responsible ectoparasite use is needed to preserve the efficacy of these drugs for the future as well as help reduce their environmental impact. Responsible use needs cooperation between vets and owners to identify the risk and implement the most appropriate treatment regimen for each patient. This is not possible with online or over-the-counter sales or one-size-fits-all regular health plan treatments.
“Oral medications are for systemic use – i.e. they are absorbed and incorporated into blood, plasma and skin etc. Normally, parasites need to bite to be exposed to them, but they are killed and repelled. They are waterproof as they are internal so there is no impact on watercourses and wetting or bathing makes no difference to efficacy. However, they are excreted as active compounds in faeces and/or urine; this can still affect the environment depending on where the animal urinates and defaecates etc.
“Topical medications are in two forms – those that are absorbed and then act systemically (see above) and those that spread through the coat but are retained within the skin and outside of the body. Systemic drugs are generally unaffected by bathing or water unless this occurs quickly after administration before the drug(s) have been absorbed. True topical treatments have limited waterproofing – bathing and swimming etc. with therefore reduce efficacy and contaminate water sources.
“Different worming (endoparasiticide) drugs will work on different types of worms. Drugs effective against lungworm are moxidectin (available in topical and oral formulations) and fenbendazole (oral only). Both act systemically (see above).”
Prof Dave Goulson, Professor of Biology (Evolution, Behaviour and Environment), University of Sussex, said:
“There is abundant and convincing evidence that these parasiticides are polluting English rivers, with every river testing positive, and the large majority of rivers containing concentrations far exceeding those deemed to be safe for aquatic life. There are no other plausible sources of this contamination other than use on pets, since neither chemical has been approved for agricultural use for nearly a decade. That pets are the main source of river contamination is confirmed by Sussex PhD student Rose Perkins, who studied how the chemicals pass from treated pets down the drain and through sewage works into rivers (https://www.sciencedirect.com/science/article/pii/S0048969724003103).
“Dogs and cats treated with these chemicals remain a source of environmental contamination for at least four weeks after treatment (see Perkins paper above). It seems highly likely that these widely used and persistent chemicals are also contaminating other habitats such as soils and garden flowers. No one has yet looked, other than one study from Sussex University (my postdoc Cannelle Tassin de Montaigu) which found the pesticides in the hair lining of birds’ nests (https://www.sciencedirect.com/science/article/pii/S0048969725000737).
“This new report is most welcome and suggest some obvious steps which should urgently be adopted to reduce environmental contamination with these potent neurotoxic pesticides.”
Dr Cannelle Tassing de Montaigu, Research and Associate Fellow, University of Sussex, said:
The report says, ‘circumstantial evidence suggests that the true scale of ecological harm could be significant’. What is this circumstantial evidence?
“The evidence comes from several independent observations that point in the same direction: these chemicals are widely used, are consistently detected in rivers and bird nests, and are highly toxic to many non-target organisms. Together, these findings suggest that environmental exposure is widespread, although further work is needed to determine the full ecological consequences.”
What does the evidence say about how effective a more precautionary approach to PPM use would be?
“There is good evidence that environmental exposure is linked to the scale of pesticide use, but comparatively little evidence directly comparing different prescribing strategies. This means the principle of targeted treatment is scientifically reasonable.
“Although current evidence has largely focused on spot-on treatments, the lack of data on oral flea and tick medications does not mean they pose a lower environmental risk. These compounds are still excreted through urine and faeces, providing a potential pathway for environmental contamination. The concerns highlighted in the report therefore remain relevant, particularly the need for a Phase II environmental risk assessment, alongside a more targeted and precautionary approach to treatment that ensures the benefits to animal health are balanced against potential risks to the wider environment.”
‘Paws for concern: pets, parasites and pollution’ by the Authority of the House of Lords was published at 00:01 UK Time on Wednesday 5 August 2026.
Declared interests
Prof Oliver Jones: “I have previously published research on pesticides in food and the environment. I have in the past received funds from the Environment Protection Authority Victoria and various Australian Water utilities for research into environmental pollution.”
Prof Guy Woodward: “no declarations of interest”
Prof Leon Barron:
I am one of the founding members of the PREPP team at Imperial’s Grantham Institute for Climate Change and the Environment studying this issue and I supplied written evidence to the Environment and Climate Change Committee for this inquiry
I am Chair of the Royal Society of Chemistry interest group on separation science
Projects in my lab have been co-funded by industry, including agrochemical and pharmaceutical industry partners, as well as the UK government. None of these work in the pet parasiticide industry sector that I know of.
Prof Hany Elsheikha: “I am Head of ESCCAP UK & Ireland. ESCCAP UK & Ireland is an independent organisation, and its activities receive support from companies within the animal health sector. I do not receive personal remuneration from industry related to this topic.”
Professor Tim Nuttall: “Within the last 5 years I have received consultancy fees, lecture fees, research funding and/or other support from: Virbac Animal Health, Ceva Animal Health, Zoetis, Aesculight/VetScalpel, Elanco, Royal Canin, Krka Animal Health and Merck/MSD.”
Prof Dave Goulson: “no conflicts of interest to declare”
Dr Cannelle Tassing de Montaigu:“no conflicts of interest”
For all other experts, no reply to our request for DOIs was received.