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Parasite Treatments From Swimming Dogs Pollute Pond Waters

A golden retriever sits in the water at the edge of a pond.
Credit: Barnabas Davoti / Unsplash.
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A new study of the ponds on London’s Hampstead Heath has confirmed the presence of two pesticides that are harmful to aquatic life.


The pesticides – imidacloprid and fipronil – are banned in the UK for agricultural use but are still allowed in some veterinary medicines, including common “spot-on” treatments for flea and tick infestations in cats and dogs.


The study, published in the journal Science of the Total Environment, confirms that dog swimming activity is linked to concerningly high levels of these pesticides in Hampstead Heath’s ponds, with most pet owners being unaware of the potential environmental impact that their pet’s flea treatments may have on pond life.


“Swimming can be very good for dogs, and bringing them to such beautiful places also encourages fitness and wellbeing for their owners,” said study co-author Dr. Tilly Collins, deputy director of Imperial College London’s Centre for Environmental Policy. “Stopping dog swimming in open spaces is not the answer, but we need to reduce risk, for example by providing more complete advice so pet owners can make informed decisions.”

Paddling pups and problematic pesticides

The research team studied six of Hampstead Heath’s ponds – three where dogs swam regularly and three where dog swimming is prohibited – as well as six connecting streams running above, below and between the ponds. Checking these streams allowed the researchers to assess whether there might be any other sources of contamination.


In the dog swimming ponds, the average concentrations of imidacloprid and fipronil were 309 ng/L and 32 ng/L respectively. This is around 20 times larger than the recognized threshold levels at which these pesticides are considered harmful to invertebrates.

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Neither imidacloprid nor fipronil were detected in significant amounts in the ponds where dog swimming was prohibited; the concentration of these pesticides was either below the limits of detection or the limits of quantification for both compounds. Some contamination was detected in the connecting streams; however, the researchers calculated that there persisted a strong positive correlation between dog swimming activities and elevated concentrations of imidacloprid and fipronil.


“While the chemical concentrations we measured were very high in these ponds, thankfully they were not found to significantly contaminate the wider water networks downstream of Hampstead Heath,” added Dr. Leon Barron, leader of the Emerging Chemical Contaminants team within the Environmental Research Group at Imperial College London. “The main source of these pet parasiticides in rivers generally still comes from our homes and streets through wastewater.”

Pet owners are largely unaware of the risks

For the study, the researchers also surveyed 101 dog owners who were visiting the Heath with their pets and who normally allowed their dogs to swim in the sampled ponds.


Of these 101 owners, 86% said that they were unaware of the potential environmental impacts of pet parasiticides, with 94% saying that protecting nature would be an important consideration for them in selecting a pest control product. Around one-third of the dog owners also answered that they were unaware of advice from pet parasiticide manufacturers suggesting that dogs should not swim or be bathed within a few days of spot-on treatment application.


“This work adds to the growing body of evidence that will hopefully lead to changes in prescribing practices and advice given to pet owners,” Barron said. “But as the public can buy these products without restriction in supermarkets, we need to raise awareness among dog owners that while these chemicals are useful treatments, using them preventatively may create other serious issues such as environmental pollution and pest resistance.” 


First author Lauren Yoder, a recent MSc graduate from the Royal Veterinary College, said: “Dog owners can play an important part in reducing the overall impact on the water that dogs enjoy by checking if they really need to use these products. If more owners moved to a ‘when needed’ approach, then this parasiticide pollution could be reduced.”


While spot-on treatments for flea and tick infestations are common, more than half of the dog owners surveyed reported using tablet parasiticides for dogs to swallow, instead of directly applying insecticides to their pet’s fur. Comparatively little is known about the fate or impact with these tablets, the researchers say, with more investigations needed to determine whether they might also pose a risk.

The dangers of spot-on flea treatments

Imidacloprid and fipronil have been used in monthly spot-on treatments for flea and tick infestations since the 1990s. These pesticides were also used as agricultural pesticides for many years, until it was discovered that they pose a lethal threat to honey bees and have played a major role in the decline of other invertebrate populations. Subsequently, the use of these pesticides in outdoor agriculture has been heavily restricted in the United Kingdom and the European Union.


However, both imidacloprid and fipronil are still widely detected in British surface waters at concentrations high enough to harm aquatic life. Recent studies have suggested that their continued use in pet parasiticides could account for an estimated 20–40 % of this wastewater pollution, as dogs are bathed and washed too soon after treatment. 


"Many veterinary associations recommend against routine blanket parasite treatments for pets, yet this practice remains widespread, especially through subscription pet healthcare plans,” said veterinarian and study co-author Dr. Rosemary Perkins, from the University of Sussex. “Pet parasiticides are highly ecotoxic substances and we are only now beginning to understand their environmental consequences.”


Reference: Yoder LE, Egli M, Richardson AK, et al. Dog swimming and ectoparasiticide water contamination in urban conservation areas: A case study on Hampstead Heath, London. Sci Total Environ. 2024:176686. doi: 10.1016/j.scitotenv.2024.176686


This article is a rework of a press release issued by Imperial College London. Material has been edited for length and content.