Worms, fleas and their treatments in the environment – it’s worse than I thought

The article was originally written in 2021 and revised in 2024

The majority of dogs and cats routinely receive flea and worm treatments containing toxic chemicals – the ‘active ingredients’. Many worm and flea treatments now come together in a single treatment either as a (easy to administer and very popular) ‘spot-on’ or as orally administered treatments. Spot-ons are finding their way into watercourses, polluting rivers and poisoning aquatic life and orally administered treatments are killing soil organisms when they pass into the environment along with urine and faeces. 

Once I started to research this article I realised how little I actually knew about something I thought I knew enough about. This is humbling. I thought that residue of flea and worm treatments from companion animals would literally be a drop in the ocean when compared with parasite treatments used in agriculture. Unfortunately, this is not what I have found and the negative environmental impact of companion animal parasite treatments is much more significant than I’d imagined. Parasite treatments for livestock are regulated and according to the ‘agriculture and horticulture development board’, there is detailed guidance for efficient and targeted parasite treatments meaning that residues are kept to a minimum. It has to be said that these stringent safeguards were brought in to limit resistance to the chemicals by the parasites rather than to protect the environment but we have to take our wins where we find them! As part of the licensing process, manufacturers are required to provide detailed information on the environmental impacts of veterinary medicines for use on livestock. Companion animal treatments are exempt from this level of scrutiny, ostensibly because they are used as individual treatments and usually in much smaller quantities. However, because in small animal medicine parasiticides are used prophylactically for prevention as well as treatment and because they are being widely used on a growing population, the quantities are very significant. In contrast with livestock treatments, companion animal parasite treatments are prophylactic and broad spectrum, they are not targeted.

There 10 million dogs and about the same number of cats in the UK and that’s a hell of  a lot of worm and flea treatments, some of which are ending up in the biosphere, causing harm. 

I checked the ‘active ingredients’ in the ‘advocate’ spot-on I had in the cupboard. Advocate cat and dog use the same active ingredients, imidacloprid and moxidectin. It turns out that imidacloprid is a neonicotinoid and in case you don’t know, neonicotinoids are responsible for the collapse of bee populations amongst other ecocides. I had thought neonicotinoids to be totally banned in the EU (and still at the moment the UK), but it turns out that they are ‘restricted’ rather than banned. In this case, ‘restricted’ means they are completely banned in farming (flowering crops in 2013 and all outdoor crops in 2018) and they are now only permitted for inside use – or on companion animals!

Spot on treatments are designed to sit within the lipid layer of the skin – essentially within the fat. The Advocate data sheet explains that after ‘spotting-on’, imidacloprid is rapidly distributed over the animal’s skin and from here, it gets into the plasma. Within one day of application, Imidacloprid can be found on the body surface throughout the treatment interval. This is harmful for the pet and also everyone and everything that comes into contact with them.

Moxidectin is also absorbed through the skin, reaching maximum plasma concentrations approximately 1 to 2 days after treatment in cats and approximately 4 to 9 days after treatment in dogs. Following absorption from the skin, moxidectin is distributed systemically throughout the body tissues, but because it’s fat soluble, its concentrated mainly in the fat. It is slowly eliminated from the plasma as manifested by detectable moxidectin concentrations in plasma throughout the treatment interval of one month.  Presumably from here it will enter the environment through urine and faeces.

All licensed medications have datasheets.The Advocate one says dogs are not to swim for 4 days after treatment and thats all it says in terms of protection of the environment! For UK medications, the datasheets should be accessible using the NOAH compendium, https://www.noahcompendium.co.uk/home. Its not helpful that you can only access them on line, surely they should be included inside the medication packaging too?

Meanwhile, Jade Urquhart-Gilmore of vet sustain tells me it is important to not wash pets or let them go out in the rain or in water for approximately a week after treating with a spot-on to avoid the worst contamination of water courses. Unfortunately, she believes there to be little research on residue from spot-on or orally administered parasiticide treatment in faeces, making it difficult to know how long to bin faeces rather than compost it. As an aside, because of the potential for not just parasitcide residues in pet faeces but also parasites themselves pet keepers should always pick up, even if on agricultural land or in the middle of nowhere see ‘environmentally friendly disposal of pet waste’ article. Clearing up after cats that go outside and clearing up urine are more difficult considerations.

Whilst my research started with Advocate, after discovering the full horror of imidacloprid, I realised that whilst there are an alarming array of active ingredients it might be best to focus on the worst offenders in terms of damage to the biosphere as the scope of this article is obviously limited. Further research rapidly indicated that along with imidacloprid, fipronil is probably the other most toxic chemical currently used in companion animal parasiticides in the UK. Fipronil isn’t actually a neonicotinoid, but it is a close approximation of one. Both fipronil and imidacloprid are widely used in spot-ons and they are both particularly damaging to water courses. In managing the excesses of the worst offenders it is perfectly possible to manage the worst excesses of the other active ingredients, both the spot-on and orally administered types (and hopefully ‘cocktails’ too). There has been growing public awareness around fipronil recently, I was circulated a petition in the summer of 2021 by a friend. Unfortunately, despite my recirculating it, it only got 2,817 signatures, falling way short of the 1000 required for a parliamentary question. 

A recent study cited in the bva-bsava-and-bvzs-policy-position (see later), found fipronil and imidacloprid at levels exceeding chronic toxicity in English rivers. The study tested up and down river of sewage treatment works and postulates that these chemicals were likely to have come from household drains because of people washing pets they have recently treated with spot-on. 

Whilst imidacloprid is only available from the vets, fipronil is available over the counter in petshops with no checks or advice other than what’s in the packet. Shockingly fleas, the target, developed resistance to fipronil a long time ago and products containing this active ingredient like frontline are no longer even effective!

The bva, bsava and bvzs state that since the advent of ‘spot-ons’ parasiticide sales have gone through the roof.

• one spot on treatment for a medium sized dog contains enough pesticide to kill 60 million bees! (BVA report)
• Total sales data from the Veterinary Medicines Directorate (VMD) shows that 27,471 kg of fipronil and 33,036 kg of imidacloprid have been sold as flea treatments since the products were first authorised as veterinary medicines in 1994 and 1997, respectively
• imidacloprid is 7000x more toxic than DDT (BVA and webinar vet)

I think we need that parlaimentary question more than ever.

Companion animal parasiticides are broad spectrum:

• Advocate cat ‘protects’ against fleas, hookworm, round worm, heart worms, otodectes, eucoleous, notoedres, aelurostrongylus, and thelazia (and everything else in soil and water).

• Advocate dog ‘protects’ against fleas, hookworm, whipworm, round worm, heart worms, angiostrongylus, creneosoma, otodectes, thelazia, sarcoptes, demodex, louse, skinworms, microfilaria, spirocerca and eucoleous

I’ve no idea what half of these are and whether we and our pets need ‘protection’ or not. These products are so ‘broad spectrum’ that their ‘friendly fire’ kills pretty much anything without a back bone!

At the same time, we need to keep our pets parasite free – flea infestations (and I speak from personal experience) are not much fun and worm infestations are not very pleasant either. Worm infections can cause health problems in people too (and are spread by fleas…). Although this is rare and according to a vet sustain webinar on the subject, over emphasised https://thewebinarvet.s3.eu-west-1.amazonaws.com/Webinar_Video_2021/TWV_VetSustain%20-%20The%20environmental%20fate%20of%20pet%20flea%20products.mp4.

Although I have always used these nasties, it’s always seemed sensible to me to only use them  as and when the need has arisen. We have used ‘spot-ons’ for fleas only when we have seen signs of fleas and wormers every 3-6 months. I can’t say that this is because I’d considered their impact on the environment though, I don’t think anyone’s been thinking about that – until recently. Writing this article has made me reappraise the way we do things – that’s for sure.

If we are going to use these chemicals then we need to use them sparingly and in a targeted way.

Parasite treatment advertising relies on generating fear to sell them. The posters for parasite control up in vet waiting rooms are frankly frightening. When still very young, my daughter was frightened that our pets would get lung worm after reading all about it in the vets. I would rather take an educated approach rather than scare people including children.

However, many vets and corporate veterinary practices promote monthly pet care clubs with parasite treatments included. These have been widely taken up by the public. This has led to routinely treating pets for fleas and worms, whether or not they are infected. One of my veterinary colleagues private practice vet club (2024) includes: bravecto (fleas, ticks, mites) and milprazon (roundworms, tapeworms, heart worm, lungworm) and vaccinations plus 10% off stuff. The pet club isn’t variable and everyone gets the same unless an animal won’t take tablets or has a reaction

Subscribers get discount on the products, that said, discount on something you don’t need isn’t really discount at all. The larger proportion of veterinary practices are now corporates, making the priority shareholders dividends above human or animal welfare and the environment. Non corporate vet practices find themselves having to compete.

I am reviewing this article 3 or so years after the publication of the of the BVA, BSAVA and BVZS policy position on responsible use of parasiticides for cats and dogs in 2021 https://www.bva.co.uk/take-action/our-policies/responsible-use-of-parasiticides-for-cats-and-dogs/. The article was  widely disseminated in the veterinary press. Three years on, I have been speaking to some of our pets vets and other veterinary colleagues and I have been heartened by the response. They definitely understand the problem, feel there is a need to act responsibly and are doing their best within various constraints.

The situation is code red urgent but, on the bright side, with decisive swift action it could be immediately drastically improved and ultimately made better.

What Government can do: These chemicals need to be subject to and environmental impact assessment. It would also make sense to me to ban at least the most damaging ‘active ingredients’. However, this would require regulation – lets hope the new Labour Government is more interested in regulation than the last! It has also become clear that pet shops must be banned from selling these products making them only be available through vets. I have been against the new RCVS rules making a vet check compulsory before being prescribed these products as I thought it mostly a money making exercise. I can now see the sense in it – but only if petshops are banned from selling these toxic products. If vets tailor the plans to the individual risks with the risk to the environmental on equal footing to the risk of infection to pets and pet keepers that could work. Legislation also has to be enforced although I expect the veterinary profession is more able to comply than some professions.

What vets can do: It makes sense in every way that if we are going to use these products we must use them efficiently as much of the damage these chemicals do can be controlled through correct usage. This is where vets come in. When I wrote this article in 2021, around the time of the publication of the BVA, BSAVA and BVZS policy position mentioned above, I felt comforted by how seriously the veterinary bodies appeared to be taking this issue.

The document is comprehensive and doesn’t underestimate the environmental impact of spot-ons, acknowledging that “some of the most commonly used parasiticide products that are used in companion animals which are neurotoxic to other species are the pesticides fipronil and imidacloprid either alone or in  combination with other parasiticides. These products are effective because of their potent toxicity even at low dose rates towards a wide range of invertebrates, their persistence and their water solubility. These same properties also increase the environmental hazards associated with their use”.  Perhaps unsurprisingly for vets, the report emphasises the risk to pets and pet owners of contracting parasites above environmental concerns. It also emphasises lack of evidence and the need for more research which is laudable, but not if the veterinary profession exploits these gaps in our knowledge. The report uses words like “may” too often for my liking. In summary, the correct advice is there but it is downplayed. There is no ambiguity in the harm done by these chemicals to our rivers.

The report acknowledges that prophylactic treatment for parasites often forms part of veterinary practice health plans for small animals, which provide important income for veterinary practices. It accepts that moving away from blanket treatment will pose challenges to veterinary practices. The report very sensibly suggests that if monthly clubs were adjusted they could be pet, vet, client and planet friendly. Using less parasiticide treatments should help to ensure affordability, as clients are buying less product. In-clinic risk assessments and worm count laboratory testing would be helpful for practices working to adapt their health plans. In this way, health plans can be restructured to ensure costs continue to be spread out and affordable over time, customer loyalty can be maintained and vet practices can still makes money.

Of course, this will also add to workloads for stretched veterinary teams, and the additional time needed could have cost implications for clients.