PART ONE: Where are we coming from and where are we going?
One of the more hotly debated topics in veterinary medicine at the minute is the use of parasiticides to protect our pets both from parasites and the diseases that these parasites may transmit to our pets and ourselves.
On one side of the debate, we have the parasitology experts who readily acknowledge that there is evidence that some of the products may be causing an environmental impact, and we should take care with their use, but point out we have a social obligation to be conscious of the impact that not treating for parasites may have. Not only on the welfare of our pets, but also the impact they can have on other animals including farm animals, wildlife and disease risks to the more vulnerable members of the human population.
On the other hand, those studying the environment are rightfully concerned that there is definite evidence that levels of the older generations of parasite drugs are building up in the environment and that these levels are having a negative impact on wildlife and the biodiversity of our planet. They are also concerned that, while there is currently little evidence to support the argument that the newer generation of parasite products are having an impact, if we wait for that evidence to arrive, it may be too late to act. As such we also have a social obligation to minimise the use of parasiticides wherever possible.
What we have tried to provide for you in this two-part article is a summary of what we know, what we suspect, and where we may need to gather more information. Part One will look at how we have approached parasite treatment in the past, what we know about the build-up of these products in the environment and what information we need to seek moving forward. Part Two will give more information about the parasites we would regularly treat in small animal medicine and the potential risks owners should be aware of when considering their personal treatment strategies.
A Quick Summary
As this is obviously a complex issue, we have started with a summary of the main three points:
- We must stop using Fipronil and Imidacloprid products immediately
While the use of parasiticide drugs is under hot debate, there is little doubt from either side that these two drugs are causing real harm to the environment. They are possibly not the only issue but are certainly the two that there is a significant amount of evidence to show there is a problem. Their use in agricultural settings is already heavily regulated, however, there is significant evidence that their use in pet products is harming the environment. The levels found in wastewater coming from households is considered to be high enough to be causing an impact, and the drugs survive sewage treatment to make it into water environments. In addition to being found in our waterways, a new study has shown the levels of these drugs present in pet hair used to line nests may be causing a reduction in hatching of eggs in songbirds in the UK. In the absence of better government regulation in this area, it is down to vets and pet owners to ensure we are no longer using these products. Fipronil has been at the front line of treatment for fleas in dogs and cats for over a decade. It is currently present in several different products on the market. While these products are rarely prescribed in veterinary surgeries these days, Fipronil products are still one of the most popular “over the counter” products now available at pet shops and online without a prescription. On the other hand, Imidacloprid is still present in some parasite treatments prescribed by veterinary surgeons although many will no longer advocate their use due to environmental concerns.
- If we want to reduce the amount of parasite products used, we will need to test more
For decades we have been using preventative treatment to reduce the risk of our individual pets catching parasites. It has been cheaper for the owner, easier and safer for the pet to do so. Now that we are moving away from the overuse of these drugs, it is important to realise there is a responsibility to ensure our pets are not infected. The only way we can do that is by regular testing for disease. The balance between regular testing and regular treatment is something that can be discussed with your vet to decide on an individualised parasite plan suitable to your pet.
- We need to become more comfortable with the understanding that a reduction in use of preventative treatment for our pets may increase the level of risk to our pets and the wider human population
There is no getting away from the fact that the parasites our pets carry can result in disease risks both to themselves, their owners and the wider community. To be clear, these disease risks in the UK are low but the true level of their impact is hard to measure. It is important that any pet owner deciding on whether or not to treat their pet for parasites is clear on what these disease risks are, and how to minimise them. This is covered more in second part of these articles.
The Detailed Article
With the summary done, if you want to know more, please read on… It’s not going to be a short story… (maybe sit down with a cup of tea!) but we hope it will give a clear and full explanation of the challenges faced in this topic. This information is mainly considering the UK pet population and is a summary of where we are at in March 2025.
Where have we come from?: “Evidence based medicine” and “Do Least Harm”
Two of the tenants that form the foundation of responsible veterinary medicine are:
- To make treatment plans based on all available evidence,
- In the absence of sufficient evidence, to precede in a way that does the least harm.
One of the major challenges with deciding on appropriate parasite treatment protocols has always been the lack of definitive evidence on localised parasite risks. For example, we know Lyme disease, a disease transmitted by ticks, used to be mainly located in areas including the New Forest, the west coast of the UK and the highlands of Scotland. However, there is now evidence of it being found in tick populations throughout the UK, but little specific data of how prevalent it may be in any given area. We also know that lungworm has become established in the UK, especially in the south of England, but we don’t know how prevalent it is in local pockets around our local environment.
Given the serious health consequences of developing some of these parasitic diseases and the risk to human health in some circumstances, we as veterinary professionals and pet owners have traditionally decided the “least harm” route is to treat our pets with preventative treatment against these parasites to reduce the possibility of disease developing. This choice has been exacerbated by the fact it is usually cheaper for the owner to give preventative treatment to treat for parasites, than it is to test for the presence of the parasites themselves. Further to this, to make this treatment easier for most owners to give, companies have developed products that aim to treat all parasites in one tablet or spot on which can lead to over-treating for parasites that aren’t necessarily a concern for the individual animal.
What has changed?
The environment around us!
Again, the arguments around this area are heated and often contradictory. What we do know is that studies have shown a dramatic drop in the insect population within the UK along with a reduction in the biodiversity in our waterways. In addition, there are studies showing the level of drugs contained within some of the parasite treatments we use in the pet population have been found in our waterways (mainly fipronil and imidacloprid).
Now it is important to clarify that fipronil and imidacloprid have both been used in the agricultural industry for some time and this would have a significant impact on the levels found in waterways, but there is also evidence of these drugs present in waterways that were not near treated farms. A further study done in 2022 summarised that there was currently no clear evidence that parasiticides used in our pets were having a negative effect on the environment, but strongly stressed that this may be due to lack of evidence and called for in-depth regulatory investigation of the environmental risks of these drugs. A single study in 2017 also found moxidectin in waterways, and one Dutch study has shown evidence of flulaner in a water environment following a swim test with a dog that had been treated with that drug. Ultimately, a lot more research needs to be done to assess whether these drugs are truly having an impact on the environment.
We also need to see changes in the regulations surrounding what drug companies publish about their effects. While the use of parasiticides in agriculture are heavily regulated with regards to their environmental impact, there is less regulation regarding their use in pets. The argument being that the treatment of one dog or one cat has negligible effect on the environment, not taking into account the millions of pets being treated across the UK. This is something we believe needs to change.
So what to do next?
So while the cogs slowly turn in the regulatory bodies of the government and pharmaceutical industry, the question remains, what should we as pet owners be doing next?
Consider our individual household risk:
While the debate rages on as to whether and how much impact the use of parasite treatment used in the pet population has on the environment. It would seem prudent to reconsider the way we approach its use. We need to shift our thinking from simply “do least harm to the pet in our house” to also include “and do least harm to the environment around us” while also considering any potential health risk to the people living in our households and the local area.
However, this involves some difficult decisions and will ultimately come down to owners’ personal preference. There are risks that owners who chose to reduce their pet’s parasite prevention strategies should be aware of. Many of those risks are low, but again, it is an area that is hard to fill with absolute facts or figures.
With regards to discussing these risks, we have tried to summarise them as clearly as possible in Part Two of this article. Our goal has been to provide the information as clearly as possible, without being too alarmist! Our suggestion would be for you to read the information available, discuss it further on a one to one basis with your veterinary surgeon, and devise a parasite control strategy that you feel is best suited to your household.
Gather more information:
I suspect if you’ve read this far, you would agree that the most frustrating aspect of looking at the use of parasite treatments in pets is the persistent lack of information. We lack reliable information on:
- how many cases of human illness derived from pet parasites are seen in the UK every year;
- we lack data from the drugs companies about how these drugs are excreted into the environment and in what form (which they are currently under no legal obligation to research or provide);
- we don’t have enough information about the prevalence of these diseases in our local environment.
We are collectively trying to navigate a sensible solution to the treatment of our pets while dealing with some significant information gaps. Now some of these measures are out of our hands. We need a clearer national (human and pet) disease monitoring systems; we need legislation changes for the drug manufacturers to publish more information about how these drugs are excreted; and we need continued environmental research into whether the use of these drugs are impacting the environment and to what level. Monitoring of parasiticide levels in waterways are still on going, but as far as we are aware, there is no testing being done on soil.
Test not treat:
However, the only way we can learn more about the prevalence of disease in local area is by testing our pets. This will take a shift in the mindset of pet owners and veterinary professionals alike. For example, it has been far cheaper for us to treat for intestinal worms rather than to test for them, so that is what we have generally suggested, and what clients have happily embraced. Similarly, we know that lungworm tends to exist in pockets in local areas, but it is only by testing our patients and forming a database of where pets have been, that we can start to form a map of where those pockets are.
Ultimately, the cost of these tests would need to be borne by the pet owners themselves. In an environment where the costs of providing veterinary medicine have escalated heavily in the last 4 years, we appreciate that the discussion of increased costs for veterinary care won’t be welcome to many. However, it is worth looking at the UK horse population as a good example of how positive change can be achieved.
Twenty years ago, horses were routinely treated for intestinal worms, but horse owners often struggled with accurately weighing and dosing their horses correctly. This led to a situation where horses may have been treated too often and with dosages of medications that were too low. We know that under dosing of parasite treatment can lead to the development of resistance and we were certainly seeing a developing issue in the horse population at that time. However, the industry responded very well and there has been a clear shift from “regular worming treatments” to “regular worm egg counts”.
This has subsequently led to appropriate dosing only when necessary which is not only beneficial to the horses themselves, but also to the reduction of developing resistance. While we would need to be more careful replicating this system in the pet population as there is a greater risk to people that are exposed to eggs produced by intestinal worms from our pets than those from intestinal worms in horses, a switch to regular worm counts rather than medicating could be possible if done correctly.
In summary:
We need to consider making some major changes from funding of studies and regulatory changes from the government, right down to the way we discuss parasite treatment in our consult rooms and in our households with our pets. In the absence of concrete information, a lot of these decisions will come down to our personal assessment of risk. There are no right or wrong answers, but we would encourage you all to read the information provided, (avoid the misinformation found online!) then come and discuss your personal situation with a member of our vet team.
In mean time, please click here to find Part 2: What are the Risks?
Further information
Among other sources:
To Flea Or Not to Flea: survey of UK companion animal ectoparasiticide usage and activities affecting pathways to the environment
Rosemary Perkins and Dave Goulson
Pet dogs transfer veterinary medicines to the environment
N.J. Diepens, D. Belgers, L. Buijse, I. Roessink
https://www.sciencedirect.com/science/article/pii/S0048969722066499
- https://unsplash.com/photos/brown-horse-on-green-grass-field-near-trees-during-daytime-pqX8m4ME5yw ↩︎