Towards more rational use of parasiticides – but how?

Towards more rational use of parasiticides – but how?

Earlier this year, Chris Little and Alistair Boxall, writing in the Vet Record (25 January) called for vets to “stop the blanket prophylactic use of antiparasitic drugs” to minimise environmental exposure. 

If blanket use means getting all cats and dogs in the practice on to regular year-round parasiticides to control any or all of the five main parasites (roundworm, tapeworm, fleas, ticks, lungworm), this does seem to be the aim of pet health plans and client reminder services, and there is an obvious business case for this approach.

Of course there are important health risks to pets and, from certain parasites, to humans. And there is a constant reminder about these – through regular articles in veterinary publications and product advertising. Viewed from the position of preventing parasitic disease alone, there is a simple solution  – banish parasites from pets with year-round prophylaxis. But what we don’t hear much about are the costs of this approach, including the potential harm to the environment through the widespread use of the products. The information needed to make a judgement about the overall benefit-harm ratio is generally missing. For example, the risk of parasitic diseases to animals and humans is not usually quantified; how common is ocular toxocariasis in humans and is there evidence that prophylactic therapy in pets has an effect on the incidence?; what is the prevalence and mortality rate of Angiostrongylosis vasorum in dogs? In other words is the cost of the harm we might be doing to the environment outweighed by the benefits of a blanket prophylactic approach?

A risk-based approach to parasiticide use – is it the answer?

The veterinary parasitology organisation ESCCAP promotes risk-based use of parasiticides. This sounds like it might lead to a more targeted and rational use of the drugs. For example, if a risk-based strategy is followed, tapeworm control will not be needed by many dogs, and 3-monthly roundworm treatment is sufficient unless the animal is at high risk of roundworm infection or the animal lives with people at high risk from infection. But, in practice, it might not be possible to rationalise use because of the features of the available authorised products. For instance, if you want to prevent lungworm (Angiostrongylus vasorum) monthly therapy is the authorised dose regimen. There is no available evidence to show that dosing less frequently than monthly is either adequate or inadequate for preventing lungworm disease (and such evidence is probably unlikely to come from the pharmaceutical industry). So, if your aim is to prevent lungworm, your roundworm prevention is also going to be monthly. 

Although there is a very wide choice of products, the lack of single-drug products, makes for a lack of flexibility in the choice of therapy. For example, there is no milbemycin-only product (which would be also useful for treating angiostrongylosis), so lungworm prevention (monthly remember) must go hand in hand with tapeworm control (unnecessary for many dogs) or flea and tick control (which may be unecessary all year round). This drives vets to use more than one combination product. My own dog was prescribed a Seresto collar (imidacloprid + flumethrin), and monthly Advocate spot-on (imidacloprid + moxidectin) and monthly Milquantel (milbemycin + praziquantel): this is belt and braces parasite protection indeed! Such overuse of parasiticides means more product ends up in the environment as well as increasing the likelihood of adverse effects.

Veterinary prescription is only part of the picture

If we want to reduce the potential environmental harm from parasiticides we need to look at parasiticide use more broadly. Veterinary prescription is not, of course, the only source of parasiticide use: imidacloprid, fipronil and other parasiticide drugs are available in a wide range of non-prescription products.

Keeping insecticides out of the environment depends in part on pet owners observing warnings (on prescription and non-prescription products) about not letting dogs into rivers if they wear a parasiticide collar or have been treated recently with a spot-on. Unfortunately the product information provided with veterinary medicines is rarely suitable for general reading. Usually the package leaflet includes the same wording as the SPC, and so is densely written in highly technical language in which important information about safe use is hard to find. In any case, the warnings about staying out of water for a few days after applying a spot-on may be inadequate for some products, because there is some evidence from a study of the use of fipronil as a spot-on indicating that the drug continues to leach from dogs’ coats for several weeks and that wash-off into wastewater is another route into the environment (Teerlink et al 2017).

We also need to know more about the comparative effects of different parasiticides. Imidacloprid has been in the spotlight, but is it worse than other topical insecticides (like fipronil), and are the spot-ons that are minimally absorbed inherently worse than systemically-acting spot-ons or oral products in their effect on the environment? I do not think this has been explored enough. 

Broad thinking needed

So asking vets to use parasiticides more rationally may be too simplistic and impossible to achieve as things stand. What is needed is a more thorough consideration of the implications of companion-animal parasiticide use from all angles, and more information about the comparative effects of parasiticides to inform prescribing decisions.

Andrea Tarr, Founder and Director of Veterinary Presciber

You might also like to read:

Time to think about the evironmental effects of medicines

Environmental effects of pet parasiticides

Is there a pet parasiticide that is safer for the environment?

Advice on worming cats and dogs depends on whose giving it

References

Little CJL, Boxall ABA. (2020) Environmental pollution from pet parasiticides. Vet Record 25 January [letter].

Teerlink J et al. (2017) Fipronil washoff to municipal wastewater from dogs treated with spot-on products. Science of the Total Environment 599: 960-6.

Our purpose...  

......is to provide busy veterinary professionals with impartial information on veterinary medicines with which to make treatment decisions in the best interests of animals, their owners and the environment. We mainly do this through the Virtual Veterinary Medicines Academy where our evidence-based peer-reviewed appraisals are the result of a rigorous research and editorial process and are presented succinctly in our multi-media CPD modules. We’re independent: we don’t sell ads, or receive commercial support. We’re funded by subscribers so you can be sure the information we provide is completely objective. Subscribers get unlimited access to the Virtual Veterinary Medicines Academy.