They are called fluralaner, afoxolaner, lotilaner and sarolaner, four technical names that probably mean nothing to most pet owners. Yet, if you have a dog or cat and you treat it against fleas and ticks, it is very likely that at least one of these anti-parasitic active ingredients has already entered your animal’s body.
We are talking about isoxazolines, the most recent class of veterinary ectoparasicides, much loved for their convenience (they come in chewable tablets) and duration of action but now at the center of an important environmental alarm.
A study, just published on Environmental Toxicology and Chemistry by researchers from Vetagro Sup, Campus Vétérinaire in Marcy-l’Étoile, France, precisely measured how long these active ingredients remain in the feces of treated dogs and cats. The result is worrying: we are talking about weeks, in some cases months. An emblematic case is that of lotilaner detected in dog feces up to 204 days after administration.
Isoxazolines are eliminated from the body mainly via the faeces. The researchers monitored 20 dogs and 20 cats in the period following the antiparasitic treatment, collecting samples at regular intervals for three months (and longer), and analyzing them with mass spectrometric techniques. The results show very long fecal elimination half-lives: approximately 23 days for fluralaner in dogs, 25 days for lotilaner, with similar values in cats.
In simpler words: every time your dog or cat defecates after a flea treatment, its feces contain measurable quantities of active insecticidal substances. And this feces ends up in the ground, in parks and lawns.
Who loses?
The most at risk are coprophagous insects, i.e. those that feed on feces or lay their eggs in them: dung beetles, manure flies, some species of diptera. These are animals that are fundamental for the health of the soil, because they break down excrement and accelerate its recycling into fertile organic matter.
The researchers used statistical simulations to estimate the probability that a coprophagous insect, randomly feeding on contaminated feces, would be exposed to a toxic dose. The results were alarming: in the case of fluralaner and lotilaner, this probability exceeds 87-92% if the concentration affecting 50% of the insect population (EC50) is considered as the threshold. If, however, the more conservative threshold of the no-effect concentration (PNEC, i.e. EC50 divided by 100) is used, all isoxazolines lead to a probability of toxic exposure close to 100%.
The good news, so to speak, is that afoxolaner and sarolaner seem less persistent, probably because they are metabolized more rapidly and their fecal residues are lower. The bad news is that the underlying problem is structural: isoxazolines are administered continuously, often for preventive purposes, to millions of pets across Europe. This means a constant and uninterrupted flow of insecticidal substances entering the environment through feces.
The researchers also point out that the most recent antiparasitic products often combine isoxazolines with other active ingredients such as moxidectin or milbemycin oxime, anthelmintic substances which are also eliminated in the feces and can affect arthropods. Moxidectin, in particular, is classified as persistent, bioaccumulative and toxic in the European Union.
What scientists ask
The study concludes with some concrete recommendations. First of all, the researchers call for information on prolonged fecal elimination to be included in veterinary drug information leaflets, along with practical guidance for owners. Secondly, in countries where waste is incinerated, they suggest disposing of the feces of treated animals in the garbage (and not in composters or in the garden) for the duration of the treatment.
It is an invitation to awareness, rather than a condemnation of isoxazolines. Knowing that your dog or cat, after the flea tablet, “emits” active insecticide for weeks may seem like an uncomfortable discovery, but it is the first step towards making more informed choices, and perhaps to urge the regulatory authorities to carry out more stringent environmental assessments.
