For me, the month of September comes with a creeping sense of dread: the inevitable email from the primary school secretary announcing that the classroom has a new visitor: Pediculus humanus capitis, also known as head lice.
I still get itchy thinking back a few years when my kid’s class fell victim to a particularly relentless infestation. It seemed, at first, manageable – we all treated our kids’ hair with medicated shampoos, and the lice appeared vanquished for a few blissful days. But those tiny sesame-seed-sized parasites had other plans. With their six legs, they silently scuttled and crawled from head to head across the classroom, reinfesting each child. Using their hook-shaped claws, they climbed up strands of hair like rope until they reached the scalp, where they’d feed on blood. Within hours, a well-fed female louse can lay 10 eggs a day.
Each day and night, I meticulously combed my son’s hair with intense focus, just like the chimpanzee in that unforgettable BBC Trials of Life scene with David Attenborough. Nothing worked. The parents’ WhatsApp group descended into madness. One mother texted me to say she’d smothered her daughter’s hair in mayonnaise; a father, who left me unsure if he was joking, suggested we should “freeze their heads”.
I resorted to covering my son’s thick, short hair in tea tree oil, which achieved nothing beyond filling the house with a scent best described as “old field hospital”. Ultimately, I reached for the simplest weapon, a pair of clippers, and gave him a tight buzz cut.
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But then, there’s my dog. I did a double-take a few weeks back when she sat on the grass and twisted herself into a pretzel, scratching furiously at her side with a hind leg. I hadn’t seen her pull off those moves in a few years, and they felt eerily familiar, like she was battling an army of Ctenocephalides canis, dog fleas.
Though only distant cousins in the insect world, both head lice and dog fleas share a fondness for sucking blood, which is full of protein and fat to fuel their growth and reproduction. Fleas, larger than lice, have long, strong legs which propel them from one host to another. The female dog flea can live for around 100 days, laying up to 50 eggs each day in the fur.
Although resistance is becoming more common, pesticides are effective against these parasites. Human head lice are often treated with lotions with permethrin. For dogs and cats, spot-on flea treatments containing fipronil or imidacloprid are dabbed on the back of the neck. Fleas cause discomfort to pets and, although very rare, can spread zoonotic diseases such as bartonellosis, so it’s vital to get rid of them when they appear.
However, the unnecessary overuse has a consequence: the pesticides wash down our drains and into rivers, devastating aquatic life. A recent study by University of Galway scientists found permethrin in waterways at concentrations considered a “moderate to high risk” to the environment.
Fipronil and imidacloprid are so toxic to wildlife that their use in agriculture has long been outlawed. In 2013, the European Food Safety Authority labelled fipronil a “high acute risk” to honeybees. Researchers at Imperial College London estimate that one monthly flea treatment for a dog contains enough imidacloprid to kill 25 million bees.
Despite these risks, flea treatments containing these chemicals are common, and veterinarians recommend continuous use, even when pets show no signs of infestation. The European Scientific Council Companion Animal Parasites, an industry-sponsored body that provides veterinary guidance on parasite control, advises “year-round” application. For manufacturers, it’s a profitable business, but research shows the hidden toll on aquatic life – and the cost of remedying this – could be enormous.
[ The unanswered questions on impact of pesticides on the health of ecosystemsOpens in new window ]
Researchers at the University of Sussex – Rosemary Perkins, Dave Goulson and Martin Whitehead – have spent years working out how these pesticides get into, and impact, rivers and the wider environment. In one survey of English waters, fipronil appeared in 99 per cent of samples, with one showing levels 38 times the safety limit.
Dogs treated with fipronil or imidacloprid continue to release these pesticides into water for up to a month after treatment, at levels hazardous to insects, fish and other species. The problem extends beyond dogs swimming in rivers and lakes after treatment; residues enter waterways “down the drain” when owners wash pet bedding or clean their hands after handling the animals. In one US study, elevated concentrations were detected in the drains of dog-grooming facilities.
Uisce Éireann does not test drinking water for imidacloprid or fipronil. The Environmental Protection Agency does not routinely monitor them either, but in recent years, they have carried out a small number of spot checks in four rivers – the Nore, Suir, Liffey and Annalee – and Lough Derg. Exceedances were detected in the Liffey (which supplies Dublin’s drinking water) and the Annalee.
It’s not just life in the water that’s affected. In spring, birds such as blue tits often use bits of animal fur to build their nests. Earlier this year, a separate research team found fipronil, imidacloprid and permethrin in nests lined with fur, along with a higher incidence of dead chicks and unhatched eggs.
Until recently, I had no idea. I had dosed my dog for fleas without questioning whether it was necessary or considering the broader impacts. Mossie loves swimming in the water – I never thought to stop her after being treated.
According to Martin Whitehead, who’s also a vet, there’s no evidence that the large majority of dogs or cats need year-round flea treatment. Vets profit from selling these chemicals, he says, but for owners, he advises a more cautious approach: only treat when there’s an actual infestation, or if your pet is at high risk (for example, if they’re allergic to fleas). For four weeks after treatment, keep dogs away from rivers and, to avoid the pesticides going down the drain, don’t bathe them or wash their bedding at home, and don’t send them to a dog groomers.
As for my dog Mossie, all I could find was a patch of dry skin – thankfully, no fleas. But she’s just one of half a million dogs in Ireland. If we follow the advice to treat pets continuously, whether or not fleas are present, we inadvertently contribute to significant water pollution. Scientists warn that a few drops of these pesticides can kill millions of insects. As much as I love my dog, I plan to use these treatments very judiciously from now on.
