By Professor Steve Ormerod, Cardiff School of Biosciences / Water Research Institute
Anyone interested in pets or pesticides will have noticed that chemicals used in some veterinary flea or tick treatments occur as pollutants in streams, rivers and ponds. Focus has centred on imidacloprid (neonicotinoid) and fipronil (phenylpyrazole) – applied as ‘spot-ons’ to domestic dogs, cats, rabbits or ferrets. Concern ramped up a few years ago when Rose Perkins, Dave Goulson and others used Environment Agency data to show that both chemicals occurred widely in English rivers. Work by environmental NGOs, notably the invertebrate charity Buglife, added to the evidence. Against a background of otherwise declining concentrations in rural UK rivers (as agricultural uses of imidacloprid and fipronil were banned or restricted), the most plausible explanation was unintended contamination from more than 3.5 million doses (about 6000 kg) of imidacloprid or fipronil products applied annually to the UK’s ~22 million household dogs and cats.
In small, enclosed ponds, dogs swimming after treatment is a likely source of contamination. In rivers, however, the quantities present could not be explained so simply, and further work began to implicate wastewater. Specifically, owners who washed their hands, their pets or their pets’ bedding after application were inadvertently loading chemical residues into the sewer network. Wastewater treatment, in turn, barely intercepted the onward flow.
Dog having tick and flea treatment applied. Credit Chutima Chaochaiya (Shutterstock.org)
With data lacking from Welsh rivers, in 2021 we began a three-year programme to compare the occurrence of pet flea-treatment chemicals among i) urban rivers receiving wastewater; ii) rural rivers and iii) small urban streams at risk from misconnected sewers. The results generally reinforce patterns in England. Imidacloprid and fipronil were widespread, occurring respectively in 77% and 44% of all our Welsh samples. Flea-treatment chemicals were also more likely to occur in urban rivers, where concentrations exceeded safe levels in almost half of the samples, for example downstream of wastewater treatment plants.
A typical surface drain discharging into Cardiff’s Roath Brook – and a likely pathway for wastewater transport via misconnected sewers. Credit S. Ormerod.
But our study detected other wastewater routes. The highest imidacloprid and fipronil concentrations were in a small Cardiff stream, Roath Brook, at 11 and 45 times safe levels, respectively. Reductions in numbers of sensitive insects in this stream accompanied increased imidacloprid concentrations suggesting ecotoxicological effects from this substance alone or as part of a pollutant cocktail. With no inputs from wastewater treatment or sewer overflows, only misconnected sewers could explain such elevated concentrations. Estimates suggest that around 5% of UK properties could be misconnected in this way – where drainage from washing machines, dishwashers, showers, sinks or toilets have been mistakenly plumbed to surface drains rather than foul sewers. In some hot-spots, as many as 20% of properties could be affected. We suspect that such misconnections are potentially an important contamination route that has been overlooked. As further evidence of links with this and other wastewater pathways, concentrations of imidacloprid and fipronil in Welsh rivers correlated strongly with wastewater indicators, notably caffeine.
The Welsh study has been extensively reported in the media – along with positive responses from the British Veterinary Association about straightforward options through which pet owners can reduce spot-on use. Sewer misconnections, meanwhile, are a focus of ongoing investigation by water companies and regulators.
Find out more about PAN UK’s work on the pesticides in pet tick and flea treatments here.
