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Pyrethroid Insecticides Were the Most Likely Cause of 12 Barn Swallow Deaths in a Dutch Stable

Barn Swallow fledglings at a sheltered nest
Conservation · Threats

Twelve barn swallows (Hirundo rustica) died during an acute mortality event in a riding stable at Noordwijk in the Netherlands on 3 August 2023. The affected birds showed neurological signs similar to those recorded in an earlier mortality event involving sand martins (Riparia riparia) near Utrecht.

Researchers examined carcasses from both mortality events using post-mortem examinations, virus tests and pesticide analyses. The birds tested negative for Usutu, West Nile, Japanese encephalitis, Sindbis and influenza A viruses. Most were emaciated, but the pathological examinations did not identify disease or trauma that could explain the mortality pattern.

Pyrethroids were detected on every analysed feather sample

A broad pesticide screen detected two pyrethroid insecticides, permethrin and tetramethrin, on every feather sample that could be analysed. The compounds were absent or occurred at comparatively low concentrations in stomach and liver samples, but they were also detected in brain tissue.

Skin contact or inhalation was more likely than ingestion

The contrast between high residues on feathers and lower levels in the digestive organs led the authors to identify contact through the skin or inhalation as the most likely exposure routes, rather than ingestion. They suggest the birds may have been accidentally exposed to a pyrethroid spray in the stable, but the exact product, use and source were not established.

The authors concluded that acute neurotoxicity from direct insecticide exposure was the most probable cause of the deaths. They could not identify the exact source of the pyrethroids, and pre-existing malnutrition could not be completely excluded. The study argues that pesticide risk assessments may underestimate hazards to wildlife when they focus mainly on exposure through food and give less attention to inhalation and skin contact.

Sources and image credit

  1. Saarloos, A. I., Schrama, M. J. J., Barmentlo, S. H., van den Brand, J. M. A., Ennen-Buijs, N. M. T., Sikkema, R. S., Bakker, W., van den Berg, T. J., & van den Brink, N. W. (2026). Acute mortality in sand martins (Riparia riparia) and barn swallows (Hirundo rustica) linked to dermal or inhalation pyrethroid insecticide exposure. Environmental Chemistry and Ecotoxicology, 8, 1779–1786. https://doi.org/10.1016/j.enceco.2026.03.019Primary peer-reviewed article first available online 26 March 2026. Screening for 648 pesticides detected permethrin and tetramethrin in all analysed feather samples; the compounds were absent or relatively low in liver/stomach but detected in brain tissue. The authors concluded acute neurotoxicity following dermal or inhalation exposure was the most probable cause, while noting limitations including no healthy controls, possible pre-existing malnutrition and semi-quantitative brain measurements.

Image: Paul Anderson — Barn Swallow fledglings at a sheltered nest, photographed 11 July 2009; Wikimedia Commons; representative species image, not birds from the Dutch mortality study · CC BY 4.0 · Source

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