Chronic exposure to a common azole fungicide is associated with reactivation of chronic Plasmodium infections in farmland birds

Over the past decades, the widespread use of pesticides has raised concerns about their effects on non-target organisms. Among pesticides, fungicides remain comparatively understudied, despite representing nearly 40% of total pesticide sales in Europe. Wild birds living in or close to farmlands are frequently exposed to tebuconazole, a triazole fungicide widely used in agroecosystems. While its effects on avian physiology and life-history traits have been the subject of several studies, the downstream effects that this agrochemical may exert on host–parasite interactions remain poorly understood. Here, we investigated whether chronic exposure to tebuconazole alters the dynamics of avian malaria chronic infection in house sparrows ( Passer domesticus ). Wild-caught adults were maintained under semi-natural conditions and malaria infection was monitored before and after an eleven-week exposure period to tebuconazole ingestion, starting in late autumn and ending in mid-winter. At the beginning of the experiment, 70% of the birds were chronically infected with malaria. Then, while prevalence declined in control individuals during winter, as typically expected in passerines living in temperate climates, it increased by 20% in tebuconazole-exposed individuals. No significant difference in parasite load was observed between control and exposed birds, suggesting that tebuconazole did enhance parasite replication rate but may instead have promoted the reappearance of parasites from exo-erythrocytic stages. Our findings highlight that exposure to tebuconazole can influence chronic malaria dynamics. Future work should assess how combined exposure to multiple pesticides and environmental stressors might exacerbate these effects, with broader implications for host–parasite interactions in agricultural landscapes. • Chronic exposure to tebuconazole altered avian malaria infection dynamics in birds. • Malaria prevalence declined in controls but rose by 20% after fungicide ingestion. • Parasite load was unchanged, suggesting reactivation of chronic infections • Tebuconazole can affect host–parasite interactions in agroecosystems.