Decade-long persistence of high levels of pyrethroid resistance in Triatoma infestans populations of the Argentine Chaco.

The emergence of insecticide resistance is one of the major challenges vector control programmes face worldwide. For Chagas disease and its main vector Triatoma infestans, pyrethroid-resistant foci have severely compromised vector control and elimination efforts across sections of northern Argentina and Bolivia. We assessed the status and spatiotemporal distribution of pyrethroid resistance in T. infestans populations in Castelli (Argentine Chaco), where it had reached record levels, and tested whether the village-level prevalence of house infestation was associated with pyrethroid resistance. We also monitored the infestation and pyrethroid resistance status of a reinfested (index) house in an adjacent municipality (Pampa del Indio) under sustained triatomine surveillance and control. Eighty-three georeferenced populations of T. infestans collected in 13 villages over 2018-2024 were tested for susceptibility to deltamethrin by discriminant-dose assays. Widespread pyrethroid resistance occurred in 12 villages; 45% of the tested bug populations had very low mortality (<45%). Pyrethroid resistance levels were spatially heterogeneous between and within villages, were negatively associated with house infestation at the village level and reached record resistance ratios (RR₅₀ > 2400). Pampa del Indio's index house remained persistently infested with high resistance levels despite pyrethroid applications and other interventions (i.e., repeated control failures) over a 7-year period. Castelli's highly resistant focus has persisted for >10 year despite the near absence of government-sponsored pyrethroid applications. Alternative insecticides effective against pyrethroid-resistant T. infestans populations are urgently needed to suppress them and curtail their propagation. Sustainable triatomine control in this high-risk scenario requires an integrated vector management framework including housing and peridomestic modifications combined with community-based triatomine surveillance.