Population-level variation shapes West Nile virus transmission potential in the Culex pipiens complex mosquitoes.

West Nile virus (WNV; formally Orthoflavivirus nilense) is a globally distributed arbovirus maintained mainly by mosquitoes of the genus Culex. Since its introduction in Argentina in late 2004, efforts have focused on identifying which species of this genus are involved in its transmission. This study evaluated vector competence and transmission dynamics of Culex pipiens and Culex quinquefasciatus populations from central Argentina. Mosquitoes were fed using an infected bloodmeal containing increasing viral doses (from 2 log10 to 7.3 log10 PFU/mL). Infection, dissemination, and transmission were determined in Vero cells through viral plaque detection in the abdomen, legs, and saliva samples. Dose-response relationships were quantified using mixed modelling approaches incorporating population-level variation, and temporal dynamics were evaluated to estimate the extrinsic incubation period. Both species exhibited parallel dose-response patterns, indicating comparable susceptibility and transmission potential. Infection probability increased progressively with viral dose, with a median infectious dose (ID₅₀) estimated at 4.63 log₁₀ PFU/mL. Dissemination required higher viral exposure (DD₅₀ = 6.45 log₁₀ PFU/mL), while transmission showed the highest threshold (TD₅₀ = 8.16 log₁₀ PFU/mL), suggesting the presence of sequential biological barriers during the infection process. Population-level variability was most pronounced during the infection stage (14%) but became minimal once viral dissemination was established. Temporal analyses revealed that transmission began around 8 days post-infection at 28°C, with an estimated median extrinsic incubation period of 11 days. These results demonstrate that Cx. pipiens and Cx. quinquefasciatus populations exhibit comparable vector competence for WNV in Argentina and highlight infection-stage variability as a major source of heterogeneity among populations. The identified dose thresholds and incubation dynamics indicate that both species could contribute equally to local transmission cycles, providing key biological parameters for understanding WNV persistence near its southern distribution limit.