Comparative vector competence of post-2015 St. Louis encephalitis virus in Culex tarsalis and Culex quinquefasciatus mosquitoes

The human pathogenic orthoflavivirus St. Louis encephalitis virus (SLEV) reemerged in the western United States in 2015 after more than a decade of apparent absence and has since expanded throughout California with sustained, interannual transmission. This shift from the historically sporadic pattern of SLEV activity prior to 2003 raises the question of whether contemporary SLEV strains differ in fitness in Culex vectors compared with earlier strains. To determine whether reemerging SLEV possess augmented infectivity and transmissibility that may have facilitated reestablishment, we compared the vector competence of five genotype III SLEV strains detected in California between 2016 and 2023 with a historic genotype V strain from 2003. Laboratory colonies of the two primary California vectors, Culex (Cx.) tarsalis and Cx. quinquefasciatus, were orally exposed to bloodmeals containing 3, 5, or 6.7 log10 plaque forming units (PFU)/mL of SLEV, and infection, dissemination, and transmission were assessed 13,14, or 15 days later by quantifying SLEV RNA in individual mosquitoes. Both species exhibited strong dose-dependent responses, with minimal infection at 3 log10 PFU/mL and uniformly high infection, dissemination, and saliva positivity at 6.7 log10 PFU/mL. At 5 log10 PFU/mL, genotype III strains infected Cx. quinquefasciatus more efficiently than the historical 2003 strain, which failed to infect this species. In Cx. tarsalis, fitness differences among SLEV strains were more modest and strain-specific. These findings demonstrate that multiple genotype III SLEV strains exhibit equal or greater vector infectivity in Cx. quinquefasciatus than the 2003 genotype V strain, suggesting that enhanced fitness in this vector may contribute to the persistence and geographic spread of SLEV in California since its reemergence and underscoring the need for continued vector surveillance and targeted control efforts to reduce SLEV transmission to humans. AUTHOR SUMMARYSt. Louis encephalitis virus (SLEV) is a mosquito-borne virus that can cause neurologic disease in humans. After being absent for more than a decade, SLEV reappeared in California in 2015 and has since spread across the state. We wanted to understand whether the newly circulating SLEV strains are better able to infect and be transmitted by local mosquitoes than older strains, which could help explain why the virus has reestablished. In the laboratory, we exposed two common California vector species, Culex (Cx.) tarsalis and Cx. quinquefasciatus, to recent SLEV strains from 2016-2023 and to a historic 2003 strain. We found that the recent strains infected Cx. quinquefasciatus mosquitoes more efficiently than the historic strain, while differences in Cx. tarsalis were smaller and varied by strain. These results suggest that enhanced mosquito infectivity of contemporary SLEV strains may have contributed to the persistence and spread of SLEV in California. Our findings highlight the importance of ongoing mosquito surveillance and targeted control to reduce the risk of SLEV transmission to humans.