Comparative fitness of reemerging St. Louis encephalitis virus in vertebrate and mosquito cells, Culex tarsalis and Culex quinquefasciatus mosquitoes, and mice

ABSTRACT The human pathogenic orthoflavivirus St. Louis encephalitis virus (SLEV) reemerged in the western United States in 2015 after more than a decade of absence and has since expanded throughout California with sustained interannual transmission. This shift from the historically sporadic pattern of SLEV activity before 2003 raises the question of whether contemporary strains differ in fitness from earlier strains. To assess whether reemerging SLEV possesses enhanced infectivity or transmissibility, we compared five contemporary genotype III strains from California (2016–2023) with a historical genotype V strain from 2003. Growth kinetics were evaluated in mammalian, duck, and mosquito cells; vector competence was assessed in laboratory colonies of Culex tarsalis and Culex quinquefasciatus vectors; and SLEV RNA levels in blood were measured in collaborative cross recombinant intercross mice. Some genotype III strains produced higher titers than the historical genotype V strain in avian, but not mammalian or mosquito cells. Several genotype III strains infected and transmitted SLEV RNA more efficiently than the historical strain in both mosquito species, although no temporal trend in fitness was observed. SLEV fitness was comparable or greater in Culex quinquefasciatus than in Culex tarsalis . Sequencing identified no shared amino acid substitutions associated with vector infection phenotypes. Although genotype III strains exhibited a delayed peak relative to the historical strain, murine SLEV RNA levels in blood were comparable across strains. These findings show some contemporary strains exhibit equal or greater fitness than the historical strain, which may contribute to SLEV persistence and spread in California, underscoring the need for continued surveillance and targeted vector control. IMPORTANCE St. Louis encephalitis virus (SLEV) reemerged in California in 2015 after more than a decade of absence and has since established sustained transmission and expanded geographically. The factors underlying this reemergence remain poorly understood. By comparing contemporary genotype III SLEV strains with a historical genotype V strain, we found that several contemporary strains exhibit equal or greater fitness compared to the historical strain in avian cells and are transmitted more efficiently by the two principal California vector species, Culex tarsalis and Culex quinquefasciatus . We also demonstrate that Cx. quinquefasciatus can transmit infectious genotype III SLEV, supporting its role in SLEV maintenance and spread. Despite differences in mosquito infection and transmission, we found no evidence that fitness in mosquito vectors or mice has continued to rise among strains detected more recently, suggesting that enhanced transmission is not driven by ongoing directional adaptation in these models. These findings indicate that contemporary genotype III SLEV strains possess transmission competence in mosquito vectors that may have contributed to successful reestablishment and persistence of SLEV in California. Improved understanding of the characteristics of reemerging SLEV strains can inform surveillance, risk assessment, and vector control efforts aimed at reducing human exposure to prevent disease caused by SLEV.