Submitted by the MVCAC VVBD Committee
M. Arturo Flores Rodriguez, et al. 2026. Journal of Virology 100, 10 https://doi.org/10.1128/jvi.00952-26
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; 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.