# Transmission dynamics and fitness of reemerging St. Louis encephalitis virus

> **NIH NIH R21** · UNIVERSITY OF CALIFORNIA AT DAVIS · 2023 · $240,000

## Abstract

St. Louis encephalitis virus (SLEV) is a mosquito borne pathogen that causes febrile illness and sometimes
fatal encephalitis and was a leading cause of infectious encephalitis in the US during the 1970s. After an 11-
year absence in activity in California, SLEV reemerged and reestablished in CA from 2015-2021, and caused
an outbreak in nearby Phoenix, AZ. SLEV reemergence is marked by increasing human cases and positive
mosquito pools across more counties each year, including areas with concurrent activity of West Nile virus
(WNV), a related flavivirus that invaded CA in 2003 and which shares avian reservoirs and Culex mosquito
vectors with SLEV. Genetic tracing by our team showed that reemerging (‘contemporary’) SLEV in the Western
US is genetically distinct from pre-2003 (‘historical’) SLEV and that it likely originated in South America.
However, other than these studies, drivers of SLEV reemergence, including into WNV-endemic areas, have
not been examined. Spread of WNV across the US was facilitated by augmented avian reservoir infection and
vector competence. We propose that similar fitness gains may have enabled SLEV reemergence. The
objective of this project is to assess the extent by which SLEV reemergence is promoted by augmented
infectivity and transmissibility in mosquito and avian cells and mosquito vectors. Higher SLEV activity in
different regions may be explained by differential vector competence for 3 primary Culex vectors (pipiens,
tarsalis, and quinquefasciatus) that favor different environments across the state. We hypothesize that SLEV
reemergence in the Western US since 2015 was mediated by augmented fitness of the introduced genotype in
cells and vectors and that geographic localization is influenced by relative vector competence of 3 primary
Culex species. Viral fitness can be assessed experimentally by comparing SLEV strains using vector
competence and competition assays. The project hypotheses will be tested in 2 project Aims: 1) Determine
transmission competence of contemporary versus historical SLEV in Culex vectors, and 2) Compare relative
fitness of contemporary versus historical SLEV in avian and mosquito cells and vectors. This project is
significant in that it will provide a unique opportunity to compare viral factors involved in the sequential invasion
and spread of 2 Culex-borne flaviviruses, where understanding co-circulation dynamics can be applied to other
sympatric flaviviruses. By defining the role Culex vector species play in SLEV transmission this project will
determine whether augmented infectivity and transmissibility is a factor enhancing reemergence and spread.
To reduce disease, vector control districts can allocate resources towards spatial targeting of the Culex vector
species that exhibit high SLEV competence.

## Key facts

- **NIH application ID:** 10645668
- **Project number:** 1R21AI176187-01
- **Recipient organization:** UNIVERSITY OF CALIFORNIA AT DAVIS
- **Principal Investigator:** Lark L Coffey
- **Activity code:** R21 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2023
- **Award amount:** $240,000
- **Award type:** 1
- **Project period:** 2023-07-03 → 2025-06-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10645668

## Citation

> US National Institutes of Health, RePORTER application 10645668, Transmission dynamics and fitness of reemerging St. Louis encephalitis virus (1R21AI176187-01). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/10645668. Licensed CC0.

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