Eilat virus (EILV) causes superinfection exclusion against West NILE virus (WNV) in a strain specific manner in Culex tarsalis mosquitoes.

West Nile virus (WNV) is the main cause of mosquito-borne disease in the United States. In the absence of a human vaccine or WNV-specific antivirals, vector control is the key strategy to reduce WNV prevalence and transmission. The WNV mosquito vector, Culex tarsalis, is a competent host for the insect-specific virus Eilat virus (EILV). EILV and WNV potentially interact within the mosquito host, and EILV can be used as a safe tool to target WNV in mosquitoes. Here, we characterize the ability of EILV to cause superinfection exclusion (SIE) against two strains of WNV-WN02-1956 and NY99-in C6/36 cells and Cx. tarsalis mosquitoes. EILV suppressed both superinfecting WNV strains in C6/36 cells. However, in mosquitoes, EILV enhanced NY99 whole-body titers at 3 days post superinfection and suppressed WN02-1956 whole-body titers at 7 days post superinfection. Vector competence measures, including infection, dissemination, and transmission rates and transmission efficacy, as well as leg and saliva titers of both superinfecting WNV strains, were not affected by EILV at both timepoints. Our data show the importance of not only validating SIE in mosquito vectors but also testing multiple strains of viruses to determine the safety of this strategy as a control tool.