North American Culex pipiens mosquitoes are competent for Usutu virus transmission

Usutu virus (USUV, Flaviviridae) is a mosquito-borne, neuroinvasive flavivirus that is maintained in a transmission cycle between Culex spp. mosquitoes and passerine birds. USUV shares close ecological and antigenic similarity to West Nile virus (WNV). Our study focused on North American Cx. tarsalis and Cx. pipiens, both important in WNV transmission. At day 10, we found that Cx. pipiens were competent for USUV while Cx. tarsalis had limited evidence for transmission. Additionally, Cx. pipiens transmission significantly increased by day 21 to 33%. We found the minimum threshold for infection of Cx. pipiens to be 5 log₁₀ PFU/mL when using an artificial bloodmeal and 6 log₁₀ PFU/mL when using a live avian host. These results provide support that North American Cx. pipiens are competent vectors for USUV but require a high USUV dose for infection. Together, these data provide insight into the potential efficiency of North American Cx. pipiens for USUV transmission.