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A Newly Emergent Genotype of West Nile Virus Is Transmitted Earlier and More Efficiently by Culex Mosquitoes

Studies examining the evolution of West Nile virus since its introduction into North America have identified the emergence of a new dominant genotype (WN02) that has displaced the introduced genotype (NY99). The mechanistic basis for this displacement, however, remains obscure. Although we found no detectable difference in vitro between the genotypes in either replication or fitness, there were significant differences in vivo in Culex mosquitoes. After peroral infection, the extrinsic incubation period (EIP) of the WN02 genotype was up to 4 days shorter than the EIP of the NY99 genotype; however, after intrathoracic inoculation, there was no difference in EIP between the genotypes, suggesting that differences in genotype interaction with the mosquito midgut are likely to play a role in this phenotype. These results suggest a model for the displacement of the NY99 genotype, where earlier transmission of WN02 viruses leads to higher WN02 infection rates in avian reservoir hosts.

Received February 27, 2007. Accepted for publication April 27, 2007.

Acknowledgments: The authors acknowledge the excellent technical assistance provided by Amy Lovelace and Pamela Chin. Kristen Bernard provided helpful advice and discussions. The authors thank the Wadsworth Center Tissue Culture Core facility for providing Vero, C6/36, and DF-1 cells.

Financial support: This project was funded in part with federal funds from the Centers for Disease Control and Prevention Grant 1RO1AI069217-01. RM is funded by National Institute of Allergy and Infectious Disease, National Institutes of Health Grant T32-AI055429-03.

^* Address correspondence to Robin M. Moudy, Arbovirus Laboratories, Wadsworth Center, New York State Department of Health, 5668 State Farm Road, Slingerlands, NY 12159. E-mail: rmoudy{at}wadsworth.org

Authors’ addresses: Robin M. Moudy, Mark A. Meola, Laura-Lee L. Morin, and Laura D. Kramer, Arbovirus Laboratories, Wadsworth Center, New York State Department of Health, 5668 State Farm Road, Slingerlands, NY 12159, Telephone: 518-862-4330, Fax: 518-869-4530, E-mails: rmoudy{at}wadsworth.org, mmeola{at}wadsworth.org, llm09{at}health.state.ny.us, and lkramer{at}wadsworth.org. Gregory D. Ebel, University of New Mexico, MSC08 4640, 1 University of New Mexico, Albuquerque, NM 87131, Telephone: 505-272-4814, Fax: 505-272-8084, E-mail: gebel{at}salud.unm.edu.