Differentiation of West Nile and Usutu Virus Infections by Antibodies Directed to the Non-Structural Protein 1

) comprises several important pathogens that are widespread across the globe, often co-circulating in many regions. In Austria, the closely related mosquito-borne West Nile (WN) and Usutu (USU) viruses have been detected since the early 2000s. Orthoflavivirus-neutralizing antibodies primarily target the major envelope protein E. However, due to their antigenic relationship, recurring contacts with different orthoflaviviruses can lead to the induction of broadly cross-reactive E-specific antibodies. These can pose a problem in the diagnosis and differentiation of orthoflavivirus infections. Therefore, we established immunological assays based on the non-structural protein 1 (NS1) to differentiate infections caused by WN and USU viruses. The NS1 protein is secreted during acute infection, and NS1-specific antibodies have been reported to be less cross-reactive than those against E. Using sera from individuals with a confirmed WN or USU virus infection, it was possible to distinguish between the two virus infections with high accuracy, specifically when IgM and IgG results were combined.