Usutu virus (USUV) and West Nile virus (WNV) are closely related pathogens circulating between mosquitoes and birds, but also infecting mammals as dead-end hosts. Both viruses share the same susceptible hosts, vectors, and even distribution areas in Central Europe. The aim of the study was, therefore, to understand their amplification potential and interference upon a successive
infection. Two-week old geese were initially infected with an USUV isolate from Germany and with a German WNV isolate17 days later. The geese were susceptible to the USUV and the
WNV infections, as evidenced by specific flavivirus
antibodies in all of the birds. Furthermore, in half of the USUV-inoculated geese, USUV genomes were detected in the blood and swab samples 2-4 days post-
infection. Additionally, most of the examined organs contained USUV genomes and showed signs of
encephalitis and ganglioneuritis. Interestingly, upon a sequential
infection with WNV, the genome copy numbers in all of the examined samples were significantly lower and less frequent than after a WNV mono-
infection. Similarly, the histopathological lesions were less severe. Therefore, it can be concluded that a previous USUV
infection can protect birds from clinical disease in a subsequent
WNV infection.