Shared viral burdens: Evidence of active Usutu virus circulation and multi-arbovirus exposure in migrant and resident birds at wintering locations in Nigeria

BackgroundWest Nile (WNV), Usutu (USUV), and Sindbis (SINV) virus were initially detected in the African region, and subsequently across temperate regions where they were absent. Wild birds are primary reservoirs for these arboviruses and are considered major contributors to their global spread through seasonal migration. To understand the transmission dynamics of arboviruses in wild birds and the potential of migratory birds to spread the viruses at an intercontinental scale, we investigated arboviral infections and exposures in African resident and Palearctic migratory birds at wintering locations in Nigeria. Methodology/Principal FindingsOropharyngeal- and cloacal swabs, feathers and blood were collected from resident and migratory birds at two wintering locations (Amurum and Ngel-Nyaki Forest Reserves). Swabs and feathers were tested using RT-PCR for WNV, USUV and SINV, and blood with ELISA and FRNT90 or PRNT80 for antibodies. 573 birds were sampled between 2021 to 2024 across months coinciding with arrival and departure of migratory birds. USUV RNA was detected in 2.6% of feathers including a positive Icterine warbler and a garden warbler sampled prior to spring migration. None of the swabs was positive for viral RNA but neutralizing antibodies to WNV and USUV were detected in 4.5% of birds. SINV antibodies were also found in 34.1% of birds sampled across the wintering locations. Conclusions/SignificanceOur findings showed that migratory birds can become infected with USUV, and potentially with WNV and SINV during their overwintering periods in Africa and highlighted a wider arbovirus risk in Nigeria. In addition, detections of viral RNA in feathers, but not swabs, suggest feathers may be a suitable matrix for surveillance in the absence of a reliable cold chain. The overall detections in wild birds at these locations highlight the need for further surveillance to define the epidemiology and public health risks of these arboviruses in the region. SynopsisVector-borne viruses like West Nile, Usutu, and Sindbis, were once limited to parts of Africa, but now appear in Europe and other temperate regions. Migratory birds are believed to help spread these viruses across continents. To understand the role of birds in this, we tested resident and migratory birds in Nigeria for evidence of infection. Between 2021 and 2024, we sampled over 570 birds during their winter stay. We found genetic material of Usutu virus in feathers of six birds including two migratory birds just before their return to Europe. While swab tests did not detect active infections, antibodies to these viruses, which indicate past exposure, were found in several birds, especially for Sindbis virus. Our findings show that migratory birds can pick up and possibly carry these viruses across continents. We also found that feathers may be a practical tool for virus detection in areas without reliable refrigeration, supporting easier and broader surveillance of these viruses in Africa and possibly beyond.