Zoonoses: West Nile and Usutu viruses are circulating among bird populations

For the first time, neutralizing antibodies against two zoonotic flaviviruses—West Nile and Usutu—have been detected in passerines in western Spain. The discovery that these viruses are circulating among local and migratory bird species has implications for both animal health and public health. Future outbreaks among avian reservoir hosts may indeed occur, and passerines must now be considered priority target species in flavivirus surveillance programs.   West Nile and Usutu are emerging vector-borne zoonotic arboviruses with expanding incidence and distribution. The West Nile virus is regularly isolated in both humans and equines, generally in southeastern Europe, in an area stretching from the Camargue to Central Europe. Italy is the most affected country in Western Europe. For its part, the Usutu virus is endemic in several European regions. Surveillance conducted by the European Centre for Disease Prevention and Control (ECDC) includes animals, as they are not only considered reservoirs of these viruses but also serve as sentinels for the circulation of flaviviruses during the epidemic season.   The fact that 82 wild birds, representing 8 different species (12.7% of the sample), were exposed highlights the urgent need to include passerines in routine surveillance programs for zoonotic diseases. Indeed, the West Nile and Usutu viruses are known to require mosquito-to-bird transmission in their cycle of spread, with humans and mammals serving only as opportunistic hosts. So why is this new publication significant? It is the first study on flavivirus exposure in wild birds living freely in their natural habitats, confirming the role of passerines as reservoirs for the West Nile virus and Usutu virus and as potential amplifying hosts for the infection. Furthermore, these birds come into contact with humans in both agricultural and urban settings, which increases the zoonotic risk and the risk of epidemics. In 2020, another study had already concluded that the number of asymptomatic carriers within avian populations was underestimated.   Regarding the results, of the 645 samples collected from birds captured in a single region, 75 tested positive via an ELISA test (11.6%) and 51 were inconclusive (7.9%). Only 104 samples (71 positive and 33 questionable) could be analyzed using a viral seroneutralization test. In total, 19.23% of the birds that tested positive had antibodies specific to the West Nile virus, 0.96% to the Usutu virus, and 58.65% to an unidentified flavivirus. The 82 positive birds were confirmed to have been infected with flaviviruses. The seroprevalence is therefore 12.7%, similar to the 10% reported in poultry and wild birds in northeastern Spain in 2021.   The next step is to analyze the viral strains implicated in these passerines to determine their origin, potential pathogenicity, and risk to human health. Hence the importance of developing systematic wildlife surveillance strategies, integrated into comprehensive data analysis systems using a “One Health” approach.   The West Nile virus, which originated in East Africa, is now endemic around the Mediterranean, in Central Europe, and in North America. Every year, it causes human deaths. It is transmitted through the bite of the Culex mosquito: “After biting infected birds, female mosquitoes multiply the virus and then transmit it to humans during a blood meal. ” In horses, it can cause encephalitis. It is currently under constant surveillance by the Equine Pathology Epidemiological Surveillance Network (Respe). It is a Category 1 disease, meaning it is a notifiable disease and is managed by the government. The Usutu virus also originates in Africa, and it is scientifically recognized that its primary hosts are passerines. In fact, cases of infected domestic animals (including chickens) are extremely rare. The virus has been the focus of intense attention since its impact on wildlife shifted from a few isolated fatalities to mass die-offs of bird populations beginning in the 2000s. Infection in humans represents an epidemiological dead end but requires monitoring in immunocompromised patients (due to the risk of meningitis or meningoencephalitis). In France, the epidemiological turning point was reached in 2018.