Usutu Virus: First Detection in France in Blackbirds Found Dead

Mosquito-borne viruses are becoming a growing threat in Western Europe, and France is no exception. Closely related to the West Nile virus currently circulating in the Camargue, the Usutu virus has just been isolated for the first time on French soil, in blackbirds found dead in the Haut-Rhin department. An initial alert occurred in 2009–2010 among magpies during a serological survey on the circulation of flaviviruses in southern France*.   An abnormal mortality rate among blackbirds in the Haut-Rhin in early August 2015 alerted the Sagir network. About twenty of these birds were found dead in the same municipality over the course of about ten days. Two of them were then subjected to histopathology at the departmental diagnostic laboratory. They exhibited hepatomegaly and splenomegaly, consistent with a viral infection. Since similar mortality events have been reported among the avifauna in neighboring countries in recent years—particularly in Germany**—samples were sent to the laboratory of the French Agency for Food, Environmental and Occupational Health & Safety (ANSES). RT-PCR analyses subsequently detected the presence of a flavivirus in two blackbirds. Sequencing confirmed that it was indeed the Usutu virus. First isolated in 1959 in South Africa, the Usutu virus is an arbovirus of the genus Flavivirus (family Flaviviridae ). It circulates cyclically between mosquitoes (vectors) and wild birds (natural reservoirs), infecting mammals (humans, horses) only accidentally, via a mosquito bite . Genetically and antigenically similar to the West Nile virus—which, for its part, decimated thousands of birds in North America after its introduction in 1999— the Usutu virus appears to be much less pathogenic for mammals, and particularly for humans. Only two cases of neurological infection have been reported in immunocompromised patients in Italy in 2009. In most cases, the risk of transmission to humans is negligible and, when it does occur, causes only fever, headaches, and, rarely, meningitis. For nearly 40 years, this virus was identified only on the African continent. But in 1996, a strain emerged in Italy, subsequently spreading to Austria in 2001, where it caused massive mortality among certain species of passerines in Vienna, primarily the blackbird. The virus then spread rapidly to other European countries: it was detected in dead birds or mosquitoes in Hungary, Switzerland, Spain, Germany, the Czech Republic, Belgium, and the United Kingdom. In the avifauna, the Usutu virus is associated with episodes of mortality—either isolated or in clusters—primarily among passerines (blackbirds, titmice, house sparrows, robins, and European starlings) and owls. Central nervous system disorders have also been reported, but these symptoms are not very specific: weakness, prostration, disorientation, motor incoordination, and weight loss. The organs are generally congested, with foci of necrosis, gliosis, and inflammatory infiltrates consisting mainly of lymphoid cells and histiocytes. Macroscopically, Usutu virus infections cause splenomegaly and hepatomegaly. To better understand the spread of this virus in France, the Sagir network requests that all cases of abnormal mortality among passerines and nocturnal raptors observed in the field be reported to it . Laboratories should now include Usutu virus infections in the differential diagnosis of neurological disorders in susceptible species.   * M. Vittecoq et al.: Recent circulation of West Nile Virus and potentially other closely related flaviviruses in Southern France, Vector-Borne and Zoonotic Diseases, 2013, No. 8, doi: 10.1089/vbz.2012.1166.** U. Ziegler et al.: Epidemic spread of Usutu virus in southwestern Germany from 2011 to 2013 and monitoring of wild birds for Usutu and West Nile viruses, Vector-Borne and Zoonotic Diseases, 2015, vol. 15, no. 8.