Researchers are taking stock of the emergence of Usutu virus in Europe, a mosquito-borne virus that circulates between birds. It can attack the nervous system of certain birds, such as blackbirds, and cause significant mortality.
The virus belongs to the genus Flavivirus, which includes dengue, yellow fever, Zika, Japanese encephalitis and West Nile fever.
The virus was suspected in a patient hospitalized in Montpellier, France in 2016, a diagnosis confirmed and published in May 2018 in the journal Emerging Infectious Diseases . Two strains of the virus have been detected in mosquitoes in Camargue, one of which is assumed to have caused the human case in 2016 .
Usutu virus is largely unknown compared to other viruses with a similar transmission cycle. Researchers from INSERM, CIRAD, ANSES, the University of Montpellier and the Montpellier teaching hospital are collaborating to better understand Usutu virus transmission. Is it endemic to the Mediterranean or merely brought by infected migrating birds coming from the tropics? What is the potential for transmission by Culex mosquitoes in France?
“We came across Usutu virus when we were looking for viruses associated with mosquitoes in the Mediterranean, and decided to take a closer look,” says Serafin Gutierrez, a virology researcher with CIRAD’S ASTRE research unit.
In Camargue, researchers from CIRAD detected it in mosquitoes of the species Culex pipiens, which also carry other arboviruses. “We showed that the virus was circulating actively within this mosquito species in Camargue in 2015, since we repeatedly observed it at several sites between June and September”, Serafin Gutierrez adds. Two strains of the virus have been detected, one of which is assumed to have been responsible for the human case in Montpellier and the other in bird deaths in Europe in 2016  .
Just 28 cases of human infection have been reported in Europe in the past decade, none of them fatal, although some have caused serious neurological damage. But this emerging zoonotic arbovirus of concern must be monitored, given recent outbreak experiences with other arboviruses such as Zika and Chikungunya.
 Human Usutu virus infection with atypical neurologic presentation, Montpellier, France, 2016, May 2018
 Emergence of two Usutu virus lineages in Culex pipiens mosquitoes in the Camargue, France, 2015, July 2018
Source: CIRAD, adapted