Usutu Virus Antibodies in Blood Donors and Healthy Forestry Workers in the Lombardy Region, Northern Italy

Usutu virus (USUV), a member of the genus Flavivirus, is known to circulate at low prevalence in Northern Italy, and has been reported to cause overt infection. USUV was first reported in Europe in 2001, but a retrospective study showed that it has been present in Italy at least since 1996. Seroprevalence data for USUV antibodies in sera are being collected in different European countries, showing circulation at low prevalence in human populations. Interestingly, two consecutive studies in Northern Italy indicate a possible increase in the presence of the virus, from 0% to 0.23% seroprevalence in blood donors. In this study, antibodies against USUV were measured in 3 consecutive blood samples collected from October 2014 to December 2015 from 33 forestry workers in the Po river valley, while samples from 200 blood donors from the same geographical area were tested in parallel. Neutralizing and IgG antibodies were found in six forestry workers (18.1%) and in two blood donors (1%). Our results indicate that USUV circulation in the examined area, part of a highly populated region in Northern Italy, is higher than expected. Healthy subjects exhibit a higher prevalence than what was found in a previous report in an adjoining region (0.23%), while the population at risk shows a much higher prevalence value (18.1%).