Emergent arboviruses in Istria County, 2012-2014

Objectives: Several arboviruses have emerged in Croatia in the last few years such as dengue virus (DENV), West Nile virus (WNV) and Usutu virus (USUV). In addition, antibodies to DENV and chikungunya virus (CHIKV) were sporadically reported in travelers to endemic areas. We analyzed seroprevalence of the emergent arboviruses in humans and animals in Istria County during a three-year period (2012-2014). Methods: Human serum samples were tested for DENV, WNV, USUV and CHIKV IgG antibodies. Horse and chicken serum samples were tested for WNV IgG antibodies. All tested chickens were less than one year old. Serologic tests were performed using commercial enzyme-linked immunosorbent assays. Except in chickens, all IgG reactive samples were further tested for the presence of IgM antibodies for confirmation of acute/recent infection. Results: In 2012, 2/136 (1.4%) human samples from inhabitants of Pula and Labin tested positive for CHIKV IgG antibodies while IgM antibodies were not detected. Antibodies to WNV, DENV and USUV were not found in any tested sample. Among horses, WNV IgG seropositivity was 1.8% (5/276) in 2012, 1.7% (5/289) in 2013 and 2.3% (2/87) in 2014. Acute infections were not detected during the tested period except one documented seroconversion during 2014. Chicken seropositivity to WNV was 33.3% (3/9) in 2013 and 90% (18/20) in 2014. Conclusions: Our results indicate circulation of WNV among horses and chickens in Istria County during three consecutive transmission seasons. Although USUV infections were not detected in Istria County, emergence of human USUV neuroinvasive disease in Croatia highlighted the importance of flaviviruses surveillance. In addition, current data from Europe and Croatia indicate that indigenous transmission of DENV and CHIKV is possible. Permanent vector control measures should be regularly performed, particularly in regions with established Ae. albopictus population.