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Serosurveillance of the emergent flaviviruses of the Japanese encephalitis virus serogroup in equine in Croatia

Flaviviruses comprise more than 70 different viruses, many of which are arthropod-borne and transmitted by either mosquitoes or ticks. Taxonomically, they form a genus in the family Flaviviridae, which in addition includes genera Hepacivirus and Pestivirus. According to vectors, the members of genus Flaviviruses are assigned to three groups: tick-borne viruses, mosquito-borne viruses and viruses with no known arthropod vector. Several human pathogenic mosquito-borne Flaviviruses (including Yellow fever, Dengue, Japanese encephalitis, West Nile and Tick-borne encephalitis viruses) have a significant public health impact in different parts of the world and the potential of emerging in previously non-endemic regions. The International Committee on Taxonomy of Viruses (ICTV) assigns mosquito-borne Flaviviruses to seven groups, based on genetic and antigenic relationships. During the last two decades the emergence of mosquito-borne Flaviviruses was reported in many European countries. Among them, the most important is West Nile virus, the member of the Japanese encephalitis virus serogroup. Of other Flaviviruses, sporadically imported Dengue virus infections were reported in many European countries, but the spreading of the main vector (Aedes albopictus) in some parts of Europe led to first recorded autochthonous cases. Autochthonous cases of Dengue virus infections were reported in some Mediterranean countries. In Croatia, first autochthonous cases of Dengue fever were recorded on Pelješac in 2010. During the same year, a high seroprevalence was recorded in neighbors of the autochthonous cases. West Nile virus, a member of the Japanese encephalitis virus serogroup, has been known for a long time in Southern Europe, causing sporadic outbreaks usually involving localized regions, with few clinical (human and/or animal) cases, separated by long periods of epidemiological silence. Sporadic outbreaks were explained by the pathogen being introduced by migratory birds. During the last two decades, changes in the epidemiology of West Nile in Europe were observed. The outbreaks with a large number of clinical cases in humans and animals were reported with a short period of epidemiological silence and during the last five years the presence of virus in consecutive years in same countries and region implicates the endemisation of the disease in Europe. Finally, the endemisation was confirmed with the evidence of virus overwintering in mosquitoes and spreading in autochthonous bird populations. The highest viral activity level was reported during the transmission season 2012 with hundreds of infected people in southern Europe. The wide spread of WNV and clinical cases with fatal end present this virus as an emerging zoonotic pathogen whose impact on animal and public health is considerable. In accordance, many European countries have implemented different surveillance systems of WNV. Opposite to the situation in North America, massive bird mortality was not reported in Europe so passive surveillance system is based on horses and mosquitoes. Also, mosquitoes’ infection rate is much lower than in North America, which shows that active horse surveillance is a sufficient model for the detection of WNV activity in Europe. In Croatia, active horse surveillance started in 2010 and WNV activity was confirmed in at least eight counties in the continental Croatia and the westernmost Istarska county. In the following year, 2011, the results of the active surveillance confirmed again the viral activity in the continental Croatia and Istra. A general increase in seroprevalence was reported and the highest rate was again observed in the east part of the continental Croatia. The exception was the decrease of seroprevalence, which was observed in Istra county. During the last transmission season, 2012, the significant increase of seroprevalence was observed in Eastern Slavonia counties and in July in the same counties the acute infection of horses was confirmed. The significant increase in seroprevalence and the acute infection implicate an increase of viral activity in this region and a high risk for public health. Unfortunately, it was confirmed when first clinical cases of neuroinvasive WNV infections in humans were reported during August and September 2012 in the same region. Presented data of human clinical cases in the region where seroprevalence in horses is significantly increased and reported human clinical cases two to three weeks after the confirmation of acute infection in horses prove that the WNV active serosurveillance in horses is an important and useful early warning system for public health in Croatia. Furthermore, during performing of the WNV active serosurveillance in horses, the circulation of Usutu virus (USUV), another emergent Flavivirus, a member of the Japanese encephalitis virus serogroup, was confirmed for the first time in Croatia. This virus is also a potential zoonotic pathogen which caused infections in two humans in Italy with fatal end. Despite very few information about epidemiology and pathogenesis of USUV infection in humans, active serosurveillance in horses again presents an important control measure for this new potential threat for public health in Croatia and Europe. To conclude, emergent mosquito-borne Flaviviruses are today endemic in Europe and with every season they spread to new regions. The control of these emergent zoonotic pathogens and preparing for the new challenges is complex, but the collaboration of experts in human and veterinary medicine could prevent further outbreaks in accordance with the well known One Health initiative.

Flaviviridae; West Nile Virus; USUTU; Surveillance; Horses; Public Health

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