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Outbreak of West Nile virus infection in humans and horses, Croatia, 2012

During the last two decades, several West Nile virus (WNV) outbreaks have occurred throughout the world. In Croatia, clinical cases of WNV infection have not been documented until now, although cases have occurred in the neighboring countries(Italy, Hungary, Romania and Greece). Moreover, in July 2012, the first outbreak of WNV disease occurred in Serbia with 44 laboratory diagnosed cases and six deaths. We are reporting the first outbreak of WNV neuroinvasive infection in humans and evidence of acute WNV infection in horses in Croatia (August-September 2012). WNV IgM/IgG testing and IgG avidity were performed using commercial ELISA and confirmed by plaque-reduction neutralization test (PRNT) and micro-neutralization test (micro-NT). Samples were also tested for other flaviviruses (tick-borne encephalitis virus-TBEV, dengue virus-DENV and Usutu virus-USUV) to rule out cross-reactions. Seven human cases of WNV neuroinvasive disease were laboratory confirmed between 06 and 21 September 2012 in Croatia. The median patient age was 62 years (range 48-77 years). Five patients presented with meningoencephalitis and two patients with meningoencephalitis followed by acute flaccid paralysis. Most of them had underlying diseases, the most common being hypertension. In six patients, WNV-specific IgM antibodies with low IgG avidity antibodies were detected while one patient showed only IgM antibodies. Crossreactivity was seen mainly for DENV IgG. WNV infection was confirmed by PRNT and micro-NT in all samples. Patients with WNV neuroinvasive disease are residents of three counties located in eastern part of Croatia, near the Serbian state border. The first case was imported from Serbia while the others represent the first autochthonous WNV cases. Five patients fully recovered. Before human cases were reported, acute WNV infection had been demonstrated by detection of IgM antibodies in twelve asymptomatic sentinel horses in counties where human cases occurred. In September 2012, WNV neuroinvasive disease in humans was documented for the first time in Croatia. Asymptomatic acute infection in sentinel horses had been reported in the same geographic region before human cases occurred. Adulticidal and larvicidal treatments were administered immediately in the respective place of residence. Epidemic control measures prevented further spreading of disease.

West Nile Virus; outbreak; human; horses; Croatia

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