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West Nile Virus – an upcoming threat from the South of Europe

West Nile Virus (WNV) is an emergent pathogen of different species including horses and humans as incidental hosts. Horses are particularly susceptible to West Nile virus (WNV) infection showing clinical signs of acute encephalomyelitis, such as ataxia, rear limb paresis, muscle tremors and fasciculation, and recumbency. Although 80% of affected horses recovers in 3–4 weeks, a small proportion may suffer from persistent neurological deficiency. The disease can also follow a fatal course and the mortality rate in recumbent horses can reach up to 60%. In the past few years WNV became endemic in Southern European countries from Greece to Spain(1, 2). In Croatia, between 2010 and 2012, seroprevalence of WNV infection in horses, on a state level, was 3.43%. Seroprevalence was declining from the eastern to the western part of the country during the years 2010 and 2011, which clearly illustrates the rapid spread(3). During 2012, 1472 equine serum samples were collected from randomly selected healthy horses with no signs of clinical disease reported in the last 6 months. Animals were from six Croatian counties with the highest seroprevalence of WNV infection in 2010 and 2011. All samples were evaluated for IgG and IgM antibodies to West Nile virus (WNV) by commercial ELISA tests (West Nile Competition Screening Test ; ID Screen® West Nile IgM Capture, ID.VET, Montpellier, France). The results showed that IgG seroprevalence was significantly higher compared with the past season in four eastern counties. For the first time in Croatia, acute infections were confirmed in 12 horses (IgM positive). Furthermore, in the 2012 season for the first time clinical cases of WNV infection in humans were confirmed. Important to note is that clinical cases of neuroinvasive disease in humans were reported in the same area where increased viral activity in horses was encountered. These results and epidemiological data from neighboring countries, confirmed an increase of viral activity in this part of Europe. Despite relatively low numbers of clinical cases in horses in Europe, further spread of WNV infection to the western European countries can be expected. Moreover, it can be expected that by the introduction of such a pathogen to a naive horse population a clear increase of clinical cases can manifest itself. With confirmed endemisation and significantly increased viral activity in Southern Europe and highly probable spread to the rest of Europe, WNV represents an important emergent and zoonotic pathogen and the need for a surveillance program to be implemented in all European countries becomes urgent. WNV infection therefore should be kept in mind in differential diagnosis of horses presented with neurological signs.

West Nile virus; horses; epidemiology; Croatia

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