West Nile Virus on the rise in Germany; horses and humans infected

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A Culex species mosquito. Photo: Muhammad Mahdi Karim GFDL 1.2 via Wikimedia Commons

Researchers have sounded a warning over potential risks around the rise of West Nile Virus (WNV) in Germany, after an outbreak last year involving birds, horses and people.

In 2018, the mosquito-borne disease infected 12 birds and two horses, in cases of local transmission. Last year, probably fueled by the second very warm summer in a row, there were 76 confirmed cases in birds (involving a large number of different species), 36 in horses and five human cases.

The virus is maintained in wild birds, and spread between them by mosquitoes. Humans, horses and other animals can become infected if bitten by mosquitoes harboring the virus.

About a quarter of people infected by WNV develop a fever, headache, or muscle pain.

Less than 1 percent of infected individuals go on to develop severe neurological problems, with about 10 percent dying.

WNV is distributed in wide areas of Europe, mostly circulating in southeastern Europe and Italy.

However, low WNV activity is also seen in the nearby countries of Germany, France, Austria, and the Czech Republic.

Over the last decade, different monitoring programs were implemented in Germany to screen for anitbodies and genetic material from the virus in birds, horses, mosquitoes and chicken eggs.

The German cases in 2018 represented the first epizootic emergence of the disease in the country, Ute Ziegler and her colleagues wrote in the journal Viruses.

Their research showed that Germany has experienced at least six WNV introduction events and that strains of a distinct group (Eastern German WNV clade), which was introduced to Germany as a single introduction event, dominated mosquito, birds, horse and human-related virus variants in 2018 and 2019.

Virus strains in this clade are characterized by a specific mutation which might lead to an increase in viral fitness, they said.

At the same time, a large WNV outbreak was observed in south-eastern and southern Europe, testing suggested the German infection may have arrived in the country from the Czech Republic, probably before 2018.

The extraordinary high temperatures in 2018/2019 likely triggered the 2019 epidemic, they said.

It is now known that European birds are susceptible to WNV infections and Culex mosquitoes in Europe are competent to transmit the virus.

Discussing their findings, the study team stressed that less than 25% of infected humans develop noticeable symptoms, and far fewer run the risk of developing severe neurological problems.

Given that three of the five confirmed cases in Germany developed neurological problems gives rise to speculation that hundreds of undetected human WNV infections occurred in the country during the 2019 epidemic.

The ongoing circulation and dominance of the sub-clade detected in 2019 indicates successful overwintering of the virus in Germany through hibernating mosquitoes throughout the winter season.

The study team says its findings highlight the risk of a further spread in Germany in coming years, with additional human WNV infections.

“Thus, surveillance of birds is essential to provide an early epidemic warning and thus, initiate targeted control measures,” they report.

The risk of human WNV infections also poses risks for transfusion and organ transplantation safety, they note.

“Therefore, intensive surveillance of mosquitoes, birds, horses and humans should remain a public health priority.”

Such surveillance would be useful in developing targeted control mechanisms.

“Our study also highlights the need for international cooperation in the area of WNV surveillance and monitoring, especially across national borders and as a “one-health” approach for improved risk analysis.

“This should also include the generation of higher numbers of whole-genome sequences, allowing for a more precise molecular epidemiology and strain characterization.”

Members of the 26-strong study team are affiliated with a range of German institutions.

Ziegler, U.; Santos, P.D.; Groschup, M.H.; Hattendorf, C.; Eiden, M.; Höper, D.; Eisermann, P.; Keller, M.; Michel, F.; Klopfleisch, R.; Müller, K.; Werner, D.; Kampen, H.; Beer, M.; Frank, C.; Lachmann, R.; Tews, B.A.; Wylezich, C.; Rinder, M.; Lachmann, L.; Grünewald, T.; Szentiks, C.A.; Sieg, M.; Schmidt-Chanasit, J.; Cadar, D.; Lühken, R. West Nile Virus Epidemic in Germany Triggered by Epizootic Emergence, 2019. Viruses 2020, 12, 448.

The study, published under a Creative Commons License, can be read here

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