2024-01-30 16:25:44
According to a study, the spread of West Nile virus in Europe is linked to bird migration, cities and intensive agriculture. What is particularly important are the living conditions of the common mosquito, which is widespread in Europe (Culex pipiens), which transmits the virus, reports a research team led by Lu Lu from the University of Edinburgh.
The subline of the virus, WNV-2a, which is particularly common in Europe, is spreading there at a comparatively high speed of 88 to 218 kilometers per year, the researchers write in the journal “PLOS Pathogens”. The spread correlates more strongly with the movement of birds than with the migration of Culex mosquitoes of 0.5 to 2 kilometers per year. The research team analyzed data from viral genomes, epidemiological studies and environmental surveys over the past 20 years.
The direction and speed of spread of the viruses often corresponded to areas of intensive agriculture and urban areas. The more crops, pastures and livestock there were, the greater the speed of expansion. The mosquito Culex pipiens But they also prefer cities because there are many artificial water areas and warmer temperatures as well as fewer natural enemies, the researchers write. But wetlands and bird sanctuaries are also attractive to mosquitoes.
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The results of the genetic analysis are very interesting, says co-author Ute Ziegler from the Friedrich Loeffler Institute (FLI), who contributed information from Germany. The study showed, for example, that the West Nile virus came to Germany not from Italy, but from the Czech Republic and Austria, with birds that only migrate shorter distances.
It was first discovered in this country in 2018 in a great gray owl in Halle an der Saale and in 2019 in a human, also in eastern Germany. In these years there was a humid, warm spring and a hot summer, says Ziegler. The mosquito eggs develop in the water. The sexually mature female mosquitoes ingest the virus with a blood meal from birds, which in hot summers only takes 12 to 14 days to develop in the mosquito instead of the usual three weeks or longer. Therefore, in hot summers there are more generation cycles and faster reproduction of the viruses in the mosquitoes.
Transmission of West Nile virus: Several factors must be correct
According to Ziegler, many factors must have come together for it to be established in East Germany: suitable mosquitoes, favorable climatic conditions and birds that can be infected with the virus, such as owls, small songbirds or hawks. Only a few generations of mosquitoes that emerge in late summer could then infect both birds and mammals and thus also transmit the virus to humans. In order to survive in a region in the long term, the virus must be able to overwinter there.
To this day there is a lot of evidence in Germany, especially in the east. The occurrence there shows that under certain circumstances the conditions for the virus to establish are present, adds co-author Marion Koopmans from the Erasmus University Medical Center in Rotterdam. “Whether this is the case still depends on accidental introductions and the time of introduction,” emphasizes Koopmans.
The study found a clear connection between the spread of West Nile virus and agricultural land use, she says. “The analysis is a first indication that we need to go deeper, but that could mean, for example, that certain birds that harbor the viruses are attracted to agricultural areas or that there are effects on the abundance of mosquitoes there.”
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Mosquitoes and birds can pass on the virus, but not humans and other mammals. They are so-called false hosts. West Nile fever is one of the few mosquito-borne diseases in Europe. According to the Robert Koch Institute (RKI), most infections have no symptoms; around 20 percent of those infected develop a feverish, flu-like illness that lasts a few days, and around one in a hundred develops brain or meningitis.
In Europe, West Nile virus was detected in blood samples from the 1950s, according to the health authority ECDC. Since 2008 it has been spreading rapidly in Central Europe and the Eastern Mediterranean. In 2023, the authority had counted over 700 cases in people by mid-December, most of them in Italy and 6 cases in Germany. Not all of them are recognized.
According to the authors, there are significant gaps in the available data across Europe. “In Germany, together with our cooperation partners, we examine a lot of birds and look at where they died and whether they were infected with the West Nile virus,” says Ziegler. The data is evaluated in a national wild bird network at the FLI. The presence of West Nile virus in birds in previously unaffected regions could be used as an early warning system for humans. In this way, general practitioners in the respective region could be made aware of the disease in a timely manner.
The mosquito species Culex pipiens occurs wherever people live, says Ziegler. “The common mosquito practically lives around the corner from us. It is therefore illusory to eradicate them.” But you can reduce their numbers by avoiding water accumulation, which ranges from toys left lying in the sandpit to flower coasters and cemetery vases.
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