Agricultural practices and bird migration patterns linked to West Nile virus spread in Europe

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A study published in the journal PLoS Pathogens Agricultural land use and bird movement are the main factors responsible for West Nile virus outbreaks in Europe.

Study: West Nile virus spread in Europe: analysis of phylogenetic patterns and key drivers.  Image credit: Kateryna Kon/ShutterstockStudy: West Nile virus spread in Europe: analysis of phylogenetic patterns and key driverss Image credit: Kateryna Kon/Shutterstock

Background

Infection of humans and animals by mosquito-borne viruses has become a major public health concern worldwide. In Europe, the prevalence of West Nile virus (WNV) has been increasing steadily in many geographic regions over the last few decades. This virus can cause serious infections in humans.

WNV is an enveloped, single-stranded RNA virus transmitted by mosquitoes as vectors and birds as amplification reservoir hosts. Nine distinct lineages of WNV have been identified worldwide, with WNV-1 and WNV-2 being the predominant strains identified in humans and animals. So far, WNV-3 to WNV-9 strains have been detected in mosquitoes, birds, horses and amphibians.

WNV was first identified in Europe in the 1960s. Since 1996, an emergence of WNV outbreaks has been observed in southeastern and central Europe. In recent years, an outbreak of WNV-1 and WNV-2 has been detected in Europe, which can significantly affect human and animal health.

In this study, scientists explored the transmission dynamics of WNV in Europe and assessed factors responsible for WNV transmission.

Research design

The scientists combined WNV genome sequences, ecological data, and epidemiological data into phylodynamic models to map the evolution and transmission history of WNV in Europe. They developed spatially precise phylogeographic models to determine the influence of various factors on viral dispersal direction and velocity.

Furthermore, they used a Skygrid-Generalized Linear Model (GLM) to assess how changes in environmental temperature and biodiversity over the past two decades could predict changes in viral genetic diversity.

Important observation

The study found distinct evolutionary trajectories for WNV-1 and WNV-2 lineages and WNV-2a and WNV-2b sub-lineages in Europe. Of the six lineages found in Europe, WNV-2a was identified as the major sublineage, accounting for 73% of the publicly available viral sequences obtained from Europe. This sublineage spread to at least 14 countries.

Phylogenetic analysis of WNV complete and partial nucleotide sequences identified from Europe.  Evolutionary distances were calculated using the optimal GTR+I model, with the phylogenetic tree constructed using the maximum likelihood (ML) method.  Bootstrap values ​​are given for 1000 replicates.  (a) ML trees of all genera found in Europe.  The branches of the lines are all collapsed and shown as rectangles;  (b) subtree of WNV-2 sequences;  (c) WNV lineage distribution over time using the same color as shown in the tree;  (d) Geographical distribution of WNV lineages.  Map with a small pie chart showing the total number of sequences detected in each country (on a logarithmic scale), with each slice proportional to the number of distinct WNV lineages within that country.  The European shapefile used in the study was obtained from Data & Maps for ArcGIS (formerly Esri Data & Maps, https://www.arcgis.com/home/group.html?id=24838c2d95e14dd18c25e9bad55a7f82#overview) under a CC4B0Y.  License.Phylogenetic analysis of WNV complete and partial nucleotide sequences identified from Europe. Evolutionary distances were calculated using the optimal GTR+I model, with the phylogenetic tree constructed using the maximum likelihood (ML) method. Bootstrap values ​​are given for 1000 replicates. (a) ML trees of all genera found in Europe. The branches of the lines are all collapsed and shown as rectangles; (b) subtree of WNV-2 sequences; (c) WNV lineage distribution over time using the same color as shown in the tree; (d) Geographical distribution of WNV lineages. Map with a small pie chart showing the total number of sequences detected in each country (on a logarithmic scale), with each slice proportional to the number of distinct WNV lineages within that country. The European shapefile used in the study was obtained from Data and Maps for ArcGIS (formerly Esri Data and Maps, https://www.arcgis.com/home/group.html?id=24838c2d95e14dd18c25e9bad55a7f82#overview) under a CC-BY 4.0 license.

Research results revealed that WNV-2a has evolved into two major co-circulating clusters (clusters A and B) over the past two decades and transmitted westward (cluster A) and southward (cluster B). Both clusters originate from Central Europe and show distinct dynamic histories and transmission patterns.

Scientists hypothesized that WNV-2a was first introduced to Europe via long-distance migratory birds. During its circulation in native bird populations and other hosts, WNV-2a evolved, diversified, and spread throughout the European continent.

Dispersal velocities of WNV-2a were estimated to be as high as 88 to 215 km/year, which correlates with bird movements. Agricultural land use was identified as a strong factor driving the spread of WNV.

Specifically, factors related to crop and livestock production, such as agricultural land coverage, pasture, cultivated and managed vegetation, and livestock density, showed positive correlations with WNV dispersal velocity and directionality of infection. A positive correlation between WNV transmission direction, wetland coverage and migratory bird flyway was also observed.

The scientists highlighted that regions with high-level agricultural activities could influence the speed of WNV’s spread and the direction of its transmission in Europe. As noted by scientists, high-level agricultural activities are associated with significant damage to natural ecosystems, reduction of mosquito and bird diversity, and inclusion of aquatic habitats. All these factors can increase the transmission of vector-borne pathogens.

Furthermore, changes in migratory routes of birds due to habitat loss may affect the transmission of WNV to new areas. Studies have found higher transmission of WNV in urban locations, where the abundance of common house mosquitoes is significantly higher due to the availability of artificial aquatic habitats, the presence of warmer climates, and the lower abundance of predators.

Significance of the study

Studies have found high lineage diversity of WNV in Europe. Agricultural land use has the greatest impact on the direction and speed of WNV transmission, which is directly linked to urbanization and bird habitat change.

Scientists highlight the need to strengthen virological surveillance in Central Europe, where WNV outbreaks are most likely. Increased surveillance is required in areas with high cultivation density.



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