Spatial patterns of West Nile virus distribution in the Volgograd region of Russia, a territory with long-existing foci

Abstract

Southern Russia remains affected by West Nile virus (WNV). In the current study, we identified the spatial determinants of WNV distribution in an area with endemic virus transmission, with special reference to the urban settings, by mapping probable points of human infection acquisition and points of virus detection in mosquitoes, ticks, birds, and mammals during 1999–2016. The suitability of thermal conditions for extrinsic virus replication was assessed based on the approach of degree-day summation and their changes were estimated by linear trend analysis. A generalized linear model was used to analyze the year-to-year variation of human cases versus thermal conditions. Environmental suitability was determined by ecological niche modelling using MaxEnt software. Human population density was used as an offset to correct for possible bias. Spatial analysis of virus detection in the environment showed significant contributions from surface temperature, altitude, and distance from water bodies. When indicators of location and mobility of the human population were included, the relative impact of factors changed, with roads becoming most important. When the points of probable human case infection were added, the percentage of leading factors changed only slightly. The urban environment significantly increased the epidemic potential of the territory and created quite favorable conditions for virus circulation. The private building sector with low-storey houses and garden plots located in the suburbs provided a connection between urban and rural transmission cycles. Author summary West Nile fever is a zoonotic disease caused by flavivirus infection that dispersed circumglobally by the end of the 20th century. There is the long-existing endemic area in southern Russia (Volgograd region) that remains affected by West Nile virus (WNV) with the maximum number of reported cases in the country. The natural features of the region formed favorable conditions for long-term WNV circulation. At the same time, the urban environment could pose greater hazards for WNV infection in humans. Road density, distance from water bodies, land surface temperature and altitude were identified as the most significant spatial determinants of WNV distribution. The private building sector with low-storey houses and garden plots located in the city suburban area provided connections between urban and rural transmission cycles. Climate change also may favor WNV transmission as increased temperatures affect the sum of effective temperatures necessary for virus extrinsic replication. A statistically significant correlation between WNV human cases and the sum of effective temperatures was found during the last 20 years.