In a recent study published in One Health, researchers performed a systematic review to identify environmental factors related to alterations in WNV (West Nile virus) transmission and vector distribution in the EU (Europe) and the MR (Mediterranean region).
WNV is an extensively prevalent flavivirus across the globe and is considered to be emerging in several EU and MR regions. The Culex mosquitoes and birds are the key vectors and vertebrate host species, respectively; however, WNV may infect humans. Environmental variables could regulate WNV vector prevalence and abundance, and the EIP (extrinsic incubation period) and landscape changes due to human residences could provide conditions favourable for the breeding of mosquitoes. Studies have reported increased WNV transmission in MR in recent times; however, data on the factors related to epidemiologic alterations of WNV are not clear.
About the study
In the present systematic review, researchers evaluated environmental and climatic factors related to WNV epidemiology and transmission dynamics in mosquitoes, bird species, human hosts, and animals in MR and EU.
The study was conducted between 1 June 2021 and 30 June 2021. Original research articles of studies performed in MR and Europe on the West Nile virus and its vectors, and environmental factors associated with changes in WNV virology and transmission, were included. The studies were published between January 1, 2000, and December 31, 2020, in English, Portuguese, French, Spanish, German, and Italian.
Studies conducted in non-European and non-MR regions and those that did not evaluate the role of the environment on WNV dynamics were excluded from the analysis. The quality of the included studies was assessed using a 12-item tool to evaluate the validity of the included studies. Data extracted included the first study author, study year and publication year, vectors, diseases, reservoirs, hosts, study setting, study design, sources of data, and climatic and/or environmental factors.
In addition, the approach used for analysis, (correlation/association modelling, abundance/ density modelling, predictive modelling, spatial modelling, transmission modelling), result summaries, effects on WNV infection incidence, and the projected WNV prevalence. The included studies were categorized according to the vectors, reservoirs and hosts.
A total of 65 records were considered for the final analysis, comprising 62 records obtained from databases and three supplemental records, of medium or good quality. Of the records analyzed, 32% (n=21) belonged to Italy. Culex mosquitoes were studied in 40% (n=26) of records, humans were studied in 29% (n=19) of records, and animal hosts were addressed in 25% (n=16) of records. In addition, avian hosts were studied in 8.0% (n=5.0) of records.
The team observed a positive and statistically significant association between thermal and precipitation alterations and WNV epidemiology, albeit contrasting findings were observed. Other factors showing positive correlations to West Nile virus transmission kinetics were anthropized habitat expansion and NDVI (normalized difference vegetation index) values. Increases in temperature correlated positively with the abundance of Culex mosquitoes (especially of Cx. pipiens), Aedes mosquitoes, and the Ochlerotatus species.
Earlier temperature elevations (especially between early Spring and June) were associated with longer and earlier vector seasons. Likewise, increased precipitation was associated with enhanced abundance and growth rates for Cx. pipiens. Other factors increasing Cx. pipiens abundance included nearness to urban and population-dense locations, increased distance from aquatic habit distances, presence of caprine, bovine, and ovine farms within a 0.5km radius of mosquito trapping points and the expansion of agricultural, forestal, periurban, and rural habitats.
Other factors correlating positively with WNV presence in mosquitoes included daytime durations, evapotranspiration, and hours of insulation, whereas altitude (particularly >1700 metres) correlated negatively with Cx. modestus and Cx. pipiens abundance. Regarding WNV incidence and/or prevalence in vertebrate hosts (especially birds), temperature elevations (especially between June and October, with shorter EIP) and wetland presence showed positive correlations.
Factors associated with increased WNV presence in animals (especially hoses) included daily elevations in land surface temperatures in Israel, greater average temperatures between July and August in Germany, and greater temperatures during summer and spring seasons in South Europe. Other factors correlating positively with WNC presence in animals included elevated average yearly and half-yearly NDVI values (in Israel, Tunisia, and Morocco), proximity to wetlands in Spain and France, and lower altitudes in Israel.
Unusual thermal patterns correlated positively with WNV presence in humans. Precipitation patterns also affected WNV incidence in humans; however, contrasting results were obtained. Other positive predictors included nearness to avian migration routes, and yearly increases in NDVI values, whereas altitude correlated negatively with WNC incidence in humans.
Overall, the study findings showed that WNV epidemiology appears to be associated with global climatic variations. However, further research must be performed to elucidate the entire zoonotic cycle and to ascertain the independent effects of environmental factors on WNV transmission. WNV endemicity and expansion warrant adopting holistic strategies to improve understanding of WNV epidemiology and enhance WNV surveillance efforts and control measures.