Predicting the spatio-temporal spread of West Nile virus in Europe

Abstract

West Nile virus is a widely spread arthropod-born virus, which has mosquitoes as vectors and birds as reservoirs. Humans, as dead-end hosts of the virus, may suffer West Nile Fever (WNF), which sometimes leads to death. In Europe, the first large-scale epidemic of WNF occurred in 1996 in Romania. Since then, human cases have increased in the continent, where the highest number of cases occurred in 2018. Using the location of WNF cases in 2017 and favorability models, we developed two risk models, one environmental and the other spatio-environmental, and tested their capacity to predict in 2018: 1) the location of WNF; 2) the intensity of the outbreaks (i.e. the number of confirmed human cases); and 3) the imminence of the cases (i.e. the Julian week in which the first case occurred). We found that climatic variables (the maximum temperature of the warmest month and the annual temperature range), human-related variables (rain-fed agriculture, the density of poultry and horses), and topo-hydrographic variables (the presence of rivers and altitude) were the best environmental predictors of WNF outbreaks in Europe. The spatio-environmental model was the most useful in predicting the location of WNF outbreaks, which suggests that a spatial structure, probably related to bird migration routes, has a role in the geographical pattern of WNF in Europe. Both the intensity of cases and their imminence were best predicted using the environmental model, suggesting that these features of the disease are linked to the environmental characteristics of the areas. We highlight the relevance of river basins in the propagation dynamics of the disease, as outbreaks started in the lower parts of the river basins, from where WNF spread towards the upper parts. Therefore, river basins should be considered as operational geographic units for the public health management of the disease.

Epidemiology of West Nile Virus Infections in Humans, Italy, 2012–2020: A Summary of Available Evidences

“…In recent years, outbreaks of WNV infections have hit many European countries, with the peak notification rate of 0.3 cases per 100,000 during 2018 [ 1 , 2 , 12 , 16 , 17 , 18 ]. Even though WNV is often described as an “emerging pathogen”, when it comes to Italy, this definition may represent a sort of misnomer: as confirmed by our analyses, large areas of Northern Italy have become endemic to this infection since early 2010s [ 16 , 17 , 18 , 30 ], with around one-fifth of the total population living in areas at risk.…”

“…Even though WNV is often described as an “emerging pathogen”, when it comes to Italy, this definition may represent a sort of misnomer: as confirmed by our analyses, large areas of Northern Italy have become endemic to this infection since early 2010s [ 16 , 17 , 18 , 30 ], with around one-fifth of the total population living in areas at risk. Moreover, the pooled crude incidence rate for WNV infections 2012–2020 of 0.211 cases per 100,000 (0.090 cases per 100,000 for WNND) characterizes WNV infections as a relatively rare instance, but still exceeds the EU notification rate for locally acquired cases of 0.1 cases per 100,000 [ 1 , 11 , 12 , 21 , 25 ].…”

“…The importance of such factors is also confirmed by our analyses, as both population density, and occurrence of WNND cases in the nearby provinces were identified as significant effectors for the WNV neuro-invasive disease. Even though WNV has no inter-human spreading, higher population density increases the likelihood of being infected by pools of competent vectors, whose occurrence is in turn substantiated by higher rates in nearby areas [ 1 , 11 , 28 , 34 , 35 ].…”

“…West Nile virus (WNV) is a mosquito-borne RNA virus belonging to the genus Flavivirus (family Flaviviridae ). In Europe, WNV is usually carried by species of the genus Culex (mainly C. pipiens , C. peregrinus , and C. modestus ) and Aedes [ 1 , 2 , 3 ], being sustained in an enzootic “amplification” cycle between birds and mosquitoes. Even though WNV can infect large mammalians (e.g., horses), including humans, the latter represent rather incidental and dead-end hosts [ 2 ].…”

“…Similar to other arboviruses (e.g., tick-borne encephalitis virus or TBEV) [ 9 , 10 ], WNV is currently spreading in Europe [ 1 , 2 , 11 , 12 , 13 ], alongside the routes of migratory birds [ 1 ]. After the first European outbreak in Camargue, Southern France (1962), WNV has regularly involved larger areas in Eastern, Western, and Southern Europe, with a seasonal trend from April to November.…”