Intensive West Nile Virus Circulation in Serbia in 2018-Results of Integrated Surveillance Program

Abstract

The results of the Serbian national integrated West Nile virus (WNV) surveillance program conducted in 2018 and funded by the Serbian Veterinary Directorate are presented. The WNV surveillance program encompassed the entire territory of Serbia and was conducted by the veterinary service in collaboration with entomologists and ornithologists. The objective of the program was early detection of WNV circulation in the environment and timely reporting to the public health service and local authorities to increase clinical and mosquito control preparedness. The program was based on the detection of WNV presence in wild birds (natural hosts) and mosquitoes (virus vectors) and on serological testing of sentinel horses (WNV-specific IgM antibodies). The season 2018 was confirmed to be the season of the most intensive WNV circulation with the highest number and severity of human cases in Serbia ever reported. The most intense WNV circulation was observed in the northern and central parts of Serbia including Vojvodina Province, the Belgrade City area, and surrounding districts, where most positive samples were detected among sentinel animals, mosquitoes and wild birds. The majority of human cases were preceded by the detection of WNV circulation during the surveillance. The WNV surveillance program in 2018 showed satisfactory results in the capacity to indicate the spatial distribution of the risk for humans and sensitivity to early detection of WNV circulation in the environment.

Keywords: WNV national surveillance program 2018; West Nile virus; horse surveillance; mosquito surveillance; wild bird surveillance.

Figure 1

Distribution of WNV activity detected during the 2018 surveillance program at the district level. (a,b) show WNV infection rates as percentages (number of positive samples/number of samples tested) for a horses, and b mosquito pools. The comma instead of the point in the percentage values represent the decimal separator shown by the software that was used for the preparation of the maps. (c) presents the number of found WNV positive wild birds and (d) presents the number of clinical and laboratory-confirmed human West Nile cases reported to the ECDC. The basic administrative maps were extracted from the GADM database of Global Administrative Areas (www.gadm.org), version 3.6 (accessed on 15 February 2021), and were changed subsequently according to the data presented in them.

Figure 2

The temporal (time-line) distribution of the first positive results detected during WNV surveillance and the time of the first West Nile human cases in 2018. (a) The first positive results obtained during WNV surveillance and the first reported cases of human West Nile infection in 2018 are represented by symbols per district level. The top seven districts represent the Vojvodina Province, followed by the other districts of central Serbia are sorted in alphabetical order; (b) the time of the first positive results in horses, mosquitoes, and birds obtained during surveillance in 2018 concerning the time of the first human cases is shown. The red line represents the time when the first human case was detected per district level, and positive findings are presented in 2-week intervals periods in columns before and after the first human case was detected. Only the districts where human cases were detected are presented.

Figure 3

Categorization of districts in the Republic of Serbia according to the risk of WNV outbreak. The figure shows the geographic position of the Republic of Serbia in Europe. The districts (NUTS3) with a higher risk of WNV infection are marked in red (11 districts), and the districts with a lower risk of WNV infection are unmarked (grey). The solid black lines in the maps of the Republic of Serbia represent the borders of districts (all 25 districts in Serbia that were under WNV surveillance). The most northern seven districts represent the Autonomous Province of Vojvodina. Description of the sampling locations: districts names, geographic coordinates and web links are provided as the supporting information (Table S1). The basic administrative maps were extracted from the GADM database of Global Administrative Areas (www.gadm.org), version 3.6 (accessed on 15 February 2021), and later on, changed according to the data presented in them.