Methodology and results of integrated WNV surveillance programmes in Serbia

Abstract

Studies conducted during the past few years have confirmed active West Nile virus (WNV) circulation in Serbia. Based on these studies and the epidemiological situation, the Veterinary Directorate of the Ministry of Agriculture and Environmental Protection launched national WNV surveillance programmes in 2014 and 2015. The programmes encompassed the territory of Serbia and were conducted by the veterinary service in collaboration with entomologists and ornithologists. The objective of the programmes was early detection of WNV and timely reporting to the public health service and local authorities to increase both clinical and mosquito control preparedness. The WNV surveillance programmes were based on direct and indirect surveillance of the presence of WNV by the serological testing of initially seronegative sentinel horses and chickens as well as through viral detection in pooled mosquito and wild bird samples. The most intense WNV circulation was observed in all seven districts of Vojvodina Province (northern Serbia) and Belgrade City, where most of the positive samples were detected among sentinel animals, mosquitoes and wild birds. The West Nile virus surveillance programmes in 2014 and 2015 showed satisfactory results in their capacity to indicate the spatial distribution of the risk for humans and their sensitivity to early detect viral circulation at the enzootic level. Most of the human cases were preceded by the detection of WNV circulation as part of the surveillance programmes. According to the existing data, it can be reasonably assumed that WNV infection, now an endemic infection in Serbia, will continue to present a significant problem for the veterinary service and public health.

Fig 1. Categorization of districts in the Republic of Serbia according to risk of WNV outbreak.

The figure show the geographic position of Republic of Serbia in Europe. The districts (NUTS3) with higher risk of WNV infection are marked in red (10 districts in WNV surveillance programme in 2014 and 11 districts in WNV surveillance programme in 2015), and the districts with lower risk of WNV infection are marked in yellow. The solid black lines in the maps of Republic of Serbia represents the borders of districts (25 districts in Serbia that were under WNV surveillance in 2014 and 2015). The northernmost 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 S3 Table. The basic administrative maps were extracted from the GADM database of Global Administrative Areas (www.gadm.org), version 2.8, November 2015, and later on changed according to the data presented in them.

Fig 2. Distribution of WNV activity detected during the 2014 surveillance programme at the district level.

Figs 2a-d show WNV infection rates as percentages (number positive samples/number of samples tested) for a) sentinel chickens b) sentinel horses, c) mosquito pools and d) wild birds. The basic administrative maps were extracted from the GADM database of Global Administrative Areas (www.gadm.org), version 2.8, November 2015, and later on changed according to the data presented in them.

Fig 3. Distribution of human West Nile cases reported during the 2014 at the district level.

Fig 3 shows the number of clinical and laboratory confirmed human West Nile cases reported to the ECDC (except for one clinical case in Raška and Nišava district that were not laboratory confirmed). The basic administrative maps were extracted from the GADM database of Global Administrative Areas (www.gadm.org), version 2.8, November 2015, and later on changed according to the data presented in them.

Fig 4. The temporal distribution of the first positive results detected during WNV surveillance and the time of the first human West Nile cases in 2014.

The data for the first positive results obtained during WNV surveillance and the first reported human cases in 2014 are presented by symbols per district level. The top seven districts represent Vojvodina Province, followed by the other districts of Serbia sorted in alphabetical order.

Fig 5. Distribution of WNV activity detected during the 2015 surveillance programme at the district level.

Fig 5a-c show WNV infection rates as percentages (number positive samples/number of samples tested) for a) wild birds b) horses, and c) mosquito pools. Fig 5D 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 2.8, November 2015, and later on changed according to the data presented in them.

Fig 6. The temporal distribution of the first positive results detected during WNV surveillance and the time of the first West Nile human cases in 2015.

The first positive results obtained during WNV surveillance and the first reported human West Nile cases in 2015 are represented by symbols per district level. The top seven districts represent the Vojvodina Province, followed by the other districts of Serbia sorted in alphabetical order.