Vector control practices and strategies against West Nile virus

West Nile virus (WNV) is a vector-borne virus maintained and amplified in nature in an enzootic cycle between birds and mosquitoes which, under certain conditions, can spill over to humans and equines. To mitigate the risk of WNV being transmitted to humans and animals, European countries have been investing significant resources in vector surveillance and control interventions, but with little common knowledge on their effectiveness and no EUwide strategy or technical guidance to properly apply these methods and evaluate their efficacy. To help address these gaps, ECDC has conducted a survey to collect information on the current WNV surveillance and control capacities across European Union/European Economic Area (EU/EEA) countries, European Neighbourhood Policy (ENP) partner countries1 and EU candidate/potential candidate countries2 to identify the major challenges faced by public health authorities when implementing WNV control strategies. In addition, ECDC has also carried out a scoping literature review to collate existing knowledge and operational experience on the effectiveness of vector control practices in reducing WNV risk.

Eighty-three percent of the EU/EEA countries, ENP partner countries and EU candidate/potential candidate countries have implemented at least one method of WNV surveillance. The most common passive WNV surveillance method for all countries is the detection of human cases, followed by surveillance of dead animals. The most common method of active WNV surveillance is mosquito screening, followed by sentinel bird screening and sentinel equid screening for EU/EEA countries, ENP partner countries and EU candidate/potential candidate countries. The majority of the countries conduct routine vector surveillance (abundance monitoring). However, less than 15% of the EU/EEA countries, ENP partner countries and EU candidate/potential candidate countries perform pesticide resistance testing, and even in those countries that do, this is not implemented systematically. The majority of the EU/EEA countries that do not implement vector control have no history of autochthonous WNV human cases. Among countries that implement vector control (58%), the most widely-adopted methods are biological larviciding and public education, while the least widely-adopted method is source reduction through environmental management. Among ENP partner countries and EU candidate/potential candidate countries, the most widely-used methods are public education, followed by biological larviciding, chemical larviciding and low volume (LV) surface spray adulticiding. A substantial number of countries include adulticiding in their vector management response strategies, including ground-level ultra-low volume (ULV) space spraying, aerial ULV adulticiding and ground-level LV surface spraying. EU/EEA countries apply larviciding interventions (in any form) in response to WNV vector abundance (larval density) data, however the most important trigger by far for any ULV adulticiding treatment is the occurrence of autochthonous WNV human cases. In ENP partner countries and EU candidate/potential candidate countries, all available vector control tools (including adulticiding) are routinely implemented in response to vector abundance data.

The scoping literature review targeted published, peer-reviewed manuscripts on controlled studies assessing the impact of operational WNV vector control strategies using entomological indicators (adult vector abundance and WNV prevalence in mosquitoes), and/or veterinary indicators (enzootic circulation in sentinel/wild animals), and/or assessing the impact of vector control directly on human cases. Twelve studies were identified that satisfied the above criteria, the majority of which were conducted in the United States. According to the available scientific evidence, aerial ULV adulticiding is currently the only method directly linked to (a) reduction in WNV circulation levels using entomological/veterinary indicators, (b) interruption of WNV enzootic amplification in a natural-wetland environment and (c) interruption of WNV transmission in urban areas resulting in fewer human cases. Source reduction (when feasible) and larviciding interventions are justifiably the first and most important step in reducing and sustaining vector populations at low levels. There is, however, a critical absence of evidence linking the impact of preventive intervention (such as larviciding) to reductions in WNV circulation and transmission levels.
In general, survey participants identified limited availability of insecticidal active substances, lack of long-term registration for products/methods, a complex regulatory framework for the use of biocidal products, and lack of EU-wide technical guidelines as significant barriers to effective vector control operations. To address the above issues, the first and most important step is to increase communication between all relevant stakeholders. Moreover, studies are needed on the impact of vector control measures in order to better inform public health policy decisions on strategies for WNV management.