West Nile Virus in the United States: Guidelines for Surveillance, Prevention, and Control

Promote health and quality of life by preventing and controlling vector-borne diseases The following CDC, Division of Vector-Borne Diseases staff members formed the technical development group that prepared this report: We are grateful to representatives from the following organizations for their thoughtful review and contributions to this document: 5 Foreword As West Nile virus (WNV) spread and became established across the United States following its first identification in New York City in 1999, the responses of all levels of the public health system have resulted in a detailed understanding of WNV transmission ecology and epidemiology as well as development of systems and procedures to reduce human risk. This includes an expanded capacity to diagnose and monitor WNV infections in humans, measure WNV transmission activity in vector mosquitoes, and implement effective WNV control programs. These guidelines, which update the third revision released in 2003, incorporate this new knowledge with the goal of providing guidance to health departments and other public health entities in monitoring and mitigating WNV risk to humans. Human disease surveillance provides an ongoing nationwide assessment of the human impact of WNV, and over the past decade has demonstrated where WNV incidence and total disease burden are greatest. However, human disease surveillance, by itself, is limited in its ability to predict the large focal outbreaks that have come to characterize this disease. These outbreaks typically intensify over as little as a couple of weeks; however, human case reports are lagging indicators of risk since case reports occur weeks after the time of infection. Thus, environmental surveillance – monitoring enzootic and epizootic WNV transmission in mosquitoes and birds – forms a timelier index of risk, and is an important cornerstone for implementing effective WNV risk reduction efforts. Research and operational experience shows that increases in WNV infection rates in mosquito populations can provide an indicator of developing outbreak conditions several weeks in advance of increases in human infections. Communities that have a history of WNV, particularly metropolitan areas with large human populations at risk, should implement comprehensive, integrated vector management (IVM) programs that incorporate monitoring mosquito abundance and infection rates. Mosquito-based WNV surveillance programs should use strategies that assure data are comparable over time and space, and are designed to detect trends in WNV transmission levels. Programs should enlist quantitative indicators such as the WNV infection rate or vector index to represent WNV transmission activity in mosquito populations. Programs must …