West nile virus in the United States - a historical perspective

Abstract

Prior to 1999, West Nile virus (WNV) was a bit player in the screenplay of global vector-borne viral diseases. First discovered in the West Nile District of Uganda in 1937, this Culex sp.-transmitted virus was known for causing small human febrile outbreaks in Africa and the Middle East. Prior to 1995, the last major human WNV outbreak was in the 1950s in Israel. The epidemiology and ecology of WNV began to change in the mid-1990s when an epidemic of human encephalitis occurred in Romania. The introduction of WNV into Eastern Europe was readily explained by bird migration between Africa and Europe. The movement of WNV from Africa to Europe could not, however, predict its surprising jump across the Atlantic Ocean to New York City and the surrounding areas of the United States (U.S.). This movement of WNV from the Eastern to Western Hemisphere in 1999, and its subsequent dissemination throughout two continents in less than ten years is widely recognized as one of the most significant events in arbovirology during the last two centuries. This paper documents the early events of the introduction into and the spread of WNV in the Western Hemisphere.

Figure 1

Global distribution of flaviviruses in the JEV serocomplex in 1999.

Figure 2

Viral antigen screening panels for arboviral diagnosis used in 1999. Abbreviations: LAC, LaCrosse; SLE, St. Louis encephalitis; WEE, western equine encephalitis; VEE, Venezuelan equine encephalitis; CTF, Colorado Tick fever; DEN2, Dengue 2; POW, Powassan; EEE, eastern equine encephalitis; HJ, Highlands J; EVE, Everglades; MAY, Mayaro; YF, yellow fever; SIN, Sindbis; TBE, tick-borne encephalitis; TAH. Tahanya; INK, Inkoo; CHIK, Chikungunya; JE, Japanese encephalitis; WN, West Nile; SSH, Snowshoe hare; RR, Ross River; BF, Barmah Forest; MVE, Murray Valley encephalitis.

Figure 3

Phylogeny of WNV based on gene sequence of the envelope protein as of 1999 [21].

Figure 4

Yearly spread of WNV throughout the U.S. 1999–2002. Counties reporting WNV activity in humans (red) and non-human e.g., birds, mosquitoes, equines, and other mammals (blue), as reported to CDC ArboNet.

Figure 5

Yearly reported WNV human cases, 1999–2012, as reported to CDC ArboNet.

Figure 6

Average annual incidence of West Nile virus human neuroinvasive disease by age group, 1999–2012, as reported to CDC ArboNet.

Figure 7

West Nile virus human disease cases reported to CDC by week of illness onset, 1999–2012, as reported to CDC ArboNet.

Figure 8

Average annual incidence of West Nile virus human neuroinvasive disease reported to CDC by county, 1999–2012, as reported to CDC ArboNet.

Figure 9

Total WNV and SLEV disease cases by U.S. state (WNV/SLEV). WNV, 1999–2012 (bold) cases reported to ArboNet. SLEV cases from 1964–2000 are listed [1].

Figure 10

Total WNV disease cases by province, Canada, 1999–2012. Some cases assigned to a particular province were associated with travel outside of that province. Cases reported to the Public Health Agency of Canada [71]