Introduction

In 1999, public attention was captured by an outbreak of meningioencephalitis in the New York city area. A coincident die off of corvid birds served to further heighten concern. Initially, St. Louis encephalitis was suspected, but tests on the birds confirmed West Nile Virus (WNV). Until then, WNV had never been found in the western hemisphere.

WNV was first identified in Uganda in 1937. It has since been detected in humans, birds and other vertebrates as far north as Russia. The first human neuroinvasive cases resulting in meningitis and encephalitis were documented in Israel in 1957. In the early 1960s, the disease was found in horses in Egypt and France.

Other than the outbreak in Israel and isolated incidents prior to the mid 1990s, most infections were very mild. However, between 1996 and 2000, outbreaks in Romania, Russia, and Israel involved hundreds of people with severe neurological disease.

New York public health officials understood the threat posed by WNV. They implemented mosquito eradication programs, fearing that WNV would spread as the birds migrated south. Those fears proved to be well founded. Despite their efforts, the disease spread south and north and west as well.

By the end of 1999, WNV had been detected in 4 states; by 2000, it had spread to 8 states; and by 2001, to 22 states and Ontario, Canada. In 2002, the greatest number of cases were in Illinois; in 2003, in Colorado; and in 2004, in California, showing the inexorable march of the infection across the United States. The CDC provides a weekly updated map to show current WNV activity in the United States: http://www.cdc.gov/ncidod/dvbid/westnile/Mapsactivity/surv&control06Maps.htm

At present, WNV has spread throughout the continental United States and across Canada and Northern Mexico. Statistics have varied considerably from year to year for reasons that are not clear:

Year	Humans infected	Deaths
1999	62	7
2000	21	2
2001	66	9
2002	4156	284
2003	9862	264
2004	2539	100
2005	3000	119

A post-epidemic serosurvey following the 1999 outbreak in New York City found approximately 110 asymptomatic and 30 mild WNV (WNV fever) cases for each severe (WNV meningitis/ encephalitis) case. Thus, it is likely that the incidence of WNV has been significantly under-recognized and under-reported, especially since mild cases may mimic the flu.

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WNV has spread quickly since 1999, infecting thousands of people in North America.