Image of the Week

West Nile virus (the mosquito/tickborne encephalitic virus) has been in the news a lot recently (albeit in a much quieter way than it’s flashier cousins) – last year alone there were 5,674 confirmed cases of the virus, the highest since 2003. What is less commonly known is that West Nile virus (WNV) was only introduced into the US in 1999. Pictured below are three maps illustrating the virus’ incredible (and alarmingly rapid) spread across the country.

(all maps courtesy of the CDC)


Introduced in New York State, notice that by the end of 1999 all human cases of WNV were limited to the state of New York.

First introduced in NY, here we have the 1999 map of all cases of WNV in the US.
First introduced in NY, here we have the 1999 map of all cases of WNV in the US.


By 2001, the virus had spread to nearly 30 states, with human cases in 10 states.

WNV cases in 2001
WNV cases in 2001 – the disease exists in humans across the North East and Southern states.


By 2003, the virus was present in humans in 45 of the 48 contiguous states, with just Oregon and Washington remaining  WNV-free.

WNV cases in 2003
WNV cases in 2003

Today, cases of West Nile Virus have occured in all 48 contiguous states, with the numbers of cases often continuing to grow. The moral of the story? Viruses are very resilient. In order to so effectively and quickly spread across the country, the virus had to survive several brutal winters (known as “overwintering”) – remember, this is virus originating in the significantly warmer climates of the African continent.  We were very lucky that while WNV has the capacity to be severely pathogenic (encephalitis is no joke), 80% of those infected are asymptomatic. What if it had been Rift Valley Fever instead?

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