The following article was taken from mosquito.org
West Nile virus (WNV) emerged from its origins in 1937 in Africa (Uganda) into Europe, the Middle East, west and central Asia and associated islands. It is a Flavivirus (family Flaviviridae) with more than 70 identified viruses. Serologically, it is a Japanese encephalitis virus antigenic complex similar to St. Louis, Japanese and Murray Valley encephalitis viruses. Similar to other encephalitises, it is cycled between birds and mosquitoes and transmitted to mammals (including horses) and man by infected mosquitoes. WNV might be described in one of four illnesses: West Nile Fever might be the least severe in characterized by fever, headach, tireness and aches or a rash. Sort of like the “flu”. This might last a few days or several weeks. At least 63% of patients report symptoms lasting over 30 days, with the median being 60 days. The other types are grouped as “neuroinvasive disease” which affects the nervous system; West Nile encephalitis which affects the brain and West Nile meningitis (meningoencephalitis) which is an inflammation of the brain and membrane around it. (CDC)
It first appeared in North America in 1999 in New York (Cornell Environmental Risk Analysis Program) with 62 confirmed cases and 7 human deaths. Nine horses died in New York in 1999. In 2001, 66 human cases (10 deaths) were reported in 10 states. It occurred in birds or horses in 27 states and Washington D.C., Canada and the Caribbean. There were 733 horse cases in 2001 with Florida reporting 66% of the cases; approximately 33% were fatal. In 2001 more than 1.4 million mosquitoes were tested for WNV. In the United States (2004) over 43 species of mosquitoes have tested positive for WNV transmission, the Culex pipiens group seems the most common species associated with infecting people and horses. Currently, 65 mosquito and 300 bird species have tested positive in the United States for this virus.
During 2002, the number of areas reporting WNV grew to 44 states and 5 Canadian provinces. The only states not reporting WNV were Alaska, Arizona, Hawaii, Nevada, Oregon and Utah that year. Intrauterine transmission (CDC MMWR) and laboratory infections (CDC MMWR) were reported for the first time. In all over 3800 human cases with 232 fatalities in 39 states and Washington DC were recorded. More than 24,350 horse cases of WNV were confirmed or reported in 2002. There is a vaccine for horses. Even alligators (CDC-EID) were found infected in Georgia.
The first confirmed 2003 WNV infection was in South Carolina on July 7th, 2003. South Dakota confirmed a WNV infection in a dog. The final CDC report list 9858 cases. Nebraska had 1942, Colorado 2947 and Idaho only one (CDC) . In Florida there were 94 human cases with most occuring in the panhandle. Bay county, FL reported 14 cases and one death. Of the more than 9858 cases, 6829 were West Nile Fever ( the milder form), 2863 were neuroinvasive (the more severe form) and 166 clinically unspecified. There were over 4200 positive dead birds reported in 39 states and 4500 plus infections in horses in 40 states with more than 425 of these in Colorado. West Nile was reported in 1377 sentinel chicken flocks from 15 states. In Florida 1173 seroconversions to WNV were reported from 34 counties. More than 1950 positive mosquito pools were reported from 32 states and New York City.
In Canada (01-12-04) WNV was been confirmed in 9 Provinces. At least 10 human deaths and more than 1220 cases have been confirmed. Canada reported over 445 presumed or confirmed horse cases in 6 Provinces with over 180 in Alberta Province. Five Provinces have reported positive mosquito pools (>575) with over 290 from Manitoba. Canada confirmed over 1600 positive dead birds from 12000 tests.
Mexico (December 2003) has tested over 590 citizens in 25 states. Six have tested positive with three with the more severe form of WNV. Mexico horse data shows 2475 had positive WN returns in 29 states. Of more than 18000 birds tested 117 were positive. The Pan American Health Organization (PAHO).
Arizona and New Mexico reported the first human cases of WNV on May 26, 2004 and a week later confirmed a total of 7 cases. South Dakota reported it’s first case on June 8, 2004. In 2003 South Dakota had 14 deaths and over human cases reported. Wyoming and Florida (http://www.heraldtribune.com/) has joined the list recently. Alabama, Arizona, Texas and Virginia have reported WN V infections in horses. WNV seroconversions have been reported in 64 sentinel chicken flocks from 4 states (Arizona, California, Florida, and Louisiana), and 58 WNV-positive mosquito pools have been reported from 6 states (Arizona, California, Illinois, Indiana, Louisiana, and Pennsylvania).
As of 2014, there have been 36,437 cases of WNV reported to CDC. Of these, 15,774 have resulted in meningitis/encephalitis and 1538 were fatal. CDC estimates that there have been at least 1.5 million infections (82% are asymptomatic) and over 350,000 cases of West Nile Fever, but the disease is grossly under reported due to its similarity to other viral infections.
Canada’s 1st dead bird (a blue jay) from West Nile virus in 2004 was confirmed in Ontario in May 2004. West Nile virus was confirmed in 2 birds in Puerto Rico near the former US Roosevelt Roads Navy Base (southeastern Puerto Rico).
Britain’s Health Protection Agency has started its annual surveillance program for possible human cases of West Nile virus infection. The program, which has been used for the last three years, operates during the summer, when there is West Nile virus activity in other countries. The UK has had no reported WNV, but are developing a West Nile Virus Contingency Plan.