Although the nation as a whole has only been experiencing a moderately active West Nile Season in 2015, with the CDC Reporting 1,121 neuroinvasive cases and 94 deaths, California accounts for close to 1/3rd of the nation’s totals. Their latest report puts the number of WNV deaths at 32 for the year, breaking last year’s record of 31:
Some summers West Nile activity is admittedly much worse than others, with 2012 seeing more than 5600 cases of WNV disease reported to the CDC, including 286 deaths. That year, it was Texas that was the hardest hit (89 deaths), while California only saw 20 fatalities.
The vast majority who are infected - about 80% - experience at worst only mild, or sub-clinical symptoms. Most of the rest may experience a brief febrile illness (West Nile Fever). Both are likely highly underreported.
A very small percentage of those infected develop WNV neuroinvasive disease (WNND), a form of encephalitis that can sometimes prove fatal. Those over the age of 50 appear to be the most vulnerable to the most serious form of the illness, and as most of those are hospitalized, those numbers are the most reliable.
The West Nile Virus arrived in North America (New York City) in 1999 - likely imported by a viremic visitor - and over a period of a few short years managed to spread across the entire United States and make inroads into Canada.
From the USGS Factsheet on West Nile Virus
This year, all 48 contiguous states are reporting WNV activity (in birds, horses, or humans), and 45 have reported human infection. Hardest hit have been California, and a large swath of the central United States.
Although WNV season is ending across much of the nation, the reports of cases, and deaths, will likely to continue for a couple of more months. A final tally for this year’s impact won’t be available until sometime in the spring.
While mosquito season may be on the decline in North America, there are still plenty of place where mosquitoes are active, both here and abroad. Add in the risks of Dengue, Chikungunya, and the Zika virus making inroads into the Americas, and following the advice of public health officials on mosquito safety makes good sense.