Tuesday, July 21, 2015

West Nile Virus: California Reports 1st WNV Death Of 2015

CDC Update As of 7/14/15


# 10,342

Although 2015 hasn’t shown signs yet of becoming a banner year for West Nile Virus (WNV) infections, surveillance reports are always trailing indicators, often lagging a week or two behind the current reality.  We won’t really have a good idea of the extent of this year’s impact until late August or September.


Case in point, California – which (as of last week) had yet to report a single West Nile human infection this year – announced the death of a resident from the virus yesterday.


CDPH Confirms First Human West Nile Virus Death of 2015 

Date: 7/20/2015

Number: 15-050

Contact: Anita Gore, Orville Thomas (916) 440 7259


California Department of Public Health (CDPH) Director and State Health Officer Dr. Karen Smith today announced the first confirmed death in California due to West Nile virus (WNV). The deceased person was a senior citizen in Nevada County.

“This death is a tragic reminder of how severe West Nile virus disease can be,” said Dr. Smith. “West Nile virus activity is more widespread in 2015 than in years past. Californians need to be vigilant in protecting themselves.”

WNV is influenced by many factors such as climate, the number and types of birds and mosquitoes in an area, and the level of immunity in birds to WNV. It is possible that the drought has contributed to WNV amplification by reducing sources of water for birds and mosquitoes. As birds and mosquitoes seek water, they are coming into closer contact and amplifying the transmission of the virus.

Thirty-three California counties have reported WNV activity so far this year, four more than this time last year and above the five-year average of 22. To date, 497 mosquito samples have tested positive for WNV, which exceeds the five-year average of 330.

WNV is transmitted to humans and animals by the bite of an infected mosquito. The risk of serious illness to most people is low. However, some individuals – less than one percent – can develop a serious neurologic illness such as encephalitis or meningitis.

People 50 years of age and older have a higher chance of getting sick and are more likely to develop complications. Studies also indicate that those with diabetes and/or hypertension are at greatest risk for serious illness.

CDPH recommends that individuals prevent exposure to mosquito bites and WNV by practicing the “Three Ds”:

  1. DEET – Apply insect repellent containing DEET, picaradin, oil of lemon eucalyptus or IR3535 according to label instructions. Repellents keep the mosquitoes from biting you. DEET can be used safely on infants and children two months of age and older.
  2. DAWN AND DUSK – Mosquitoes bite in the early morning and evening so it is important to wear proper clothing and repellent if outside during these times. Make sure that your doors and windows have tight-fitting screens to keep out mosquitoes. Repair or replace screens with tears or holes.
  3. DRAIN – Mosquitoes lay their eggs on standing water. Eliminate all sources of standing water on your property, by emptying flower pots, old car tires, buckets, and other containers. If you know of a swimming pool that is not being properly maintained, please contact your local mosquito and vector control agency.

California’s West Nile virus website includes the latest information on WNV activity in the state. Californians are encouraged to report dead birds on the website or by calling toll-free 1-877-WNV-BIRD (968-2473).


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



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.  In 2014, only 85 deaths were reported, while in 2013 there were 111 fatalities (cite).


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 (perhaps 1%) 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.


A reminder that once the mosquitoes come out, it is time to take precautions.  Even if the latest surveillance maps don’t show your area in the midst of a West Nile Outbreak.  

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