California health officials have reported the first fatality of 2015 due to West Nile virus. The announcement yesterday from California Department of Public Health (CDPH) Director and State Health Officer Dr. Karen Smith said the deceased person was a senior citizen in Nevada County.

West Nile virus
Culex quinquefasciatus

“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.

Related: California reports 1st human West Nile virus cases in Butte County, detected after blood donation

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.

In 2014, California reported 801 human WNV cases, including 31 deaths.