Heat may be behind record number of severe West Nile virus cases in California

Severe cases of West Nile virus (WNV) leaped to a record high last year, according to new data from the Centers for Disease Control and Prevention.

This serious form of the mosquito-borne disease is rare: The CDC says less than one percent of  those infected with the virus will develop a serious neurologic illness, like encephalitis or meningitis.

But in 2014, California reported 561 cases of the more severe neuroinvasive West Nile disease, according to the CDC's weekly Morbidity and Mortality Weekly Report. Last year, the state reported a total of 801 people contracted the illness and there was a record 31 deaths.  

The number of neuroinvasive cases reported last year is 83 percent higher than the next highest year, which was 2005. Statewide, 70 percent of these severe cases were reported in Los Angeles and Orange counties.

State health officials say the record-high number of severe West Nile cases coincided with the state's hottest year on record. California's historic drought, they say, could also be partly to blame for the recent rise in infections. The heat and drought amplify the virus' transmission cycle, as birds and mosquitoes come into contact and seek out the same dwindling water sources.

FAQ: Could this be a bad year for West Nile virus?

So far this year, the state is reporting 108 cases of West Nile virus and two disease-related fatalities.  At this point last year, there were 178 human cases, according to state data.

Here in Southern California, the Greater Los Angeles Vector Control District is reporting a rate of West Nile infection in its mosquitoes samples that's below its five-year average, according to spokesman Levy Sun.

Still, Orange County continues to see an elevated rate of infection in its mosquito samples. It's so high, in fact, that the county Vector Control District announced this week it will be conducting aerial spraying over the cities of Orange, Tustin and Villa Park, and portions of Anaheim, Fountain Valley, Garden Grove, Santa Ana and Stanton.

The applications, which are intended to reduce adult mosquitoes and curb the spread of the disease, will take place on Sept. 9 and 10, weather permitting.

Most people who contract West Nile never get sick. Some will have symptoms such as a fever or aches and pains. The disease is transmitted through mosquito bites.

States officials recommend that everyone going outdoors use insect repellent, especially at dawn and dusk when mosquitoes are more likely to bite. They also recommend eliminating sources of standing water, where mosquitoes tend to lay their eggs.