A nonagenarian from La Jolla became the first confirmed case of West Nile in San Diego County this year, the County Health and Human Services Agency announced Friday.
The 91-year-old man was hospitalized in September for encephalitis, an inflammation of the brain, but testing by the California Department of Public Health confirmed on Thursday he contracted the West Nile virus.
He has been released from the hospital and is at home recovering, according to the county.
County health officials determined the man was infected from a local mosquito bite because he had not traveled before becoming ill. Mosquitoes trapped in the area near the man's home, however, tested negative for West Nile, according to the County's Department of Environmental Health.
West Nile virus mainly affects birds but can be transmitted to humans by certain mosquito species native to San Diego that fed on an infected bird or animal and then bit a person.
The presence of the West Nile virus was first spotted in the county in 2003 but the first confirmed human infection was in 2007.
Last year, two people were infected with West Nile in San Diego, one from acquired it from outside the county and one acquired it locally, county spokesman Tom Christensen said.
There were, however, 22 cases in 2016 and two people died.
The majority of people infected with West Nile virus don't know they were infected because they exhibit no symptoms.
About 10 percent of those infected suffer mild symptoms, such as headache, fever, nausea, fatigue, skin rash or swollen glands. In rare cases, people can become extremely ill and the disease can be fatal.
To prevent disease-carrying mosquitoes from breeding, residents were urged to dump out or remove anything inside or outside their homes that can be a breeding ground, such as plant saucers, rain gutters, buckets, old tires, wheelbarrows and toys.