San Francisco is reporting its first human case of mosquito-acquired West Nile virus in seven years, in a local man who is now recovering at home, public health officials announced Tuesday.
The man was hospitalized in late August, and doctors confirmed he was infected with West Nile virus on Sept. 14, public health officials said. He had not traveled recently, meaning it is likely he was infected by a mosquito in the Bay Area, and possibly in San Francisco.
The report comes amid what will probably be the largest outbreak of West Nile in the United States since the virus first arrived in the country 13 years ago. More than 3,000 cases have been reported nationwide so far this year, including 165 cases in California and four in the Bay Area.
“It’s unfortunate we had a case in San Francisco. But given the prevalence of West Nile throughout the country, I don’t think it’s unexpected,” said Dr. Charles Chiu, an infectious disease expert at UCSF.
West Nile virus is carried in birds, which are bitten by mosquitoes that then bite humans. The virus is not passed from human to human, although San Francisco reported a human case in 2010 from an organ transplant
Most people infected by West Nile experience no ill effects, and about 20 percent of those infected suffer flu-like symptoms that last about a week. But in about 1 in 150 cases, the virus infects the brain or nervous system and can cause severe disease and even death. More than 134 deaths have been reported in the United States so far this year; none was in the Bay Area.
Cases of West Nile are rare in San Francisco because the climate isn’t especially hospitable to the insects, which prefer warm, damp conditions for breeding. But a dead bird tested positive for West Nile in San Francisco last week, which is another rare occurrence – there have only been three such reports in San Francisco since 2007.
Given that West Nile is active in San Francisco, public health officials are advising doctors to be aware of symptoms so they know to look for and report cases.
Residents, meanwhile, should make sure there is no standing, stagnant water on their property. And they should take precautions like using insect repellent with Deet, wearing long sleeves and long pants when going to areas where mosquitoes are present, and avoiding those areas at dawn and dusk.
“We don’t want people to be extremely alarmed. People shouldn’t start applying Deet every time they go outside,” said Dr. Curtis Chan, medical director of maternal, child and adolescent health with the San Francisco Public Health Department. “But they should be aware.”