West Nile virus landed in the United States in the summer of 1999, starting in New York possibly with the blood of a sick bird on a ship or via an infected mosquito on a plane. Soon it had afflicted people in Queens with brain inflammation and killed birds at the Bronx Zoo.
Four years later, the virus had migrated across the country to California, far from Uganda, where it was first isolated in 1937. As of Sept. 1, West Nile has killed 229 people in the Golden State and sickened nearly 5,600. Last year a record 53 people died in California of the virus, and this year has the potential to end up as bad or even worse.
While the recent arrival in the U.S. of the Zika virus is getting most of the attention, public health experts consider West Nile to be a much more potent threat in California than Zika will ever be.
Through Sept. 1, the state has tallied 78 human West Nile cases in California this year – including a pair of deaths in Sacramento and Yolo counties. But it takes weeks for reporting and verification of West Nile cases to make it through the system.
1,900 Number of people who’ve died of West Nile virus since 1999
Gary Goodman, manager of the Sacramento-Yolo Mosquito & Vector Control District, expects the real numbers are far higher.
“I think in terms of human cases it’s going to be equal to or conceivably even worse than what we had last year,” he said.
Last year, 783 cases of West Nile were reported in California and 801 the year before that, the most in a decade.
More infected mosquitoes and birds have been found dead from West Nile than at this time last year, with the 398 dead birds counted as of Friday in Sacramento County, the largest number of any California county.
West Nile activity in the Sacramento area this summer appears to have peaked and begun its decline. But the peak comes later in Southern California, and the virus remains a threat into October.
It’s not clear why West Nile activity has been so strong this summer. Officials blamed the high numbers in 2014 and 2015 in part on the California drought. Birds carry the virus and mosquitoes spread it through bites. The idea is that with fewer sources of available water, birds and mosquitoes crowded together at remaining water sources and spread the virus among one another.
Goodman speculated this year’s easing of the drought led to a rebound of the mosquito population as water became more available for the first time in years – keeping West Nile activity high.
“This year we’ve seen some of the mosquitoes that breed in larger agricultural areas at higher numbers than usual, and I think that’s explainable in part to the additional water,” said Chris Barker, a specialist in the research of mosquito-borne viruses at UC Davis.
Rice fields are a huge source of mosquitoes in the Sacramento area, according to public health experts. Woodland, Davis, West Sacramento and Natomas are ringed by such fields, with acres of standing water and vegetation a prime breeding ground for mosquitoes.
West Nile is particularly hard to stop because of the role that birds play in its spread.
Goodman said the Sacramento-Yolo Mosquito & Vector Control District has sprayed over 600,000 acres for mosquitoes so far this year, mostly in rural stretches but also in Woodland and the Arden Arcade area of Sacramento. The spraying costs run more than a million dollars.
Most people who contract West Nile virus will have no symptoms or possibly a mild infection that might include vomiting and fever. But it can lead to a serious neurological illness that sometimes includes coma, seizures or paralysis, with recovery taking months. Nearly 44,000 cases of West Nile have been reported in the U.S., and more than 1,900 people have died.
West Nile is particularly hard to stop because of the role that birds play in its spread. After the virus arrived in New York, migratory birds helped carry it across the country, thwarting efforts to stop it through mosquito eradication. No one was sure which birds were the carriers.
“I am not sure there would have been any way that we could have prevented West Nile from taking hold in the state,” said Desiree LaBeaud, an associate professor of pediatric infectious diseases at Stanford University who specializes in viruses spread by mosquitoes.
The Zika virus is much different. Recently striking the U.S. with a homegrown outbreak in Florida, Zika is primarily spread by two mosquito species – Aedes aegypti and Aedes albopictus – that have not been detected in Sacramento County. Zika can also be spread through sex.
West Nile is a much bigger threat in California.
Chris Barker, UC Davis specialist on mosquito-borne viruses
There’s no reason to think Aedes mosquitoes won’t arrive in Sacramento. They’ve already been found in other areas of California including Los Angeles, San Mateo, Madera and Fresno.
Some research also suggests the far more common Culex mosquito that transmits the West Nile virus might also spread Zika, which Goodman said “would definitely complicate things.”
But he and other mosquito experts don’t believe Zika has the potential to approach the level of West Nile in California. California is a prime spot for West Nile with a bird population of corvids and jays to host the disease and an abundance of the Culex type of mosquitoes to spread it.
The state could experience small Zika outbreaks as Californians infected elsewhere spread the virus when they come home, either through sex or being bitten by a mosquito at home that goes on to bite others, said UC Davis epidemiologist Barker.
“But in terms of an endemic pathogen that is going to be with us from now on, West Nile is a much bigger threat in California,” he said.