California’s deadliest West Nile season in years could soon get worse

The deadliest year for West Nile disease in California in nearly a decade was made worse by the El Niño phenomenon that brought wet weather and even rain to parts of the state this month.

The mosquito-borne disease has reached a nine-year high so far with 210 confirmed infections and 15 deaths in the state, and cases could increase in the coming months.

Susanne Kluh, executive director of the Greater Los Angeles County Vector Control District, said she saw signs of a poor West Nile season earlier in the year thanks to a spring heat wave and an increase in bird infections. Since then, the district has moved aggressively to control the Culex mosquito population, which spreads West Nile.

Tracking the insects will be a challenge given that they can replicate exponentially: a female can lay 100 eggs, which hatch in three to five days under ideal conditions. Kluh said the early season rains that fell in Southern California this month are “an absolute disaster for mosquito control.”

“Because of El Niño, we had this rain on Sunday two weeks ago that filled all these little springs that had dried up over the summer that mosquitoes love in everyone’s backyard,” she said.

Kluh said he has seen a sudden increase in Culex numbers since the rain, in addition to an increase in the invasive Aedes aegypti mosquito that can spread dengue fever.

“We are seeing an increase in the number of mosquitoes in our local homes,” she said. “And we still have a lot of infected birds.”

The native Culex mosquito spreads West Nile disease in California. And in warm weather, the mosquito and the virus are able to replicate faster and survive longer.

About 20% of people with West Nile will develop symptoms, including fever, headache, nausea and vomiting. In 1 in 150 cases, West Nile will attack the brain and nervous system, potentially causing encephalitis or meningitis, among other life-threatening conditions. This year, 181 people have had a neuroinvasive infection, including a Wisconsin man who collapsed while visiting Lassen Volcanic National Park due to West Nile-related meningitis.

There is no cure for West Nile, but patients can receive supportive care.

Dr. Matt Willis, an epidemiologist and former Marin County Health Department official, said that normally in September, West Nile cases plateau or decline.

But on September 25, the California Department of Public Health reported an increase in cases, with 64 new West Nile cases, up from 25 in the previous week’s report, including 42 in Los Angeles County.

“It’s worrying, you know, it seems to have accelerated into September,” he said.

Both Willis and Kluh clarified that El Niño was not the main reason the West Nile season was so bad. Instead, the increase in viral activity began in winter and spring due to a low-immunity bird population and a heat wave in early March.

“There were a lot of birds that hadn’t been infected before,” Kluh said, explaining that this made them vulnerable to disease. “And because of the warm temperatures, (the mosquitoes) were able to live long enough to go from being infected to infecting the next bird.”

Kluh said the next few days and weeks will be crucial in knowing whether mosquito populations will continue to increase.

“If the nights and days get colder, the virus cannot multiply as quickly,” she said. “And then I hope that the next crop of mosquitoes that we’re going to wipe out from this rain won’t be as effective.”

This is not the first time that an El Niño arrival has been linked to an increase in insect populations. In 1998, during one of California’s wettest El Niño years on record, the Entomological Society of America warned of an insect “boom” across much of the country, with California facing risks such as warm, humid winters that “create ideal conditions for swelling populations of ticks, fleas and mosquitoes.”

This warning came years before West Nile reached the state.

Willis warned that if El Niño brings mild temperatures to the state this winter, it could become an ideal environment for both Culex and the invasive Aedes aegypti mosquito, which could lead to more transmission of West Nile and dengue.

“Warmer weather certainly correlates with greater mosquito activity and faster viral replication of the virus within the mosquito,” he said. “…That’s what we expect for El Niño.”

Still, Kluh said any El Niño-related weather won’t help the mosquito population. If the state receives huge amounts of rain, especially in Southern California, the accumulated water could be washed away, killing mosquito larvae before they can hatch.

In addition to mosquitoes, experts are studying whether El Niño could boost other insect populations. Janet Foley, a veterinarian and disease ecologist at UC Davis, studies tick populations and said humid weather could be ideal for some species, including the common deer tick.

“If it’s cooler and wetter for longer, you can expect more deer tick activity,” she said.

However, too much water could be harmful to ticks, as it could lead to insect-killing fungi in the soil. And because the life cycle of ticks spans several years, it could take months or even years to know if there is a boom in the tick population and, therefore, an increase in the diseases they spread, such as Lyme disease.

Foley said a key factor in whether we see an increase in deer ticks or soft ticks, more common at higher elevations, will be whether more rain leads to an increase in the number of animals these insects prey on.

“What they like best is rodents,” Foley said. “If we have a wet year with a lot of new vegetation in the spring, we’ll have more rodents, we’ll have more ticks.”