Estimated reading time: 4-5 minutes
LOS ANGELES – A historically wet winter and now warmer summer are leading to “pretty big” West Nile virus warning signs in the western United States, public health and mosquito control experts say, suggesting residents take care to avoid bites.
“The number of mosquitoes hatched after the spring snowmelt is pretty huge in a lot of the states, whether it’s Colorado or Utah or California,” says Daniel Markowski, technical advisor for the American Mosquito Control Association.
Many districts are starting to see West Nile virus in mosquitoes, he said, which means “you have the right temperature, the right mosquito population and the right time of year for localized outbreaks to occur.”
West Nile virus is the most common cause of mosquito-borne disease in the continental United States, with thousands of cases reported to the US Centers for Disease Control and Prevention each year, said Dr. Erin Staples, the medical epidemiologist in charge of the agency’s arboviral treatment. disease branch.
The virus doesn’t cause symptoms in about 80% of people who get it, but 1 in 5 may have a fever with muscle aches, headaches and a rash, Staples said. A smaller proportion of people – about 1%, according to Dr. Vicki Kramer, chief of the California Department of Public Health’s vector-borne disease section — develops severe disease, an infection involving the central nervous system that can cause paralysis or death.
“Some people who recover have persistent neurological symptoms for years, so it can be very serious,” Kramer said. Older people and people with medical conditions such as diabetes, high blood pressure or a weakened immune system are most at risk. There are no vaccines or treatments for the virus in humans.
Dusk and dawn
West Nile, named for the region of Uganda where it was identified in 1937, is typically spread in the United States by a mosquito called Culex that is found throughout the country, Staples said.
The mosquitoes, which can also transmit St. Louis encephalitis virus, “tends to like the dusk and dawn period,” she said, unlike the kinds of mosquitoes that transmit diseases like dengue, Aedes aegypti and Aedes albopictus, which are more active during the day.
West Nile is spread as mosquitoes bite infected birds and then humans, making both bird and mosquito populations important parts of public health surveillance for the disease.
And while human cases are still relatively low, at 90 across the U.S., August is typically the month that sees the most cases. And those monitoring systems in western states are starting to produce “pretty big” warning signs for a potentially more active West Nile season, Markowski said.
In California, 147 dead birds had been found per July 28, nearly double what is typically seen this time of year, based on the state’s five-year average. The signals seen in “sentinel chickens” — flocks that vector control agencies use to monitor where the virus is spreading — are similarly ahead. (Chickens are especially useful for monitoring West Nile, Kramer said, because they are not susceptible to the disease but can carry antibodies against the virus in their blood.)
And the amount of virus found in mosquito samples has also increased compared to the average over the past five years.
What to watch out for
“When you start seeing increases in West Nile virus in these mosquitoes, it raises the alarm that people need to take measures to prevent being bitten by mosquitoes,” Staples said.
However, she noted that elevated surveillance signals do not automatically translate to higher human cases, as the chain of transmission involving mosquitoes and birds is complicated.
But the weather cycle of the past year has created particularly unfavorable conditions, Kramer said, after record-breaking winter rainfall.
“Mosquitoes thrive in standing water, so the more standing water you have, the more habitat there is for mosquitoes, especially this year early in the season because of snowmelt and precipitation,” she said. She also noted that residents may use more water for landscaping and other purposes with drought restrictions lifted.
And now, with recent temperatures hitting record highs in the West, “the warm weather is speeding up the transmission cycle of West Nile virus because mosquitoes are developing faster and the virus is multiplying faster in the mosquitoes,” she said.
Mosquito control authorities are targeting the mosquitoes to try to limit the population in areas where West Nile has been detected. Kramer also encouraged California residents who see dead birds to report them to WestNile.ca.gov.
“The public can really help us detect and monitor West Nile virus activity,” she said.
To protect against mosquito bites, experts recommend using repellent or wearing long sleeves and pants if tolerable, especially in the early morning and evening; keep screens tightly fitted to doors and windows; and be aware of draining standing water where mosquitoes lay their eggs.
Mosquitoes, says Kramer, “are, after all, the most dangerous animal in the world.”