May 24, 2019 -- Mosquito season has already begun in the South and will spread north in the next few weeks, says Janet McAllister, PhD, CDC entomologist at the arboviral diseases branch of the division of vector-borne diseases in Fort Collins, CO.

Local disease control officials in Louisiana have already begun routine trapping of mosquitoes to test for West Nile even though the peak of West Nile and St. Louis encephalitis season isn’t until late July or early August, McAllister says.

“But mosquitoes are out and birds are starting to migrate,” she says.

As the climate changes, spring has been warmer and wetter than average across the nation, which could mean many Americans will deal with more than the usual number of mosquitoes this summer.

“If you have a rainy, warm season, it is always a good thing for ticks and mosquitoes,” says Dina M. Fonseca, PhD, molecular ecologist and professor at Rutgers School of Environmental and Biological Sciences in New Brunswick, NJ.

The average U.S. temperature for April was almost 2 degrees above average, and precipitation was about an inch above average.

Mosquito activity starts to pick up when temperatures move above 50 degrees, and if there are lots of wet places for them to breed, that can translate into large populations of the pest, scientists say.

Predictions and Guesses

Fonseca says predictions for any particular mosquito season are challenging because a sudden change in the weather, such as a long drought, can dampen, increase, or shift where mosquito populations emerge and whether they are more aggressive biters.

“Mosquito life cycles are such that a period of very wet and then very dry can make a difference,” she says.

Some mosquitoes carry potentially dangerous infectious diseases like West Nile and Zika. Between 2004 and 2016, the number of people sickened by mosquito, tick, and flea bites tripled, according to the CDC.

The mosquito-borne illness that has caused the most disease in the U.S. is West Nile virus. Culex mosquitoes are the primary source of the virus, which mosquitoes pick up after biting migrating birds.

“Everyone should be vigilant,” McAllister says. “Each year, hundreds of thousands of people are bitten by infected mosquitoes and get West Nile infection.”

West Nile disease has been found in every state except New Hampshire and Hawaii. In 2018, there were 2,544 cases of disease caused by West Nile virus, up 21% from 2,097 in 2017. The agency says that the number of people infected with West Nile is likely much higher -- from 42,750 to 99,750 -- but many cases aren’t reported.

In its mildest form, West Nile infection can cause flu-like symptoms, including fever and fatigue, which can resolve within a few days. In its most severe form, the virus can invade the nervous system and cause encephalitis and meningitis, or West Nile poliomyelitis, which can lead to paralysis and in rare cases, death. In 2018, 127 people died from West Nile.

The states with the highest number of cases of West Nile in 2018 were Montana, North Dakota, South Dakota, Nebraska, Iowa, and Mississippi, which are along the flight path of birds from south to north.

Other disease-carrying mosquitoes in the U.S. include Aedes aegypti, Aedes albopictus (also called the Asian tiger mosquito) and Aedes triseriatus. The aegypti and albopictus species are found in much of the U.S. and carry and transmit dengue fever and eastern equine encephalitis. They also both carry the Zika virus, which can cause severe birth defects in an unborn child. In 2018, no one got Zika from a mosquito bite in the U.S., though it has continued to spread in U.S. territories.

The triseriatus species, found in the upper Midwest, mid-Atlantic, and Southeastern U.S., can carry the La Crosse encephalitis virus. This disease is rare. In 2017, there were 63 cases of La Crosse encephalitis.

Testing and Spraying

Mosquito control programs in states, counties, and cities across the country are in charge of watching, testing, and spraying to control these disease-carrying mosquitoes. Disease control programs are also testing biological alternatives to spraying.

The federal government allowed 20 states in 2017 to experiment with releasing mosquitoes infected with a naturally occurring insect bacteria that is safe for humans, pets, and the environment and reduces overall Aedes aegypti populations. Researchers are also developing genetically modified mosquitoes that could be used to kill fellow mosquitoes.

“Both of these techniques show promise, but logistically they are difficult to deploy,” says Joe Conlon, technical advisor to the American Mosquito Control Association. “They are unlikely to serve as the sole means of control but will be utilized as adjuncts to known mosquito control products.”

Most state programs are in need of more resources to adequately control mosquito populations, says E. Oscar Alleyne, DrPH, chief of programs and services for the National Association of County and City Health Officials.

In October 2017, Alleyne’s organization published a report that says 84% of mosquito control programs in the U.S. need improvement. Since then, Alleyne says his organization has worked with programs nationwide to boost their surveillance and testing.

Mosquito control programs are also working closely with communities to address environmental concerns about spraying, including worries that sprays and chemical deterrents kill bees and other pollinators.

“We are acutely aware of our product’s risk to pollinators and go well out of our way to ensure that beekeepers are notified and that we spray at times when bees would not be exposed,” Conlon says.

Given that many state and local disease control programs may not be able to eliminate mosquito risks, public health officials urge people to pay attention to mosquito disease information posted by the CDC and local public health departments.

The CDC also urges people to:

  • Use insect repellent. DEET is an effective repellant, but “you don’t have to use DEET,” says the CDC’s McAllister. “If  you don’t want to use repellent because you don’t like DEET, find one you like that is EPA-approved and use it." Besides DEET, other repellents with EPA-approved and effective ingredients include picaridin, IR3535, oil of lemon eucalyptus (also known as para-methane-3,8-diol), and 2-undecanone.  But note that oil of lemon eucalyptus is not a do-it-yourself product. It’s sold as the main chemical in certain brands of repellent, but it is not the same as the essential oil sold in natural food stores. The National Pesticide Information Center has a list for EPA approved products.
  • Cover up: Wear long-sleeved shirts and long pants, especially at dawn or dusk when some mosquitoes bite. Wear clothing treated with permethrin.
  • Keep mosquitoes outside: Use air-conditioning or window and door screens. If you cannot, sleep under a mosquito bed net. Stop mosquitoes from laying eggs. Check your property for standing water, including in flower pot saucers, bird baths, tires, toys, buckets, trash containers, and plant containers.

Show Sources

NOAA: “National Climate Report – 2019.”

CDC: “Illnesses from Mosquito, Tick, and Flea Bites Increasing in the US;” “Protection against Mosquitoes, Ticks, & Other Arthropods;” “West Nile Virus and Other Nationally Notifiable Arboviral Diseases — United States, 2017;” “West Nile Virus: Preliminary Maps and Data for 2019;” “West Nile Virus: West Nile Disease Cases by State 2019;” “Saint Louis Encephalitis: Statistics and Maps;” “West Nile Virus: West Nile Virus Neuroinvasive Disease Incidence by State 2018;” “West Nile Virus: Potential Range in the US;” “Zika Virus: 2018 Case Counts in the US;” “La Crosse Encephalitis: Epidemiology & Geographic Distribution.”

NACCHO: “Mosquito Control Capabilities in the US.”

National Pesticide Information Center: “Find Your Local Resources;” “Choosing and Using Insect Repellants.”

Janet McAllister, PhD, entomologist, CDC. Dina M. Fonseca, PhD, molecular ecologist, professor, Rutgers School of Environmental and Biological Sciences, New Brunswick, NJ.

Joe Conlon, technical advisor, American Mosquito Control Association.

E. Oscar Alleyne, DrPH, chief of programs and services, National Association of County and City Health Officials.

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