Aug. 30, 2024 – Mosquitoes. You are certainly familiar with the high-pitched whine that portends red, itchy welts. These blood-sucking, flying insects are found almost everywhere across the world, and if you truly want to escape them, there are only two places that are completely mosquito-free: Antarctica and Iceland.
Most of the 200 types of mosquitoes that reside in the U.S. mainland and its territories are relatively harmless. But every few years, there are several types that abandon their comfortable, “fly-under-the-radar” existence and make themselves known in national headlines.
Take West Nile virus for example, which landed former National Institute of Allergy and Infectious Diseases Director Anthony Fauci, MD, in the hospital. (He is now home and recovering.) West Nile cases are also climbing in Maryland and Washington, D.C., and have been found in 33 states.
Western Massachusetts health officials are warning residents of certain towns to avoid outdoor activities at dusk and after dark due to two reports of eastern equine encephalitis in humans and two in horses. Cases of the sometimes deadly but rare virus have also been reported in New Hampshire, where one person died from the infection. The sloth fever virus (aka Oropouche fever virus) has made its way into Florida and New York, carried by travelers returning from Cuba and Brazil. In recent years, mosquito-borne outbreaks like dengue, Zika, yellow fever, chikungunya, and malaria have also been reported.
Short of packing your bags (and parkas) and moving to an icy climate, what do you need to know and do to keep yourself safe, not to mention sane?
Don’t Panic
In the U.S. and its territories, only 6% of mosquitoes are known to spread disease. Mosquitoes contract viruses and parasites in two ways, said Stephen Rich, PhD, a microbiologist, professor, and director of the New England Center of Excellence in Vector-Borne Diseases at the University of Massachusetts Amherst.
“In the Northeast, they mostly transmit mosquito to bird and mosquito to bird, and don’t really very often feed on mammals, so they are not a risk to us. It becomes a problem when they bridge from that mosquito-bird cycle into mammal populations, particularly humans and horses,” he said.
In contrast, transmission via mosquito to human, mosquito to human results in things like malaria, yellow fever, and chikungunya, he said. In recent years, many of these diseases have been imported by travelers returning home or by visitors from countries where these diseases are common, although localized cases have also started to emerge.
But transmission does not happen right away; it can take 2 to 3 weeks after a mosquito has fed on an infected animal or human before it can be spread. And the likelihood of developing an infection after being bitten (even by a single infected mosquito) is low. Also, it typically takes time before cases increase to the point where health officials begin to worry.
“We usually see a particular level of infection in mosquitoes before we start to see human cases,” said Rich. “Even if someone gets bitten by a mosquito that’s infected by the virus, there’s probably a 95% chance that person – even if they are infected – won’t have any signs of illness,” he added.
The same is true for West Nile. David Sullivan, MD, an infectious disease doctor and professor of molecular microbiology and immunology at the Johns Hopkins Malaria Research Institute and Johns Hopkins Bloomberg School of Public Health in Baltimore, said that of the roughly “10,000 cases a year, about 80% are not diagnosed. There are a lot of asymptomatic people.”
Sullivan likened the odds of becoming ill from West Nile virus to the chance of being hit by lightning (which CDC places as less than 1 in a million each year).
How Likely Are You to Become Ill?
Most of these cases “are usually self-limiting and don’t have any particularly identifiable features,” said Paul Auwaerter, MD, clinical director of the Division of Infectious Diseases and director of the Fisher Center for Environmental Infectious Diseases at Johns Hopkins Medicine in Baltimore. “They’re all the same: fever, muscle aches, headache, malaise, achy joints,” he said.
One disease in particular that tends to fly under the radar is La Crosse encephalitis, which Brian Byrd, PhD, said mostly occurs in children and is very localized. Byrd, an entomologist and professor of environmental health at Western Carolina University in Cullowhee, NC, said 80% of the cases over the last 20 years have been in western North Carolina, eastern Tennessee, Ohio, and West Virginia.
“It’s persistent, and every year, we end up with kids who are sick; most parents don’t know about it until their child becomes ill,” he said.
This is not to say that all mosquito-borne illnesses are benign; severe illness, lasting neurological damage, and even death may occur, especially in older adults, who are more likely to have severe eastern equine encephalitis, and St. Louis encephalitis. (Although, Sullivan noted that kids under the age of 18 account for at least 20% of severe cases of eastern equine encephalitis). Pregnant women and women considering pregnancy should also be vigilant; both zika and Oropouche fever viruses have been linked to poor pregnancy and fetal outcomes.
Ultimately, the best advice is to pay attention to how sick you feel. “Anyone who feels very ill and has a high temperature (over 103 F) and especially behavioral changes [for example, not thinking properly or having a very severe headache] should seek care,” Auwaerter said.
Prevention Is Key
Mosquito-borne viruses are not a new phenomenon, especially in the continental U.S. “We’re seeing a reemergence of problems that had disappeared,” said Auwaerter. “Malaria was present in the United States up until the 1950s and was only eradicated when the U.S. military used DDT. Yellow fever outbreaks occurred every summer up and down the Eastern Seaboard until 1906,” he noted.
Today, things like climate change (which has extended how long mosquito season lasts), seasonal changes in weather patterns (including more droughts and flooding), as well as expanded detection/advanced technology and mosquito surveillance efforts reinforce the need to remain vigilant and aware. People are traveling more, and they're encountering diseases that they may bring home.
Fortunately, although there is not a cure for mosquito-borne diseases, they can be prevented, especially during the major times in the year when mosquitoes are present, and times of day (dusk and dark) when they like to feed. “Most people don’t want to live in a bubble and want to go outside and enjoy, especially during the summer months,” said Carla McWilliams, MD, division chair of infectious diseases at Cleveland Clinic’s Florida Region. “They just have to do it in an intelligent way.”
Like other experts, McWilliams pointed to three primary measures:
- Dress. To reduce the risk of mosquito bites, cover exposed skin with long-sleeved clothing and pants. Clothing that is treated with the insecticide permethrin prevents bites and repels mosquitos.
- Drain. Remove any standing water from your property whenever possible. Empty saucers from outdoor flowerpots, and repair cracks in pipes or septic systems. Unclog rain gutters and drainpipes, and be sure that rain barrels are covered as much as possible.
- Defend. Wear repellent like DEET, the DEET alternative icaridin (picaridin), or oil of lemon eucalyptus. The Environmental Protection Agency offers a searchable tool on its website to find a safe mosquito repellent for you and your family based on ingredient, age, and length of time you need protection (all day or an hour or two for gardening or a picnic).
Finally, don’t hit the panic button. As Rich aptly noted, “the likelihood of finding virus in a single mosquito is vanishingly small.”