There are more than 200 species of mosquitoes in the United States, but only a handful are known to be dangerous to humans because of the diseases they carry.
When the conditions are right, swarms of these bloodsuckers can fill the air, ruining picnics, ball games, and campsites. But now, the concern for mosquito-borne disease threats is rising. While a mosquito bite may have long been merely an itchy inconvenience, getting bitten today could be much more serious.
The good news is mosquitoes aren’t indestructible monsters; a well-aimed finger flick can end one forever. Most live just a few weeks and rarely venture more than a few miles from where they’re born. They aren’t fans of the cold — most mosquitoes stop biting when temperatures drop below 50 degrees Fahrenheit. Freezing temperatures can kill off adult mosquitoes, though their eggs can survive until the next warm season.
Most of us in the US haven’t needed to worry too much about vector-borne disease threats. Here, diseases such as malaria and yellow fever have dropped in the rankings as public health threats over the 20th century with concerted efforts to keep them in check.
But this progress is fragile — and it’s time to be on the lookout.
Dangerous mosquitoes are starting to regain ground. Temperatures are rising on average due to climate change. That is opening up new habitats for these bloodsuckers while making them more vivacious in places where they’re already established. In general, the ranges of the mosquito species we need to be most worried about in the United States are expanding northward. Newer invasive mosquito species that arrived as stowaways are also setting up shop across the country. It’s not just the geographic range — warmer temperatures are allowing mosquitoes to survive at higher elevations, too.
Mosquitoes are also very sensitive to humidity and rainfall. They need water to lay their eggs and for their larvae to mature, and the climate is changing in rather unfortunate ways in this regard, too; more extreme rainfall events are creating the standing water conditions that are ideal breeding grounds for some of the more dangerous disease carriers.
Humans have also stacked the deck in mosquitoes’ favor. Deforestation and ecosystem losses mean that there are fewer natural mosquito predators like bats and birds to keep a lid on their numbers, while urbanization — with its more impervious roads, sidewalks, and parking lots — creates more places where water can pool and mosquitoes can breed. International travel is also much more feasible these days, and more people are heading back and forth from places where mosquito-borne diseases are more common. If enough travelers get infected abroad and return home to the US, they can become the seeds for local disease outbreaks as mosquitoes spread the pathogens among their neighbors. Newer diseases like chikungunya and Zika have started to make inroads into the country as well.
There are seven key mosquitoes worth tracking in the US that have benefited from these changes and are known to transmit some of the most common and dangerous infections. These are some of the biters you’re most likely to see and could be the ones that make you sick.
Containing mosquitoes requires a coordinated strategy between cities, states, and the federal government to anticipate where they’re going and quench outbreaks. But knowing which mosquitoes are around you can help you track the potential threats in your neighborhood and take steps to protect yourself from annoying bites — or worse.
Meet the 7 key mosquitoes that spread disease in the US
In the grand scheme of things, it’s actually uncommon for a given mosquito to spread any kind of disease at all. First, she — only female mosquitoes bite — has to draw blood from a host that is already carrying a pathogen.
The pathogen — whether a bacterium, virus, or parasite — has to then be able to survive inside the mosquito, replicate, and make its way to her salivary glands. The mosquito then has to bite another uninfected host to transmit the germ. So, the spread of mosquito-borne disease is a numbers game, a function of how many mosquitoes and how many infected hosts are around.
In America’s growing urban and suburban centers, there are a lot of opportunities.
But because so many factors have to fall in line, it’s hard to predict outbreaks of mosquito-borne disease. Perfect temperature and rainfall conditions might create a bumper crop of mosquitoes, but if there are no infected animals or people around, there won’t be any transmission. Still, more favorable conditions for mosquitoes increase the odds that a disease will spread. Figuring out which ones are spreading and what infections they might be carrying can help health officials determine what kinds of pesticides to use, where they should target spraying, what kinds of repellents are effective, and the symptoms individuals should try to identify.
Right now, West Nile virus is the biggest mosquito-borne threat in the US. The virus was first detected in the US in 1999. Birds like crows are its main host. This disease causes headaches, fever, and body aches, though most people don’t develop symptoms at all. According to the Centers for Disease Control and Prevention, about 1,200 people in the US develop severe illness from the virus annually, which ends up killing more than 120 people per year.
Mosquitoes from the Culex genus are the main vectors for West Nile virus and can be found throughout the country, though different species have regional specialties. They can also carry St. Louis encephalitis and Eastern equine encephalitis, rare but dangerous diseases that can cause fatal brain infections. They tend to bite in the evening and at night, and they’re often found in urban areas.
Here are the seven types of mosquitoes to watch out for. Before (or maybe after) you swat one, take a look and see if you can identify the species. And remember: It’s only the females in each of these insects that you need to worry about, and that gives you a great starting point for narrowing in on potential threats. In general, female mosquitoes are larger than males and males tend to have bushy, feathery antennae, while females have straight, unadorned antennae.
Culex pipiens, the common house mosquito, originated in Europe but is now found on every continent except Antarctica. It’s a brown-colored mosquito, between 4 and 10 millimeters long, around the size of the nail on your little finger.
Culex quinquefasciatus, the southern house mosquito, looks similar to Culex pipiens and can cross-breed with it, though it tends to prefer warmer climates. Its name, “quinquefasciatus” in Latin, means “five bands” and refers to the five black stripes across its abdomen.
Mosquito illustration by Jilian Ditner for Vox
Culex tarsalis, the Western Encephalitis mosquito, is much more prevalent in the Western US, especially in wetlands and areas with irrigated farms. It has distinct black and white bands on its legs and a white band on its proboscis.
Mosquito illustration by Jillian Ditner for Vox
Mosquitoes from the Anopheles genus were once some of the deadliest insects in the US, because they spread the parasite that causes malaria in humans. The disease was a scourge for much of US history until a national campaign of insecticide spraying and destroying mosquito breeding sites drove down infection rates. Malaria was declared eliminated in the US in 1951, but the Anopheles mosquitoes that spread it are still around and rebounding. In 2023, the US saw the first locally acquired cases of malaria in 20 years in Florida and in Texas.
Anopheles mosquitoes have a pair of long appendages called palpi on either side of their proboscis, giving their heads a distinctive trident. They are often found resting with their abdomens pointing in the air and are about 5 millimeters long, less than a quarter inch.
Anopheles freeborni, the western malaria mosquito, is brownish-black with spotted wings. It tends to be found in rural areas in the West, breeding in ditches and culverts with vegetation in states like California, Oregon, and Arizona.
Mosquito illustration by Jilian Ditner for Vox
Anopheles quadrimaculatus, the common malaria mosquito, is mainly found in the Eastern US. The name “quadrimaculatus” refers to the four dark spots often found on its wings.
Mosquito illustration by Jilian Ditner for Vox
The Aedes genus can spread a variety of viruses, including the pathogens behind yellow fever, chikungunya, Zika, and dengue, which sickens almost 100 million people around the world each year. These mosquitoes are notorious container breeders, meaning they only need tiny pools of water — such as in flower pots, old tires, or bottle caps — to reproduce. That’s why they’re so prevalent on the margins of urban development and why they’re so difficult to control.
Aedes aegypti, the yellow fever mosquito, has white stripes on its thorax in the shape of a lyre, and it likes to bite people indoors.
Mosquito illustration by Jillian Ditner for Vox
Aedes Albopictus, the Asian tiger mosquito, has a striking black and white stripe coloration (hence “tiger”). It’s a daytime biter and can reach 10 millimeters in length. It likely arrived in the US in 1985 and now has a foothold in most of the country.
Mosquito illustration by Jillian Ditner for Vox
Mosquitoes are on the move
Again, a lot of things have to align to ignite a mosquito-borne disease outbreak, but the US is becoming more hospitable to the insects that spread some of the most worrying illnesses. Mosquito-borne diseases are often spotted in travelers returning from areas like the tropics where these illnesses are endemic, but in recent years, the US has seen cases of them spread to people who haven’t been abroad.
With warmer average temperatures, mosquitoes have longer breeding seasons as spring starts earlier and summer ends later. Their reproductive cycles accelerate, as well, so more generations can arise in a season. Pathogens also replicate faster inside the mosquitoes when it’s warmer, increasing the likelihood that a bite will lead to an infection.
However, the various mosquito species all have slightly different preferences for their environments. Some like the fast-paced city life and flourish in places like potholes and garbage dumps, while others prefer the idyllic country life with its ponds, ditches, and ample vegetation. Some are more tolerant of the cold (to a point). Some mosquitoes will venture miles, while others will spend their whole lives across just a couple of city blocks.
Depending on how much average temperatures continue to rise, the suitable range for each mosquito species will look different. Even under a scenario where humanity’s greenhouse gas emissions eventually taper off and global temperatures stabilize by the end of the century, it will still be warmer than it was, and most disease-spreading mosquitoes will have a much larger tolerable range across the US.
The upside is that researchers can anticipate these changes to the environment, producing early warnings of where the vector-borne disease threat is going to get worse and the new places where it may emerge. Health departments can begin surveillance to proactively find out which mosquitoes are present and what pathogens they’re carrying.
From there, they can deploy interventions like preventing water accumulation or spraying targeted pesticides. The growing range of mosquitoes also makes a compelling case for preserving habitats for mosquito-eaters like bats to control their numbers. In the long term, humanity needs to limit greenhouse gas emissions to slow changes in the climate. Mosquito-borne diseases are a threat, but they can be managed with good planning. Otherwise, there will be more dangerous infections buzzing over the horizon.
