A mosquito bite is usually just an itchy annoyance\u2014but sometimes it can lead to serious illness.<\/p>\n
That\u2019s because mosquitoes can carry a range of microbes that infect humans<\/a>, including four species of malaria parasites. Members of the genus Plasmodium,<\/em> these malaria-causing microbes hitchhike on female mosquitoes of the genus Anopheles<\/em> and get transferred to humans when those insects bite us. According to the Centers for Disease Control and Prevention, malaria killed 627,000 people worldwide in 2020. The U.S. effectively eradicated the disease by 1951, although small outbreaks, usually carried here by infected international travelers, still pop up occasionally. As of press time, this spring and summer, seven people in Florida and one in Texas were reported to have caught malaria.<\/p>\n In addition to malaria, mosquitoes carry West Nile virus, which is the most common cause of mosquito-borne illness in the U.S. and is primarily transmitted by mosquitoes in the genus Culex<\/em>. In 2022 1,126 Americans were diagnosed with the disease, and 90 people died from it. All told, the CDC tracks nine mosquito-borne diseases in a surveillance system<\/a> dedicated to monitoring so-called arboviruses\u2014viruses transmitted by arthropods\u2014that are spread by mosquitoes and ticks. (Malaria, a parasitic infection, isn\u2019t included in the database.)<\/p>\n
\u201cThere are lots and lots of mosquitoes and lots and lots of mosquito-borne diseases that humans are impacted by,\u201d says Sadie Ryan, a medical geographer at the University of Florida. Most of these infections are much more common in tropical climates and in less common in developed countries. Researchers are concerned that this might change as the geographical range of mosquitoes shifts with climate change<\/a>.<\/p>\n As peak mosquito season looms over much of the U.S., here\u2019s what to know about the blood-hungry vectors and the diseases they can carry.<\/p>\n Overall, levels of mosquito-borne disease in the country are quite low and have been since the mid-20th century, when malaria was effectively eradicated through a nationwide campaign to spray insecticides and otherwise kill mosquitoes.<\/p>\n \u201cMosquito-borne disease in the U.S. currently, compared to historical [levels], is leaps and bounds lower,\u201d says Katie Westby, a disease vector ecologist at Washington University in St. Louis. She notes that she\u2019s more concerned about rates of tick-borne diseases<\/a> such as Lyme disease and babesiosis. But Westby and other experts warn against complacency with mosquitoes. \u201cBy no means do I think that the situation is going to last forever without continued vigilance,\u201d she says.<\/p>\n West Nile, which was first reported in the U.S. in New York State in 1999<\/a>, is now the country\u2019s most common mosquito-borne disease. Since its introduction in the country, the disease has infected more than 56,000 Americans, and it has killed nearly 2,800.<\/p>\n
Unlike some pathogens, mosquitoes don\u2019t catch West Nile by biting an infected person and then transmitting the disease to others they bite. Instead a mosquito picks up the virus by feeding on an infected bird and then transmits it to a human through a bite. While the virus\u2019s avian origin reduces the odds of human infection, it also complicates management because neither vaccinating nor culling huge numbers of birds is likely to be a cost-effective and publicly palatable solution.<\/p>\n West Nile disease is \u201cdifferent for me than things like dengue and malaria because it is a spillover disease,\u201d Ryan says. For both dengue and malaria, mosquitoes get infected when they have a blood meal from an infected person. With West Nile, \u201chumans are the \u2018dead-end\u2019 hosts; we\u2019re not the reservoir,\u201d she says. \u201cAnd so the management for West Nile has to be very different.\u201d<\/p>\n Dengue also occurs in the nation at noticeable levels. As of June 1, the CDC had reports of 129 cases of dengue in U.S. states and 256 cases in U.S. territories this year. Last year the disease infected 1,188 people in states and 828 people in territories. Other recent years have seen fewer than 500 infections within U.S. states. A surge in dengue infections<\/a> in Central and South America, however, has researchers concerned that this year\u2019s infection rates may be unusually high.<\/p>\n Dengue is spread by different mosquitoes than those that carry West Nile or malaria; only Aedes aegypti<\/em> and Aedes albopictus<\/em> transmit dengue. These Aedes<\/em> mosquitoes are particularly nasty because they can also transmit Zika<\/a> and chikungunya<\/a>, among other viruses. Fortunately, the CDC has no reported cases of either of these infections being acquired locally within U.S. states for several years.<\/p>\n
Other mosquito-borne diseases also remain rare nationally. La Crosse encephalitis virus<\/a>, which is principally carried by Aedes triseriatus<\/em> mosquitoes, has caused, on average, fewer than 100 cases of human disease in the U.S. each year since 2003. And Jamestown Canyon virus<\/a> has caused fewer than 300 known cases of human disease in the U.S. since 2011. Similarly, since 2003 the CDC has reports of fewer than 300 cases of St. Louis encephalitis, which, like West Nile, is principally spread by Culex<\/em> mosquitoes that bit infected birds.<\/p>\n Louisiana has reported one case of eastern equine encephalitis<\/a> this year. The CDC has never received reports of more than 40 infections of this virus in a year, and fewer than 200 people have caught it since 2003. This pathogen has a complicated life cycle: it is mainly transmitted among birds by a particular mosquito, although multiple other species of mosquitoes can pass it to horses and humans as dead-end hosts.<\/p>\n Mosquito-borne diseases are challenging to diagnose. Many people don\u2019t display symptoms at all: for instance, only about one in five people infected with West Nile and one in four people infected with dengue show signs of infection.<\/p>\n The high rate of asymptomatic infections can make spotting and controlling outbreaks more difficult. \u201cYou can never really be sure how much transmission is going on because a lot of the transmission that\u2019s happening is kind of under the radar,\u201d says Angelle Desiree LaBeaud of Stanford Medicine Children\u2019s Health, who specializes in pediatric infectious disease and global health. \u201cYou have this population of people that isn\u2019t necessarily showing signs or taking precautions to keep themselves from spreading infection.\u201d (Because most of these diseases can\u2019t be transmitted directly between people, those precautions focus on preventing additional mosquito bites that spread the virus to new carriers.)<\/p>\n And people who do notice an infection tend to appear in doctors\u2019 offices with generic complaints such as fevers, headaches and nausea.<\/p>\n \u201cThey can present in a lot of similar ways to each other,\u201d says Shelby Lyons, an epidemiologist at the CDC. \u201cThese diseases can be difficult to identify: their clinical signs and symptoms can mimic a lot of other diseases, and arboviruses aren\u2019t always at the top of mind for clinicians.\u201d<\/p>\n There are also no specialized vaccines or treatments for mosquito-borne viruses\u2014doctors can only treat the symptoms.<\/p>\n The CDC doesn\u2019t have a way of forecasting each year\u2019s mosquito-borne disease season, Lyons says. \u201cIt\u2019s really, really difficult for us to predict from season to season,\u201d she adds.<\/p>\n Mosquito-borne diseases are most common in the U.S. from July to September or October, Lyons says. During this season, her office monitors daily case reports for any concerning trends. So far infections have been reported from the Midwest and West, as well as the Southeast; most of those are cases of West Nile, of which there were 69 cases nationwide as of July 25.<\/p>\n Lyons says that infection rate is in line with previous years. \u201cOverall, nationally, looking at the data, it really seems to not be very different from what we\u2019ve traditionally seen,\u201d she says. \u201cIt looks about on par with what we would expect.\u201d<\/p>\n First, the infection must be in the area\u2014either in animal hosts such as birds (in the case of West Nile) or in humans who had previously traveled to one of the places where these diseases are more common and were bitten by a carrier mosquito there.<\/p>\n Typically, mosquitoes prefer hotter, wetter climates. \u201cThey like warm and humid conditions; they like to be in areas with lots of places to breed for their larvae to develop. Access to hosts is really important, [as are] shady sites for resting when it\u2019s hot, access to sugar meals,\u201d Westby says.<\/p>\n More northern parts of the country have traditionally had fewer mosquitoes\u2014and fewer mosquito species\u2014but that\u2019s changing because of the climate crisis. As mosquitoes move into new territories, they will begin biting people who are less accustomed to thinking about the threat of mosquito-borne diseases, LaBeaud says. \u201cAreas that, before, weren\u2019t really ripe for these infections are now going to become ripe,\u201d she says.<\/p>\n Many Americans are protected by their lifestyle, unlike people in less developed countries. American homes are more likely to make use of window screens or even air-conditioning, which keep mosquitoes outside. And while construction, agriculture and other industries can put workers directly in insects\u2019 paths, office jobs tend to further reduce exposure. \u201cWho is exposed to mosquito-borne disease is highly related to socioeconomic conditions,\u201d says Kim Medley, an ecologist at Washington University in St. Louis.<\/p>\n If you live in a home with outdoor space, reduce places where water can accumulate, such as garbage bins, potted plant saucers, birdbaths and other home equipment. A. aegypti <\/em>and A. albopictus<\/em> are known as \u201ccontainer species\u201d because they lay eggs in small pools of standing water. The CDC recommends emptying and scrubbing out any such items once a week to reduce the overall number of mosquitoes.<\/p>\n The best step you can take to protect yourself from mosquito bites is to use an insect repellant recommended by the Environmental Protection Agency, such as DEET. And pay attention to recommendations and warnings from your local health department because that office will have the best knowledge of whether any mosquito-borne diseases have been reported in your area. If you\u2019re traveling outside of the U.S., consult the CDC\u2019s guidelines for your destination<\/a>.<\/p>\n Don\u2019t panic with every mosquito bite. Vigilance is key to ensuring that mosquito-borne diseases remain in check, but humans may never fully vanquish these pathogens. \u201cMosquitoes have been doing mosquito things for millennia, and they\u2019re really, really good at it, despite our best efforts to keep them at bay,\u201d Medley says.<\/p>\n<\/div>\n![\"A](\"https:\/\/static.scientificamerican.com\/sciam\/assets\/Image\/2023\/mosquito_1-d.png\"\/)
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\n<\/figure>\nHow common are mosquito-borne diseases in the U.S.?<\/h2>\n
How big of a risk is West Nile virus?<\/h2>\n
![\"A](\"https:\/\/static.scientificamerican.com\/sciam\/assets\/Image\/2023\/mosquito_2-d.png\"\/)
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\n<\/figure>\nWhat other infections do mosquitoes transmit in the U.S.?<\/h2>\n
![\"An](\"https:\/\/static.scientificamerican.com\/sciam\/assets\/Image\/2023\/mosquito_3-d.png\"\/)
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\n<\/figure>\nWhat are the symptoms of these diseases, and how are they treated?<\/h2>\n
What might this summer and fall look like?<\/h2>\n
What factors affect the prevalence of mosquito-borne diseases in the U.S.?<\/h2>\n
How can you protect yourself from these diseases?<\/h2>\n