The four native viruses: West Nile, EEE, La Crosse, and St. Louis encephalitis
West Nile virus arrived in New York in 1999 and has since been detected in every state in the continental U.S. Its main carriers are Culex mosquitoes, with different species dominant in the North, South, and West. Birds are the reservoir, and the virus amplifies in bird and mosquito populations through the summer, which is why most human cases are reported from July through September. About eight in ten infected people never notice, most of the rest have a flu-like illness with fever, headache, body aches, and sometimes a rash, and a small minority, more often older adults and people with weakened immune systems, develop meningitis, encephalitis, or a polio-like paralysis. There is no human vaccine and no specific antiviral, so care is supportive. Eastern equine encephalitis shares the bird-mosquito cycle but lives in freshwater hardwood swamps, where the mosquito Culiseta melanura keeps it circulating among birds. Other species that bite both birds and mammals occasionally carry it to horses and people. Human cases are rare, but the disease is severe, with a high fatality rate among those who develop encephalitis and frequent long-term disability in survivors. La Crosse virus is a different story. It is carried by the eastern treehole mosquito, cycles through chipmunks and squirrels, and mainly affects children in the Appalachian region and upper Midwest. Old tires and tree holes near the house are the classic breeding sites. St. Louis encephalitis, spread by the same Culex species as West Nile, causes sporadic cases and occasional outbreaks in central and southern states, with older adults at the highest risk of severe disease.
Dengue, Zika, chikungunya, and malaria: mostly imported, sometimes local
Dengue, Zika, and chikungunya are spread by Aedes aegypti and Aedes albopictus, the same container-breeding, daytime-biting mosquitoes that make late-afternoon patio time miserable in much of the South. People, not birds, are the reservoir, so a local outbreak needs a starting point: an infected traveler who is bitten by a local Aedes mosquito during the few days the virus is in their blood. In most of the country the season is too short and the Aedes population too sparse for that chain to go far. In South Florida, the Texas Gulf Coast, and parts of Arizona, California, and Hawaii, local dengue transmission has been documented in several recent years, and health departments respond with targeted spraying and door-to-door container checks. Puerto Rico and other territories have ongoing dengue transmission. Dengue usually causes high fever, severe headache, pain behind the eyes, muscle and joint pain, and rash. Most people recover in about a week, but a second infection with a different dengue type raises the risk of severe dengue, which is why the warning signs listed above matter. Chikungunya is known for intense joint pain that can persist for months. Zika is often mild but can cause serious birth defects when a pregnant woman is infected, and it can be transmitted sexually, so pregnant travelers and their partners should follow current CDC travel guidance. Vaccines for chikungunya and dengue have been approved in the U.S. for specific groups; a travel clinic can tell you whether either applies to you. Malaria, spread by Anopheles mosquitoes, is almost entirely travel-related, though a small number of locally acquired cases were reported in Florida, Texas, Maryland, and Arkansas in 2023.
Heartworm: the mosquito-borne disease that affects dogs everywhere
For most households, the mosquito-borne disease most likely to affect a family member is heartworm in the dog. The parasite, Dirofilaria immitis, is carried by many mosquito species, including common Aedes, Culex, and Anopheles. a mosquito picks up microscopic larvae when it feeds on an infected dog, coyote, or fox, and deposits infective larvae onto the skin of the next animal it bites. Over about six months the larvae migrate and mature into foot-long adult worms living in the pulmonary arteries and heart. Cats and ferrets can also be harmed by even a few worms. Early heartworm infection has no visible signs. As the worm burden grows, dogs develop a mild persistent cough, tire easily, lose weight, and eventually show heart failure. Because the damage accumulates silently, the American Heartworm Society recommends testing every dog annually and keeping every dog on preventive all twelve months, even in northern states, since mosquito activity can extend into unexpected months and indoor mosquitoes bite in winter. Preventives are monthly chewables or topicals, or a long-acting injection given by a veterinarian, and they work by killing larvae acquired during the previous month before they can mature. Treatment of an established infection involves a series of injections of an arsenic-based drug, strict exercise restriction for weeks, and a real risk of complications as dying worms break apart. It typically costs many times more than a year of preventive and is hard on the dog. No amount of yard spraying replaces the preventive.
How yard control and personal protection actually reduce risk
Risk from a mosquito-borne disease is the product of three things: how many mosquitoes are around, how many of them are infected, and how often they bite you. You control the first and third. Source reduction attacks the population where it starts. Aedes eggs hatch within days of being wetted, and larvae need about a week to become adults, so a weekly walk through the yard that tips, tosses, scrubs, or covers every water-holding item breaks the cycle before adults emerge. The scrubbing matters because Aedes eggs stick to container walls just above the waterline and survive drying. Culex prefer larger, dirtier water: clogged gutters, tarps sagging over firewood, neglected pools and hot tubs, and catch basins. Bti dunks handle the water you cannot drain, and a call to the municipality handles the storm drain at the curb. A professional barrier treatment addresses adults that have already emerged or flown in. The technician applies a residual insecticide to the undersides of leaves, dense shrubs, ivy, under decks, and shaded fence lines where mosquitoes rest during the day, and the product keeps killing for several weeks. Ask what product is used, whether flowering plants will be avoided to protect pollinators, whether larvicide is included, and what the technician found as breeding sources. Personal protection covers the bites that get through. EPA-registered repellent applied to exposed skin, permethrin on clothing, long sleeves at dusk, intact screens, and air conditioning each reduce bites independently. During a health department alert for West Nile or local dengue, that combination is the response CDC recommends for households, and it is the same set of habits that keeps the dog's mosquito exposure low too.
Mosquito problems by the numbers
Homeowners often ask whether a mosquito problem is unusual and whether a quoted price is normal. Public research cannot answer either question for one house, but it does set expectations. Here is what the sources report. For homeowners, disease is a real part of the mosquito question: 32% of respondents in NaturaLawn of America's March 2024 SurveyMonkey poll of 1,163 U.S. homeowners worried about West Nile reaching their family, though the provider-sponsored, self-reported survey measured concern, not infection rates. Insects change how people use their yards: 71% of respondents in NaturaLawn of America's March 2024 SurveyMonkey poll of 1,163 U.S. homeowners had avoided or left an event because of them, a self-reported, provider-sponsored finding about general insect annoyance, not any one pest's prevalence. Twelve percent of U.S. homeowners in Mosquito Squad's 2016 Fight the Bite survey said they had taken no yard precautions against mosquitoes at all, according to the TNS Global poll, which the company sponsored and whose sample size is undisclosed. Only 15% of homeowners said the products they bought effectively reduced insects in their yard, according to NaturaLawn of America's March 2024 SurveyMonkey poll of 1,163 U.S. homeowners, a provider-sponsored, self-reported survey in which effectiveness is respondent opinion, not the result of a product trial. Most homeowners reach for the hardware-store aisle first: 57% of the 2,000 U.S. respondents in ConsumerAffairs' February 2024 survey of 2,000 U.S. respondents reported using store-bought pest products, a self-reported finding from a mixed pest-and-mold survey that speaks to habits, not infestation rates. Because every figure above has a stated population and date, you can cite it when comparing providers. What none of them can tell you is what your specific property needs; that still takes an on-site inspection.