Mosquito Facts

Key Points About Mosquitoes

Every mosquito needs standing water at some stage to grow from egg to adult.
Lawns and shrubs do not breed mosquitoes, but adults often shelter in them during the heat of the day.
Bites come only from females, which need blood. Males live on nectar and other plant juices.
A female can survive for up to about three weeks in summer, and through the winter she can last for months and lay her eggs when spring returns.

Mosquito Life Cycle

Mosquitoes belong to the order Diptera, the true flies, and develop in four stages: egg, larva, pupa and adult. Depending on the species, a female lays her eggs either right on the surface of the water or on damp ground that will flood later. When the eggs hatch in water, the larvae, often called "wigglers," molt through four growth stages known as instars. Next comes the pupa, a resting stage in which the insect stops eating and turns into an adult. Like larvae, pupae can survive only in water.

After mating, adult females go looking for a host to feed on. The blood meal fuels the development of her eggs, and once she lays them the whole cycle begins again.

How Mosquito Bites Spread Disease

A mosquito can pass along an illness only by biting. What we call a "bite" is really a pierce-and-sip process. Only females feed on blood, because they need its protein to produce eggs. To feed, the female pushes her proboscis, a slender, needle-tipped mouthpart, through the skin, releases saliva that keeps the blood from clotting, and then draws blood up through the same tube. If that blood carries germs, they end up in her gut, can survive inside her, and may later be injected into the bloodstream of the next animal or person she bites. In this way mosquitoes move disease between animals, from animals to people, and sometimes from one person to another.

About seven major mosquito-borne viruses found in the United States can cross the blood-brain barrier and cause a sudden infection of the brain and spinal cord in people and animals. Three of them, West Nile virus, St. Louis encephalitis and Western equine encephalitis, have been found in Texas.

No other animal on earth causes as many human deaths as the mosquito. Around the world, close to 4 million people die each year from illnesses that mosquitoes carry. A printable guide to mosquito species in the U.S. and abroad is available (PDF, 2MB).

Mosquito-Borne Diseases

Mosquitoes carry a number of viruses and parasites that can make people and animals seriously ill. The sections below cover the three that residents ask about most: West Nile virus, Zika and chikungunya.

West Nile Virus

What is it?

The Centers for Disease Control and Prevention (CDC) identifies West Nile virus (WNV) as the leading viral cause of encephalitis, or brain inflammation, in this country. Scientists first identified it in Africa in 1937. It reached the U.S. in 1999, when it appeared in New York City, and it has caused seasonal outbreaks of West Nile fever and more serious illness every year since. People and animals catch it when an infected mosquito bites them, and mosquitoes pick it up by feeding on birds that carry it.

Where did the virus come from?

For many years West Nile virus was common among people, birds and other animals with backbones in Africa, Eastern Europe, western Asia and the Middle East, but no one had recorded it in the Western Hemisphere before 1999. How it arrived in the U.S. is unknown, although the strain here most closely resembles strains from the Middle East.

How long has it been in the United States?

No one knows the exact arrival date. CDC scientists think the virus was likely present in the eastern U.S. by early summer 1999, and perhaps even earlier.

How do people become infected?

Almost always through a mosquito bite. A mosquito becomes infected after feeding on a bird with the virus in its blood, which can last for a few days. The virus then works its way into the mosquito's salivary glands, and when she bites again she can inject it into a person or animal, where it may multiply and cause illness. In a very small share of cases, the virus has spread through organ transplants, blood transfusions, or from a mother to her baby during pregnancy.

If West Nile has been found near me and a mosquito bites me, will I get sick?

Probably not. Even where the virus is active, only a small fraction of mosquitoes carry it. And among people who are bitten by an infected mosquito and become infected, fewer than 1 percent develop serious illness, or roughly 1 person in 150.

Can I catch it from another person?

No. West Nile encephalitis does not spread from person to person. Touching or kissing someone who has it, or being treated by a health care worker who cared for an infected patient, will not infect you.

Can ticks or other insects spread it?

Mosquitoes are the main carriers. Infected ticks have turned up in Asia and Africa, but scientists are not sure whether they help spread or sustain the virus, and nothing indicates that ticks were involved in any U.S. cases.

Can I get it from birds?

There is no evidence that people get the virus by handling live or dead infected birds. Still, never pick up a dead animal with bare hands. Wear gloves, or use two plastic bags, to place it in the trash. You can also report dead birds to the city.

Can caring for an infected horse make me sick?

No. The virus reaches people through mosquito bites, and there are no documented cases of it passing from animals to people or between people.

Can someone who had West Nile virus give blood?

The virus usually stays in the body only briefly, and most people clear it fast. Blood centers screen donors carefully so that anyone who was diagnosed with West Nile virus has fully recovered before giving blood.

Is there a vaccine for people?

Not yet, although several companies are working on one.

Where can I learn more about insect repellents?

The CDC publishes guidance on choosing and safely using insect repellents, and repellent makers provide manufacturer information on products that contain DEET.

What are the symptoms?

Most infections are mild, with fever, headache and body aches, and sometimes a rash or swollen lymph nodes. Serious cases can bring a high fever, severe headache, stiff neck, confusion or stupor, tremors, seizures, muscle weakness, paralysis or coma, and in rare cases death.

How soon after infection do symptoms appear?

The incubation period is usually 3 to 15 days.

What should I do if I think I have West Nile virus?

Talk to your health care provider about any health concerns. Anyone in your household who develops a high fever, confusion, muscle weakness or a severe headache should see a doctor right away.

Can dogs or cats get sick from it?

Rarely. Fewer than 40 infected dogs and just one infected cat have been reported to the CDC. Dogs infected in laboratory studies showed no signs of illness, and some cats had mild, vague symptoms in the first week, mostly a slight fever and a little sluggishness. Most owners would never notice that a pet had been infected.

Can an infected horse pass it to horses in nearby stalls?

No. There is no documented spread from one horse to another. Even so, a horse suspected of having West Nile virus should be protected from mosquito bites as much as possible.

Where can I learn more about horses and West Nile virus?

State agriculture departments and university veterinary programs publish resources on West Nile virus in horses, and your veterinarian is another good source of advice.

Are duck hunters and other game hunters at risk?

Hunters spend long hours outdoors, so they can be exposed if they are bitten where the virus is active. How much West Nile virus is present in wild game is not yet known. Federal wildlife health researchers, along with state and local naturalists and wildlife biologists, are studying the question.

How can hunters protect themselves?

Take the normal precautions for handling wild animals. If mosquitoes are likely, put repellent on skin and clothing as the label directs. Wear gloves while handling and dressing game so blood does not touch bare skin, and cook the meat thoroughly.

Zika Virus

Zika is a relatively new mosquito-borne virus. People usually get it when an infected mosquito bites them, and mosquitoes can pick it up by biting someone who is already infected.

The CDC has issued travel warnings for countries where Zika is spreading, with special cautions for women who are pregnant.

Chikungunya Virus

Chikungunya (say "chik-en-GUN-ye") is:

a virus passed along by the bites of Aedes mosquitoes, the same species that carry dengue; and
a concern for anyone visiting parts of the world where the virus circulates.

Signs and symptoms:

Illness usually starts 3 to 7 days after a bite from an infected mosquito.
Most people develop a fever and sharp, aching joints, particularly in the hands and feet. Some people also have a headache, muscle aches, swollen joints or a rash.
Most people feel better in about a week, though joint pain can linger for weeks or months in some cases. Deaths are uncommon but do happen.
Newborns infected around the time of birth, adults 65 and older, and people living with high blood pressure, diabetes, heart disease or similar conditions face a higher risk of severe illness.

Where it is found:

Chikungunya has been reported in parts of Africa, southern Europe and Southeast Asia, and on islands in the Indian and Pacific oceans. It was first detected in the Americas in 2013 and has since spread through the Caribbean and into Central, South and North America.

Planning a trip? Check the CDC's travel health notices for the country you will be visiting before you leave. To learn how the city monitors and controls mosquitoes at home, visit Fight the Bite.

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