Mosquito season is around the corner. While no one I know has ever found this cause for rejoicing, right now there is more reason than ever to dread the arrival of these bloodsuckers. Last summer, Florida’s Key West was hit with an outbreak of dengue fever, a tropical mosquito-borne disease seldom seen in this country. It generally isn’t deadly, but it does make people very, very sick. Dengue fever joins a list of other mosquito-borne illnesses in the US, some of which are far more serious, including West Nile virus, St. Louis encephalitis and LaCrosse encephalitis. All are caused by viruses transmitted to humans through mosquito bites. I called Roxanne Connelly, PhD, associate professor at the University of Florida and president of the Florida Mosquito Control Association, to discuss what Daily Health News readers need to know to protect themselves from this array of illnesses as the buggy season kicks in…

Dengue Fever in the US

Although dengue (pronounced den-gay) is found primarily in tropical climates, there have been occasional outbreaks here in the US. Dengue is the most common virus transmitted by mosquitoes worldwide and also often the cause of fever in people who return ill from vacations in the Caribbean and Central America.

Where it occurs: Rarely in the US, outside of occasional outbreaks (recently in Florida and isolated cases reported in Texas since 1980).

When you may get sick: Four to seven days after you are bitten.

Watch out for these symptoms: Headache, high fever and muscle, bone and joint aches, nausea and vomiting. The virus usually causes mild illness—with any luck, you’ll never even know that you were infected. But, because of the potential for severe aches and pains, dengue is also known as “breakbone fever.” In the worst-case scenario, you may develop dengue hemorrhagic fever, which can be fatal. This is rare—occurring, for instance, when a person is infected with two different strains of the virus—and it occurs most often in children. What to watch for: After several days of being sick, the patient becomes irritable and sweaty, then goes into a shocklike state. Sudden bleeding (from the gums or nose) or bruising may appear. Be alert for tiny spots of blood on the skin and larger patches of blood pooled beneath.

The treatment: There are four strains of the virus. Once you have been infected with one strain, you are at risk for more serious disease if you are bitten by a mosquito carrying a different strain. Treatment is to relieve symptoms, which usually resolve on their own within a few weeks. The hemorrhagic form is a medical emergency and requires hospitalization.

West Nile Virus

The first case of West Nile virus in the US was documented in 1999, when an epidemic hit New York City. Most people who are bitten by an infective mosquito don’t get sick, but if you’re over age 50, your odds of getting sick are higher—and the older you are, the sicker you’re likely to get. There’s a mild form called West Nile fever that is usually uncomfortable at worst (only 20% of people who are infected even have symptoms), but there’s also a more severe form, West Nile encephalitis or meningitis, that affects a small percentage (less than 1%). This form may result in seizures and even death.

Where it occurs: In most states in the US. See a state-by-state map at

When you may get sick: Two to 15 days after you are bitten.

Watch out for these symptoms: Fever, headache, body aches and a skin rash (characterized by large red patches with a varied surface, both raised and flat) are signs of West Nile fever, which lasts several days to weeks. The severe and potentially fatal form, West Nile encephalitis (an inflammation of the brain), strikes in one in 150 cases and causes neck stiffness, disorientation, muscle weakness, paralysis, convulsions and coma. Severe symptoms may last for months, says Dr. Connelly.

The treatment: Treatment focuses on symptom control (e.g., acetaminophen to lower fever and oral fluids to prevent dehydration). People with West Nile encephalitis may require hospitalization for intravenous fluids, respiratory assistance and other support.

St. Louis Encephalitis

As with West Nile, most people bitten by a mosquito carrying the virus that causes St. Louis encephalitis do not fall ill. The first known epidemic of the disease broke out in St. Louis in 1933.

Where it occurs: Don’t make the mistake of thinking this is a problem only in Missouri—it occurs throughout the country, though most outbreaks have been concentrated in the southeastern and midwestern states. See a state-by-state state map at

When you may get sick: Four to 21 days after you are bitten.

Watch out for these symptoms: Fever, headache, nausea, vomiting, dizziness and tiredness for several days to a week. Severe disease attacks the central nervous system and often involves encephalitis—which may cause a stiff neck, disorientation, lethargy, and, very rarely, coma, convulsions, paralysis and death.

The treatment: As in the case of West Nile virus, treatment consists of supportive care to relieve symptoms. In rare cases, long-term disability or death can result. The risk for death increases with age.

How to Mount an Aggressive AntiMosquito Defense

Doctors do not always think to check for a mosquito-borne virus, especially when the symptoms are so similar to other illnesses such as the flu. If no major outbreak has been publicized in the news, it is a good idea to ask your doctor if your flulike symptoms could be from such a mosquito-borne virus. But in the meantime, the best way to protect yourself from mosquito-borne illnesses is to take precautions to limit mosquito bites. Here’s how…

  • Use mosquito repellent. Choose an EPA-approved mosquito repellent, advises Dr. Connelly, one that contains DEET, picaridin or oil of lemon eucalyptus. Apply a light layer to exposed skin and clothing. Caution: Do not use repellent near or in your eyes or mouth, under clothing or on cuts or irritated skin, and use sparingly on your ears (making sure that it does not get down inside the ears). Do not use products that combine sunscreen and repellent. Since sunscreen must be applied often and liberally, you can potentially get too much repellent—even a toxic dose. When you return indoors, wash treated skin thoroughly to avoid spreading repellent. (For more on safe use of repellents, see Daily Health News, “What Mosquitoes Hate Most.”)
  • Eliminate standing water. Mosquitoes lay their eggs in standing water. Reduce mosquito populations around your house by eliminating their breeding grounds. Your job: Get rid of unnecessary buckets, flowerpots and other items that hold water. Change water in birdbaths once a week. Don’t let rain gutters get clogged. (Many people think about their gutters only in autumn when leaves are falling, but they can become clogged any time of year.)
  • Avoid peak mosquito hours. Steer clear of outdoor activities around dusk and dawn. If you have to go out at these times, use insect repellent (on exposed skin only) and wear repellent-treated protective clothing such as long sleeves and pants. Don’t waste your money: Gimmicks like antimosquito bracelets and necklaces that claim to have repellent qualities don’t work, says Dr. Connelly. Likewise, there is no evidence that sound-emitting devices will keep you from getting bitten.
  • Repair damaged window and door screens, or install new ones. Only tight, well-fitting screens will keep mosquitoes outside when your windows are open.