Cases of West Nile illness are being reported at about triple the usual rate, U.S. health officials say. The Centers for Disease Control and Prevention (CDC) said there have been 1,118 human cases so far in 2012. About half have been in Texas. In a typical year, the CDC said, fewer than 300 cases are reported by the middle of August. Most cases are reported in August and September. CDC officials suggested that the mild winter, early spring and very hot summer have spurred breeding of the mosquitoes that carry the disease. They spread it by biting infected birds and then biting humans or other animals. Only 1 person out of 5 gets sick from the infection. Symptoms usually are mild. About 1 person out of 150 has a serious illness. The most severe symptoms include neck stiffness, paralysis and coma. The Associated Press wrote about the outbreak August 22.

By Howard LeWine, M.D.
Harvard Medical School

What Is the Doctor's Reaction?

In the United States, more than 1,100 cases of human West Nile infection have been reported already this year. The infection has caused at least 40 deaths so far. These are the most cases and deaths ever reported so early in a calendar year.

And the numbers are expected to rise dramatically during the next couple of months. Late August and September are the peak times for West Nile cases to be reported.

In other parts of the world, West Nile virus has been around a long time. But the virus was discovered in the United States only 13 years ago.

In prior years, clusters of cases would occur in some regions of the country. This year there are reports of West Nile virus in humans, birds or mosquitoes in 47 states. Of the lower 48, only Vermont had not reported activity as of earlier this week. Alaska and Hawaii have never reported the presence of West Nile virus.

Experts can only speculate about the reasons for this dramatic increase in cases. It probably has some link to the record warm temperatures this past winter and spring. And this was followed by a very hot early summer. The breeding cycles of mosquitoes speed up in hotter weather. The virus seems to multiply faster in warmer temperatures.

West Nile virus infection in humans almost always occurs after a mosquito bite. Because blood products are tested for the virus, infection from a transfusion is a very rare event. West Nile can't spread from human to human by casual contact.

Most people bitten by a mosquito carrying the West Nile virus won't get sick. First, the virus has to get transmitted from the salivary gland of the mosquito to a person's blood. Even then, only 1 in 5 people will have any symptoms.

When symptoms occur, they start 3 to 15 days after the bite. The symptoms almost always resemble a mild case of the flu:

• Fever
• Headache
• Body aches

Rarely, the virus can get into the brain and fluid around the brain. About 1 in 150 people will have severe illness with:

• High fever
• Confusion
• Severe headache
• Stiff neck

People over age 50 and those with altered immune systems have a higher risk of developing severe problems from the virus.

What Changes Can I Make Now?

• Stay indoors when mosquitoes are most likely to bite you. They are most active between dusk and dawn, and especially in the early evening.
• Make sure you have window and door screens to keep out mosquitoes.
• Wear long pants and shirts with long sleeves to protect you from bites.
• Put insect repellant on when you go outside. Insect repellents containing the active ingredients DEET or picaridin provide more effective and longer-lasting protection than other repellents. Check the product to see how long it will protect you. Generally, the more active ingredient a repellent contains, the longer it will work.
• Read the labels and follow printed directions for use of insect repellents.
• If you will be in the sun, apply your sunscreen first, then insect repellent.
• Spray your clothing with repellents or with permethrin products for extra protection.
• Apply insect repellant on children. Don't have them do it themselves. Choose the lowest concentration of DEET needed for the length of time you need protection.
• Don't use DEET products on infants less than 2 months old. Use mosquito netting to protect infants.
• Check around your home for standing water. Dump out any water where mosquitoes may breed.

What Can I Expect Looking to the Future?

Current antiviral drugs do not kill West Nile virus. No vaccine is available. Neither is likely to be available any time soon.