As summer plunges into its hottest months, West Nile virus and the mosquitoes that carry the disease once again make their unwelcome presence felt in the United States.

The virus is believed to have been carried into the country by a traveler from Europe or the Middle East in 1999, but it didn't cause significant problems until summer 2002. By the end of that year, the virus had infected 4,156 people in 39 US states and killed 284, according to the Centers for Disease Control and Prevention (CDC).

In 2003, infections remained low until early August, when states from Arkansas to Arizona reported a sharp spike in the number of people infected. By mid-September the CDC reported over 4,300 cases and over 80 deaths. The final tally for 2003 was almost 10,000 cases total, with 264 deaths.

While the incidence of the West Nile virus continues to be worrisome, the number of cases has dropped dramatically since its peak in 2003. By the end of 2007, the CDC reported 3,630 cases and 124 deaths. By mid-November 2008, the CDC reported only 1,301 cases and 34 deaths.

The good news is that most people who get the virus will not experience any symptoms or illness. About 20 percent will experience only mild symptoms -- including fever, headaches, and body aches. About one in 150 people infected, however, will contract the severe infection (West Nile encephalitis or meningitis) that can result in permanent nerve damage or partial paralysis. It can also be deadly. People over 50 years old are considered more susceptible to becoming ill from the virus.

"As we had anticipated, West Nile is currently really picking up momentum within the United States," said Stephen Ostroff, MD, deputy director of the CDC's National Center for Infectious Diseases, at a teleconference. "It's important to point out that based on prior seasons, we are just now getting into the peak period of West Nile activity which, over the last several years, has been in late August and early September, and it certainly looks like we will be following a similar course over the next several weeks."

Now is the time to protect yourself from mosquito bites, Ostroff emphasized. "This involves personal protective measures, such as use of insect repellents, as well as wearing light, long sleeves and long pants, and also doing what can be done around the home to reduce sources of standing water in order to reduce the likelihood of mosquito breeding," he said.

Dead birds a harbinger of virus

West Nile virus is one of many germs that mosquitoes can carry. The insects pick up the virus by feeding on infected birds and transmit the disease when they bite other animals. The virus has been found in many different species, including horses, cats, squirrels, and, of course, humans.

Humans usually catch the disease from mosquitoes, though, not from infected people, birds, horses, or pets. However, researchers at the CDC have confirmed that four organ transplant patients developed the disease after receiving organs from the same infected donor. One of the patients died. The CDC also confirmed that 23 patients developed West Nile virus after receiving blood transfusions. As of July 14, 2003, all civilian blood donations collected in the United States, including Puerto Rico, have been screened for West Nile virus.

The agency also reported one case of a pregnant woman who transmitted the West Nile virus through the placenta to her unborn child, and one case of a new mother who transmitted the virus through her breast milk. Two laboratory workers also contracted the virus after getting pricked by tools contaminated with the virus.

Originally, West Nile virus was confined to Africa, Asia, the Middle East, and parts of Europe. Somehow, the germ found a way across the Atlantic, and in 1999 an outbreak of West Nile virus in New York City introduced Americans to the disease.

Outbreaks of West Nile virus seem to affect birds first. In Washington, DC, an early warning sign for residents was an abundance of dead crows. The CDC urges people to call local animal control officers to report dead or sick birds so that they may be tested for the virus.

In the worst-case scenario, the virus travels through the bloodstream and infects the brain. The infection can lead to encephalitis, or acute inflammation of the brain. It can also cause meningitis, which is inflammation of the brain's outer covering.

However, serious complications such as encephalitis are extremely rare in people under 50. While anyone can become infected from a mosquito bite, the elderly are much more likely to become sick.

In one such case, 81-year-old Joseph Chiella died of West Nile virus several weeks after he was bitten by mosquitoes in his Melville, New York garden. (Shortly before the infection, he found numerous dead birds in his yard -- often a sign of the virus.) In an interview with The New York Times, his widow said that her husband had been robust for his age. He had, in fact, expected to live to 100. "It's hard to understand," she said, "how this tiny little bug could kill such a strong man."

Fortunately, the worst-case scenario is unlikely. According to a report in the Annals of Internal Medicine, only one in 150 people infected with West Nile virus develops encephalitis or meningitis, and 3 to 15 percent of patients with severe infections die. Fewer than one in 1,000 people who catch the West Nile virus die from an infection.

There's no cure for the virus itself, but most people are able to fight it off easily. Many who contract the virus have symptoms so mild, and their recovery is so fast, that they might not be aware of ever having the disease. As reported in the Annals of Internal Medicine, about one in five infected people develop a mild fever. Other common symptoms include muscle pain, joint pain, fatigue, headache, swollen lymph glands, and a skin rash on the trunk. These symptoms generally last for just a few days.

Know the warning signs

If you live in an area where there's a West Nile outbreak, you should know the warning signs of this potentially serious disease. If you develop a high fever, confusion, severe headache, or muscle weakness, the CDC urges you to call a doctor immediately.

You should also seek immediate medical care if you get a sudden headache and stiffness in the neck, which are possible symptoms of encephalitis.

Other symptoms of encephalitis include tremors, convulsions, hearing or vision problems, confusion, drowsiness, vomiting, unusual sensitivity to light, lack of muscle coordination or difficulty walking, irritability (and other mental disturbances), poor responsiveness, trouble speaking coherently, or coma. If you or someone you know develops these symptoms, especially in combination with a sudden headache or stiff neck, seek emergency care right away. A single mosquito bite -- or even several -- is no cause for alarm. Even if there is an outbreak in your area, only a tiny percentage of mosquitoes carry the germ. Unless you start developing symptoms of an infection, there's no reason to worry.

What precautions to take

There's no human vaccine against West Nile virus (although there is a vaccine for horses). The best way to avoid the disease is to guard against mosquito bites. Being cautious during the summer and fall, when mosquitoes are especially prevalent, may not be enough. Some areas in the south have mosquitoes all year long, and the CDC reports that one species of mosquito known to carry the virus is able to survive winter weather. One of the first things to do is make sure your house has tight-fitting window screens. The CDC and other health officials offer these additional tips for preventing mosquito bites:

* When possible, wear long pants and long-sleeved shirts sprayed lightly with the chemical DEET (short for N, N-diethyl-m-toluamide).

* Stay inside during dawn, dusk, and early evening, the times when mosquitoes are most active.

* Don't let your house become a mosquito breeding ground. Remove standing water from flowerpots, buckets, clogged rain gutters, old bottles and cans, and birdbaths.

* Drain water from pool covers and make sure that pools and hot tubs are clean and chlorinated. The best protection against mosquitoes (short of staying indoors) is a repellent containing DEET, according to the CDC. The agency recommends spraying DEET on all exposed skin of adults and children age 3 and older.

Repellents containing DEET can irritate the eyes and mouth, so keep it off of children's hands. If you're wearing light clothing, like a T-shirt, you may have to spray that also. (You can give other natural fiber clothing a light spray as well.) A few other cautions from the manufacturer: Don't soak clothing or bedding in DEET, never spray it on broken skin or in anyone's face, and avoid using it on children under 2 months of age. For all children, the American Academy of Pediatrics recommends using DEET in concentrations of less than 30 percent. (Interestingly, DEET doesn't kill mosquitoes. Instead, it makes it impossible for them to find you for several hours. Mosquitoes locate their human prey through the carbon monoxide given off by our skin and breath, and DEET disrupts their ability to target us by following our scent and exhalations.) Last, read the product label carefully, because the repellent comes in different concentrations. If you plan to be outdoors for more than three hours, you may need to reapply it or use a higher concentration. Avoid using any pesticide on your body that's designed to kill mosquitoes. These simple steps will save you from itchy, annoying bites. And at a time when the West Nile virus is again on the march, you'll also have some peace of mind.

-- Chris Woolston, MS, is a health and medical writer with a master's degree in biology. He is a contributing editor at Consumer Health Interactive, and was a staff writer at Hippocrates, a magazine for physicians. He has also covered science issues for Time Inc. Health, WebMD, and the Chronicle of Higher Education. This story was updated by Paige Bierma,a freelance journalist in San Francisco who frequently covers health and medical issues.

Further Resources

Centers for Disease Control and Prevention

The CDC has put together a list of questions and answers about West Nile virus. The site describes the symptoms, testing, and treatment of the illness. It also contains information on clinical trials for the treatment of the virus and mosquito-proofing your home.

References

First published September 4, 2002
Last updated November 25, 2008