West Nile Virus: Epidemic Emergence

The mosquito-borne virus spread throughout the country -- and by means not seen before 2002.

By Richard Trubo
WebMD Medical News

Reviewed By Brunilda Nazario, MD
on Friday, December 20, 2002

In the summer of 1999, a 60-year-old man was admitted to Flushing Hospital Medical Center in Queens, New York, complaining of fever, nausea, and weakness. Within two weeks, four more patients had been admitted to the same hospital with similar symptoms.

The cause of their illnesses turned out to be the same: the West Nile virus. In fact, these were the first cases of West Nile infections detected in the Western Hemisphere -- but far from the last. By the end of 1999, 62 people in the U.S. had become ill with the West Nile infection, and seven of them died. Today, the numbers are much larger, and some doctors are labeling the West Nile outbreaks as an epidemic.

West Nile Statistics

While the West Nile virus was confined to the Northeast in the initial cases in 1999, it has now made its way west and south, with the first cases in California reported in late 2002 -- only three years after West Nile virus was first seen in the U.S. "The virus has not remained isolated in certain small geographic areas," says J. Glenn Morris, Jr., MD, MPH, professor and chair of the department of epidemiology and preventive medicine at the University of Maryland. "It truly has become a national phenomenon, and has affected a large number of individuals."

In 2002 alone (through December 18), a total of 3,852 cases of West Nile infection, including 232 deaths, have been reported to the CDC. These cases have occurred in 39 states plus the District of Columbia, with the rates of the disease highest in Illinois, Michigan, Ohio, and Louisiana.

"We now have a disease that, rather than being something of a curiosity, has reached numbers where it is a public health concern," says Morris. "We're in the midst of a very significant epidemic."

Transmission and Symptoms

West Nile virus is transmitted to humans through mosquito bites, most often in the warm weather months. The mosquitoes pick up the virus by feeding on infected birds, then the virus is passed from the mosquito's salivary glands to humans, in whom it can multiply and trigger an illness.

Yet even though the number of reported West Nile cases is rising, it may just be the tip of the iceberg. Only about 20% of infected people have any symptoms at all, and among those who become sick, many have only a mild to moderate flu-like illness, with a headache, fever, and body aches. According to Morris, only about one in 150 infections produces symptoms serious enough to prompt laboratory testing to confirm the diagnosis.

Although most people with West Nile virus infection have only mild symptoms, or none at all, a small number (less than 1%) develop severe illness and even life-threatening complications such as encephalitis or meningitis (brain inflammation disorders). "People with weakened immune systems, or elderly men and women, are most susceptible to these complications," says Nancy McQueen, PhD, professor of microbiology at California State University, Los Angeles. But, she adds, the number of these serious infections is relatively small, and thus it's important not to become overly alarmed about these risks. "I'd be less worried about West Nile virus than other kinds of infectious diseases that don't get reported in the news," adds McQueen.

New Areas of Concern

Not only are West Nile outbreaks affecting more people, but for the first time in 2002, a small number of cases were identified in which the infection was transmitted through the blood supply or spread through organ donations.

Cases of West Nile virus infection were reported in several individuals receiving blood transfusions in communities where the virus was prevalent. In September, the CDC also confirmed that West Nile virus was transmitted to four organ recipients from a single organ donor who had died in an automobile accident in Georgia.

Even so, officials monitoring the nation's blood supply emphasize its overall safety. In 2002, there were only about 10 reported transmissions of the virus through blood transfusions during a time when about 4.5 million people received transfusions of blood and blood products.

According to a recent statement from the CDC, "Although persons needing blood transfusions or organ transplants should be aware of the risk for WNV (West Nile virus) infection, the benefits of receiving needed transfusions or transplants outweigh the potential risk for WNV infection."

Louis Katz, MD, vice president, medical affairs, at the Mississippi Valley Regional Blood Center, agrees. "People who need blood should get that blood," says Katz. "Under almost all circumstances, a necessary transfusion should not be refused or cause great consternation about West Nile virus."

In August, the FDA issued an alert to blood banks to become vigilant in excluding potential donors who have symptoms such as headaches and fever that could indicate a West Nile Virus infection. At present, potential donors are asked questions before they give blood that may suggest the presence of West Nile virus.

But because about 80% of West Nile infections cause no symptoms, questioning donors may miss many people infected with the virus, says Katz. For that reason, a screening test for West Nile Virus is now being developed, and could be available as soon as next summer (2003).

Meanwhile, in September, a mother in Michigan transmitted the virus to her infant via breastfeeding. Public health officials suspect that the mother, who developed West Nile meningitis, probably contracted the virus from a blood transfusion she received shortly after delivering her baby. The newborn did not become ill.

According to the CDC, "Because the health benefits of breast-feeding are well established, and the risk for West Nile virus transmission through breast-feeding is unknown, the new findings do not suggest a change in breastfeeding recommendations." (The American Academy of Pediatrics and the American Academy of Family Physicians recommend that infants be breastfed during the first full year of life.)

Late in December, the CDC reported a woman transmitted West Nile virus to her baby while pregnant, in apparently the first case of intrauterine transmission of the disease. The baby was delivered normally but an MRI showed severe brain abnormalities. CDC officials characterized the infant as having severe neurological damage, and tests conducted shortly after birth showed that she had the infection. They say that this case does not prove a direct relationship between infection and the brain abnormalities, but they have found no other explanation for it.

Researchers are also investigating isolated reports of sudden polio-like limb paralysis associated with West Nile virus infections, which develops after the more common, mild symptoms occur. Although scientists don't yet understand how West Nile causes paralysis, one theory is that the virus may target the nerve cells near the spinal cord.

Prevention and Treatment

No specific treatment is available for West Nile virus infections. "There aren't a lot of antiviral drugs that can be used to try treating illnesses like West Nile infection," says McQueen. For that reason, she adds, most efforts are aimed at preventing the infection from occurring.

For example, when you go outside, wear long-sleeved shirts and long pants, and use insect repellant (containing the chemical DEET) during the warm-weather mosquito season. Avoid being outdoors at the times of day when mosquitoes are most likely to bite, such as at dawn and dusk.

Eliminate breeding sites for mosquitoes around your home, such as standing pools of water in birdbaths, or beneath a dripping outdoor faucet.

"People are particularly careless with water," says Dickson Despommier, PhD, professor of microbiology and public health at Columbia University in New York, and author of West Nile Story. In the initial outbreaks in the New York area, many people had turned off their pool pumps when they left town on vacation, and their pools turned into stagnant bodies of water that served as breeding grounds for mosquitoes carrying West Nile virus, he says.

A vaccine to prevent West Nile virus is not yet available, but the National Institutes of Health is supporting research in this area on a fast-track basis. One candidate vaccine, being developed by a company called Acambis, has been shown to be safe and has produced antibodies against the virus in animals. Clinical trials are scheduled to begin in early 2003, although the immunization may not be available for several years.

Published Dec. 26, 2002.

SOURCES: CDC • Dickson Despommier, PhD, professor of microbiology and public health, Columbia University, New York • Louis Katz, MD, vice-president, medical affairs, Mississippi Valley Regional Blood Center, West Burlington, Iowa • Nancy McQueen, PhD, professor of microbiology, California State University, Los Angeles • J. Glenn Morris, Jr., MD, MPH, professor and chair, department of epidemiology and preventive medicine, University of Maryland, Baltimore, Md.

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