West Nile Epidemic: How Real Is the Threat?

July 27, 2007— -- First, the bad news: an early spike in the number of cases of West Nile virus infection reported this year has some researchers predicting a possible epidemic as the end of summer approaches.

But don't board up your windows or stock up on bottled water just yet. Infectious disease specialists say such an epidemic would not be as deadly as those involving other viruses -- and would be largely preventable, to boot.

"What we're seeing is the expected increase in West Nile virus transmission that occurs during the summer months," said Dr. Ned Hayes, a medical epidemiologist with the U.S. Centers for Disease Control and Prevention (CDC). But he added that this year, early figures appear high.

According to July 24 data, the CDC has fielded a total of 122 reports of human West Nile Virus infection in 17 states so far this year. Three people have died.

The number of West Nile reports this year so far outstrips the figure recorded at this time last year. Whether this bump will lead to an epidemic this year has yet to be determined, according to infectious disease specialists.

"West Nile virus clearly does have epidemic potential, as demonstrated in years past," said Dr. William Schaffner, chair of preventive medicine at the Vanderbilt University School of Medicine.

"The increase for this time of the year... portends further increases later in the summer to early fall when [West Nile virus infection] usually peaks."

"It is difficult to predict if the trend will continue," said Dawn Wesson, associate professor of tropical medicine at the Tulane University School of Public Health and Tropical Medicine. "Certainly if it does, it could be a bad year for West Nile virus cases and disease."

But while doctors and researchers say it is important to implement preventive measures to stem the surge in infections, a West Nile epidemic would not likely pose the same threat to human life as other viral epidemics, such as those involving influenza.

"The word 'epidemic' is scary," noted Dr. Clifford Bassett, vice chair of the public education committee for the American Academy of Allergy, Asthma and Immunology.

"The positive aspect to West Nile Virus is that most individuals infected do not go on to have life-threatening complications."

What Is West Nile?

West Nile Virus is almost exclusively spread by mosquitoes infected with the virus to birds and humans.

First discovered in 1937 Uganda, the virus is a relative newcomer to the U.S., making its debut in New York City in 1999.

A spike in cases this year would continue an upward trend that has emerged over the past few years, beginning in 2004 with just over 2,500 cases reported nationally.

In 2005, the number of reports crept up to 3,000. And last year the virus registered 4,261 CDC reports.

But the report numbers over the past few years are eclipsed by those from 2003 --when 9,862 cases of the disease were reported, and 264 were determined to have died from the disease.

Also important to note is the fact that the number of reported cases does not typically reflect the actual impact of the disease; for every documented case, more than nine go unreported. By the CDC's estimates, for example, the actual number of Americans infected with West Nile last year may well have topped 43,000.

Part of the reason for this discrepancy could be the fact that most of those infected -- about 80 percent -- show no symptoms at all.

Roughly 20 percent of people infected with the virus develop an illness known as West Nile Fever, known for high fever, headache, body aches, nausea, vomiting and a rash on the abdomen and back. These symptoms may last from a few days to a few weeks.

Less than one percent of those who are infected with the virus develop life-threatening encephalitis or meningitis. These conditions can lead to high fever, headache, seizures and even coma.

What a West Nile Epidemic Would Mean

Because the survival rate of infection with West Nile is so high, it is unlikely that widespread West Nile infection would fit the profile of what comes to most people's minds when they hear the word "epidemic" -- a nightmare scenario of quarantines and mass casualties.

"It is not a major threat," said Michael Diamond, an associate professor in the Division of Infectious Diseases Washington University School of Medicine in St. Louis. Diamond specializes in the study of mosquito-borne illnesses like West Nile.

"Even if it was the worst year, there might be 10,000 to 20,000 diagnosed cases with 250 to 500 deaths," he said. "This is small in comparison to influenza, for example."

"A higher number of cases than anticipated from past experience appear to be occurring, which is a definition of an epidemic," said Stephen Wikel, professor and acting chair of immunology at the University of Connecticut School of Medicine. "The general public often has a very different perception of the meaning of the word 'epidemic.'"

This doesn't mean, however, that such an epidemic would be without its toll. Schaffner said such an occurrence "would produce many cases of encephalitis, especially in older persons who are the most vulnerable.

"There also would be cases of West Nile Fever without encephalitis in younger persons who would have to be evaluated in doctors' offices and emergency rooms -- using substantial medical resources to rule out other causes of fever."

Schaffner adds that the fact that the spread of the virus can be controlled

"Although numerically small, deaths are potentially preventable, so obviously they are important," he said. "The need for information and public health resources is clear."

Protecting Yourself

Wikel said that since West Nile virus is carried primarily by mosquitoes, stemming its spread depends largely on public education and on the proper use of repellents and the elimination of breeding grounds for the insects.

Individuals can take other simple measures to safeguard themselves from the disease, such as wearing long-sleeved shirts and long trousers in the early morning and evening, avoiding outdoor activities in the evening, and repairing broken screens and doors.

Such measures may be particularly important for the elderly, who are more susceptible to the ill effects of the virus.

"Everyone should be warned to protect themselves from mosquito bites, but that group in particular should be emphasized," Tulane's Wesson said.