Experts predict the continued spread of West Nile virus this summer, with the potential for serious outbreaks in numerous states, including California. Last year 9,858 human cases were reported to the federal CDC —more than double the 2002 total of 4,156—with 258 deaths. West Nile first emerged in the United States in New York in 1999, when reported cases totaled 62. Since that time, the virus—spread by migratory birds—has moved relentlessly westward and southward. Mosquitoes acquire the virus when they bite infected birds; they then transmit it to humans.

According to Jonathan F. Day, PhD, who specializes in tracking mosquito-borne illnesses including West Nile, the virus will continue its spread and may reemerge in regions where it has been present, but not in epidemic proportions, as it was in previous summers. “We are very concerned about having a large number of cases in California,” said Dr. Day, who is a Professor at the University of Florida in Gainesville.“The Southeast has not seen a major epidemic of West Nile since the virus was first introduced, and southeast Florida—from Miami to Vero Beach—is particularly at risk for a major spread of the disease if the weather conditions are right,” he noted.

Dr. Day and other experts are also carefully monitoring Texas, where a high case count is possible. He suggested that disease incidence in the Northeast may be higher than in the past few years and is particularly concerned about the Delaware River Valley. “This is an area that traditionally has many mosquito-related illnesses, so we think it’s a possibility for significant West Nile activity this year,” he noted.

Lyle R. Petersen, MD, MPH, Director of the Division of Vector-Borne Infectious Diseases in the CDC’s National Center for Infectious Diseases, agreed that the disease could have a big impact again this year. “We’ve had two very large outbreaks, both in 2002 and 2003, and there’s no reason to believe that 2004 will be any better,” he said. “The best way to avoid West Nile virus is to avoid being bitten by mosquitoes,” he added. “The best way to do that is to wear insect repellents that contain DEET [N,N-diethyl-3-methylbenzamide]. These are the most effective repellents.” Dr. Petersen also recommended wearing long sleeves or long pants to provide a barrier against mosquito bites and being especially cautious about protective measures at dawn or dusk when mosquitoes that carry West Nile virus are most likely to be active.

PREVENTING MOSQUITO BITES

DEET-based insect repellents are recommended by the CDC, as well as by state and local public health agencies, to help protect against mosquito bites that may transmit the West Nile virus. The CDC says that DEET-based repellents “are very safe” when used according to label directions. The American Academy of Pediatrics added that “products containing DEET are the most effective mosquito repellents currently available” and suggests that repellents containing up to 30% DEET can be used to help protect against insect- and tick-borne disease for children 2 months or older.

Scientific research—including a study by Dr. Day and his colleague Mark S. Fradin, MD—has demonstrated that DEET is the most effective personal-use insect repellent ingredient available. It has been studied extensively by scientists and health care professionals since its introduction in 1957.

DEET-based repellents are also effective in helping to prevent tick-borne diseases such as Lyme disease, which is also expected to hit a record high in the United States this summer, and Rocky Mountain spotted fever, which is sometimes fatal, even among teens and children, the investigators noted.

PROPER USE

“As with any product, read and follow label directions when using DEET-based repellents,” said Susan Little, Executive Director of the DEET Education Program, which operates under the auspices of the Consumer Specialty Products Association. She recommends that physicians advise parents and other caregivers to apply repellents to young children rather than letting them do this themselves. Parents should be reminded to “apply [the product] only to exposed skin, not under the child’s clothing,” she said, “and do not apply insect repellent to a very young child’s hands, since these youngsters often put their hands in their mouths or rub their eyes.”

The higher the concentration of DEET in a repellent formulation, the longer the product will ward off the biting insects. Physicians should inform their patients that lower concentration products are adequate when exposure to biting insects and ticks is brief and recommend the use of higher concentration products for longer outdoor periods and/or when populations of mosquitoes or other biting pests are high. “If the humidity is high and/or it is very hot, more frequent applications may be needed, since perspiration washes the repellent from the skin,” she noted.

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Suggested Reading
Fradin MS, Day JF. Comparative efficacy of insect repellents against mosquito bites. N Engl J Med. 2002;347:13-18.