MONDAY, May 5, 2003 (HealthDayNews) -- When the West Nile virus first entered North America in 1999, 62 cases were detected in humans and seven people died. Last year, more than 4,000 people were infected, 284 died, and the virus had infiltrated 44 states.
So what does the outlook for this year hold? The predictions, according to preliminary estimates by experts, are fairly grim.
"2002 was an historic epidemic. It was the largest epidemic of mosquito-borne viral encephalitis in the Western hemisphere ever recognized," says Dr. Roy Campbell, a medical epidemiologist with the U.S. Centers for Disease Control and Prevention (CDC).
Although Campbell, who works in the CDC's Division of Vector Borne Infectious Diseases, stresses the difficulty of accurately predicting the course of the virus, he does note some disturbing trends.
"We've already detected widespread West Nile Virus activity in Louisiana for the second year in a row in mosquitoes and birds, in sentinel chickens in Florida and a few birds in Georgia," he says. "The Gulf states once again seem to have a significant amount of early season activity. We're worried that this early season activity could be followed by another large epidemic in humans there this year, although that's not for sure."
West Nile virus is spread by mosquitoes, which pick it up when feeding on infected birds and animals. About one in 200 people infected with the virus actually become seriously ill with encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord), while about 20 in 200 will experience less severe, flu-like symptoms.
The season for infections in humans stretches roughly from late spring to early fall, with peak infection rates occurring in late summer and early spring. Last year, human infections in the United States occurred as early as May 19 and as late as December 14, reports Vicki Kramer, chief of the Vector Borne Disease Section of the California Department of Health Services in Sacramento.
California only had one human case of West Nile last year, a sort of a mystery case because no mosquitoes or birds were found that tested positive for the virus. The woman, a resident of Los Angeles County, had not traveled, however, so she presumably became infected in California.
This year, the state will probably not be so lucky, Kramer predicts: "We fully expect West Nile virus to become introduced and to become established in California this summer."
Once West Nile has infiltrated an area, it's not likely to disappear. "That seems to be the pattern we're seeing, that if you had it last year you're probably going to have the virus this year," Campbell says.
Oregon, Utah, Nevada, Arizona, Hawaii and Alaska have had no West Nile activity at all thus far. Officials in the first four states are anxiously monitoring to see if they stay clean again this year. As for the two non-contiguous states, Campbell says, "We don't think it's going to be a problem in Alaska because it's too far north for efficient transmission in terms of the climate but if it gets introduced in Hawaii, it could do quite well." (Washington state, Idaho, New Mexico, Maine and New Hampshire had reports of West Nile in birds or animals, but not humans last year.)
Not only is it possible that the virus will spread further geographically, it may also start infecting humans earlier. Birds and animals, which have a much longer transmission period than humans, seem to be showing earlier activity this year.
According to Dawn Wesson, an associate professor of tropical medicine at Tulane School of Public Health and Tropical Medicine in New Orleans, West Nile-positive dead birds are coming earlier and in far greater numbers than last year.
"The state has 26 West Nile Virus-positive mosquito pools already this year in eight parishes, representing four species of mosquitoes," she says. "We had none at this time last year."
"Last year, within two weeks of seeing some of the dead, West Nile-positive birds, we were seeing human cases," Wesson adds. "If that is replicated this year, there could be people out there already infected."
Meanwhile, experts are recommending the same precautions as any other year. Individuals over the age of 50 and those with compromised immune systems are the most vulnerable to serious illness and need to take extra care.
Officials advise getting rid of standing pools of water where mosquitoes might breed, including toys and flower pots in your backyard as well as clogged gutters. Wear insect repellant, preferably one containing DEET, when you go outside along with long-sleeved shirts and long pants. Try to avoid going outside when mosquitoes are most hungry -- in evenings and early mornings. Make sure screen doors and windows are in good repair.