The nation is heading toward the worst outbreak of West Nile disease in the 13 years that the virus has been on this continent, federal health authorities said Wednesday.
But it is still unclear where and how far cases will spread. Dallas declared an emergency last week, and West Nile deaths have been concentrated in Texas and a few nearby states, including Louisiana, Mississippi and Oklahoma, as well as South Dakota.
So far this year, there have been 1,118 cases and 41 deaths reported to the Centers for Disease Control and Prevention, Dr. Lyle R. Petersen, director of the agency’s division of vector-borne diseases, said Wednesday in a telephone news conference.
“That’s the highest number of cases ever reported to the C.D.C. by the third week of August,” he added. “And cases are trending upward.”
Because it takes some time for symptoms to develop and cases to be reported, those people were probably infected by mosquitoes two to three weeks ago, he said. The agency expects cases to increase through the end of September. In 2003, there were 264 deaths.
It takes three days to two weeks after a bite for symptoms to come on, but they may then be rapid and overwhelming.
Dr. Petersen described his own 2003 bout with West Nile.
“I was out for a jog, and within one mile I went from feeling normal to where I could barely walk,” he said.
Only about one infection in 150 becomes serious enough for the patient to need hospitalization — usually when the virus gets into the brain and spinal cord. But 10 percent of those hospitalized die, and other patients are left paralyzed, comatose or with serious mental problems. A recent study by doctors in Houston found kidney disease high among survivors.
There is no vaccine, and no drug that specifically targets the virus, so health authorities advise people to avoid getting bitten.
As of noon Wednesday, Texas had recorded 25 West Nile deaths, Dr. David Lakey, the state’s health commissioner, said during the same conference call.
The Dallas area has too many miles of roads to cover with mosquito-killing spray trucks, so the state has spent about $3 million — virtually all from the federal government — flying pesticide spray planes at night, Dr. Lakey said.
It is not clear why this is turning into the worst year nationally since the virus was discovered in New York City in 1999, nor why it is particularly concentrated in the Dallas area, Dr. Petersen said. Hot weather is known to increase transmission, but much of the country has suffered from a heat wave and severe drought has gripped the Midwestern Corn Belt.
Some experts theorize that a wet winter followed by drought creates ideal conditions for the culex mosquitoes that spread the virus. They lay their eggs in dirty, nutrient-filled pools like those left when rivers dry up, and they can survive winters with the virus by hiding in tunnels or sewers. Entomologists from HomeTeam Pest Defense in Dallas have, for example, advised residents not to overwater their lawns, which can create pools of standing water.
The virus now exists everywhere in the contiguous 48 states, and all 48 — except Vermont — have found it in local mosquitoes or birds this year. Birds act as a multiplier for the virus, which is then transmitted by mosquito species that bite both birds and humans.
Generally outbreaks begin in the Southern states and move north with warmer weather. Though much of the country experienced heat waves this year, it is not foreordained that all states will have serious outbreaks.
The spread depends on other factors, including what percentage of birds in an area have never been infected and therefore can become multipliers. (In birds and people, survivors develop lifelong immunity.)
“You can have a lot of cases in one area and not in a place just 100 miles away,” Dr. Petersen said
Edited By Cen Fox Post Team
