August 30, 2012
West Nile Virus In The US Setting Records For August
Lawrence LeBlond for redOrbit.com - Your Universe Online
US cases of West Nile virus (WNV) have risen 40 percent in just one week, with 1,590 confirmed infections, and 66 confirmed deaths, up from 47 last week, according to the US Centers for Disease Control and Prevention (CDC) on Wednesday.
This is the highest case count through the last week of August since the virus was first detected in 1999, and nearly half of all infections have so far occurred in Texas, with 894 confirmed cases and 34 deaths.
Reuters´ Sharon Begley reports that half of all cases are considered serious illnesses, where the CDC says this is the best indicator of West Nile activity because most mild cases do not get reported and symptoms may not be recognized and diagnosed as West Nile due to similarities to regular flu.
“We think the numbers will continue to rise,” Dr. Lyle Peterson, director of the CDC´s Division of Vector-Borne Infectious Diseases, said in a statement. Peterson noted that even as the number of cases are setting records for the month of August, they still are below the records for a full year: 9,862 cases and 264 deaths in 2003.
Dr. David Lakey, commissioner for the Texas Department of State Health Services, said he expects numbers to rise. It looks as if 2012 will be the worst year so far when it comes to WNV cases. Texas reported 40 deaths in 2003 due to the virus, and Lakey expects the death toll will surpass that number this year.
All 48 contiguous US states are now reporting evidence of WNV, with 43 states reporting at least one person with the illness, according to the CDC.
Most people who become infected with WNV have fever, headache and body aches, and most people get better on their own in a few days. Less than one percent go on to develop neurological symptoms such as stiff necks, coma, and paralysis.
Health officials expected cases of West Nile to peak in mid-to-late August, with cases continuing to crop up through October. But, because symptoms generally take up to two weeks to appear, case reports usually lag behind when people actually become infected.
“I´m not convinced that we have peaked. We may have plateaued,” Lakey told the Associated Press on Wednesday.
As Hurricane Isaac pounds the Gulf coast, many believe that it may cause an insurgence of mosquitoes and WNV in the south, as in past years with floods and hurricanes mosquito numbers have grown. But the CDC said it does not expect the storm to have that much of an impact on cases in the south.
Heavy storms can wash out mosquito breeding grounds, although standing water can aid breeding, Petersen said. Many other factors, such as the population of infected birds, influence the severity of West Nile outbreaks, he said.
Petersen urged the public to take proper precautions to prevent getting sick regardless of whether they live in a state with high or low case counts.
“The virus is endemic at this point throughout the United States,” with the possible exception of high-altitude regions such as the Rocky Mountains, said Peterson. “There is a risk almost everywhere.”
However, the bulk of cases (more than 70 percent) have appeared in just six states: South Dakota, Mississippi, Oklahoma, Louisiana, Michigan, and Texas, which has seen the most cases for a single state.
Peterson noted that he believes, due to a low percentage of only 2 or 3 percent of cases of WNV, that the actual number of cases is 30 to 50 times higher than what is reported.
This is mainly due to the fact that 80 percent of infected people have no symptoms, said Dr. Robert Haley of the University of Texas Southwestern Medical Center in Dallas, in a report last week published in JAMA: the Journal of the American Medical Association.
There is no treatment nor vaccine for WNV. The disease is transmitted by Culex pipiens, the common house mosquito, and the only preventative measure would be to avoid being bitten by mosquitoes. People can reduce their risk by eliminating standing water on and around their property, such as that found in small pools, old tires, birdbaths, outdoor flower pots, etc.
Public health experts are baffled about why 2012 has been such a big year for WNV. Peterson, however, said a US heat wave is most likely a strong contributing factor.
“Higher temperatures foster faster reproduction of both the mosquito and the virus,” Tony Goldberg, professor of epidemiology at the University of Wisconsin, Madison, who has studied urban outbreaks of West Nile since shortly after the virus arrived in the Midwest, told Reuters.
Higher temperatures also increase mosquito activity, making them more likely to be flying around, typically at dusk and dawn, looking for a meal.
This year´s continuous cycle of droughts and downpours may also be playing host to higher onset of WNV as mosquito populations explode.
Intense rain fills drainage ditches, storm sewers and culverts, and washes grass clippings, leaves and other organic matter into those pools of standing water, explained Goldberg. Mosquitoes prefer to breed in water that has rotting organic matter.
With normal rainfall, those breeding pools are washed away in the next storm. But when heavy spring rains are followed by summer dry spells, as has been the case in much of the United States this year, the breeding pools remain for weeks or months, said Goldberg, allowing mosquitoes to continuously breed, increasing the likelihood of infections from WNV.
"As we keep getting more climate extremes," he said, "there will be more years with many more cases of West Nile."