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West Nile spreads across US but don't expect a hurricane effect, CDC says

West Nile virus is now in 48 states, has made nearly 1,600 people ill and killed 66 of them, federal health officials said on Wednesday. But don’t expect Hurricane Isaac, which is now dumping tons of rain on Louisiana, to make matters any worse, they said.

The case count keeps 2012 on track to be the worst year for West Nile since the virus first came to the United States in 1999, the Centers for Disease Control and Prevention said. And some of the sickest people will never fully recover, the CDC says.

“As of August 28, 2012, 48 states have reported West Nile virus infections in people, birds, or mosquitoes. A total of 1,590 cases of West Nile virus disease in people, including 66 deaths, have been reported to CDC,” the agency says.

“Over 70 percent of the cases have been reported from six states (Texas, South Dakota, Mississippi, Oklahoma, Louisiana, and Michigan) and over 45 percent of all cases have been reported from Texas.”

Health officials are not sure why West Nile is so bad this year or why Texas has been so hard-hit. The very hot summer may have been a factor, but viruses like West Nile have complicated and hard-to-follow life cycles, they said.

West Nile is spread by infected mosquitoes, which breed in water. But CDC officials said they doubted Hurricane Isaac would worsen the epidemic, because mosquitoes like stale, standing water, which is likely to be washed away by a hurricane.

The CDC’s Dr. Lyle Petersen, an expert on mosquito-transmitted disease, says the virus has to pass from mosquitoes to birds and back to mosquitoes to take hold in an area, and big storms mess up that cycle of transmission. “The end result is that hurricanes and floods do not have a major impact,” Petersen told reporters in a conference call.

“Heavy rainfall can certainly eliminate breeding sites rather create them,” he added. But in the weeks after a storm, pools of water can form and make new sites for the insects, he added. After Hurricane Katrina struck the Gulf coast in 2005, a few more cases of West Nile were reported but that was more likely because so many people were outside, repairing homes and cleaning up debris, he said.

“We continue to preach the message of making sure you are raining your yards,” said Texas state health commissioner Dr. David Lakey.

Heat could be a factor, also, if it incompletely dries up pools of standing water. “There has been a lot of speculation about the heat wave this year and could this partially have caused this effect, and the answer is yes,” Petersen said. But, he added, other heat waves have not led to outbreaks.  

 Officials in the Dallas area have been spraying pesticides to kill mosquitoes and Lakey and Peterson both said that should start cutting reported cases of West Nile there. It takes a few weeks for cases to be reported, so they said the number of reported cases will probably rise before it starts to fall.

More than half the cases reported so far this year have been of neuroinvasive disease – meaning the brain and spinal cord are affected. Peterson said it’s unlikely there’s an unusually high proportion of severe cases compared to years past. He notes that 80 percent of people infected with West Nile never even feel particularly sick, and it’s the serious cases that are more likely to get noticed and counted.

But people with neurological symptoms can be in serious trouble. There are three types – meningitis, which is inflammation of the spinal cord; encephalitis, which is when the brain is infected and inflamed; and acute flaccid paralysis, caused when both the brain and spinal cord are damaged. About one in 150 people infected with West Nile develop severe illness, according to the National Institutes of Health.

Patients with meningitis must be hospitalized but usually recover, Petersen said. About 10 percent of patients with encephalitis die, and those who survive may have subtle neurological effects long term. Patients who develop paralysis are in the worst trouble – a third recover, a third have some weakness long term and a third never recover, Petersen said. Some patients have had paralyzed limbs for years now.

Lakey says people of all ages have been affected, but the older people are, the more likely they are to be seriously ill. People with damaged immune systems, such as cancer patients, are at higher risk.

By the time patients with neurological symptoms get to the hospital, the virus has already invaded the nervous system, Petersen says. There’s no drug to treat West Nile virus anyway, and this effect makes it hard to develop one, because it’s hard to make a drug that can penetrate the central nervous system.

And because West Nile is so spread out and sporadic, it’s hard to even try to develop a drug or vaccine to fight it, Petersen said. Drug companies need lots of guaranteed cases of a disease to test whether a new vaccine or drug actually works.

“We are dealing with a low-incidence disease most years, (with) cases that are widely dispersed,” Petersen said. “The thought of trying to produce a phase 3 clinical trial to show efficacy is fairly daunting."


Related links:

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Five things you need to know about West Nile virus