The year started in the United States with a mild flu season but ended up being marked by deadly outbreaks of fungal meningitis, West Nile virus and Hantavirus.

Tainted steroid medication has been cited as the cause of the meningitis outbreak that killed 39 people.

Weather contributed to the worst outbreak of West Nile virus since 2003 and an unusual outbreak of Hantavirus in California's Yosemite National Park.

Transmitted by infected mice, Hantavirus is a severe, sometimes fatal syndrome that affects the lungs. West Nile can cause encephalitis or meningitis, infection of the brain and spinal cord or their protective covering.

As of December 11, 5,387 cases of West Nile virus had been reported in 48 states, resulting in 243 deaths, the CDC said in its final 2012 update on the outbreak. The 2003 outbreak left 264 dead from among nearly 10,000 reported cases.

A large number of cases this year occurred in Texas, Louisiana and Mississippi where there are large mosquito populations.

CDC and state officials have said that rainfall in the spring and record high summer temperatures contributed to the severity of the outbreak by affecting mosquito populations, which transmit the disease by biting humans and animals.

Health officials said that only a small percentage of cases of West Nile virus are reported because most people have no symptoms and about 20 percent have mild symptoms such as aches and fever. One in 150 people with West Nile virus develop other illnesses such as meningitis and encephalitis.

The biggest outbreak in nearly two decades of Hantavirus, which emerges in dry and dusty environments, cropped up during the summer in 1,200-square-mile (3,100-square-km) Yosemite National Park, killing three of 10 infected visitors.

The National Park issued warnings to 22,000 people who may have been exposed to the rare disease, and 91 Curry Village cabins in the park were closed in late August.

In early September, a 78-year-old judge named Eddie Lovelace was rushed to a hospital in Nashville, Tennessee. Thought to have had a stroke, he died a few days later.

After a large outbreak of fungal meningitis was linked to tainted steroid injections, Lovelace's cause of death was revised. He became the first documented death in a meningitis outbreak that has infected 620 people and killed 39 in 19 states.

The New England Compounding Center in Framingham, Massachusetts, was closed after investigators found that it had shipped thousands of fungus-tainted vials of methylprednisolone acetate to medical facilities around the United States. The steroid was typically used to ease back pain.

More than 14,000 people were warned that they may have had an injection of the tainted steroid. Doctors continue to see new cases of spinal infections related to the steroid, and cases of achnoiditis, an inflammation of nerve roots in the spine.

The outbreak led two Democratic lawmakers in the U.S. House of representatives to introduce legislation to increase government oversight of compounded drugs.

And what lies ahead in 2013?

"While there are some trends we can predict, the most reliable trend is that the next threat will be unpredictable," said Centers for Disease Control and Prevention (CDC) Director Thomas Frieden.

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Frederick A. Murphy / CDC handout via EPA file

A new virus that appears similar to rabies, but has the symptoms and lethality of Ebola, shown here, has been dubbed the Bas-Congo virus. It killed two teenagers in the Congo in 2009.

A virus that killed two teenagers in Congo in 2009 is a completely new type, related to rabies but causing the bleeding and rapid death that makes Ebola infection so terrifying, scientists reported on Thursday. They’re searching for the source of the virus, which may be transmitted by insects or bats.

The new virus is being named Bas-Congo virus, for the area where it was found.  Researchers are finding more and more of these new viruses, in part because new tests make it possible, but also in the hope of better understanding them so they can prevent pandemics of deadly disease.

The virus infected a 15-year-old boy and a 13-year-old girl in the same village in Congo in 2009. They didn’t stand a chance, says Joseph Fair of Metabiota, a company that investigates pathogens. Fair is in the Democratic Republic of Congo now, under contract to the U.S. Agency for International Development (USAID) to help battle an ongoing Ebola outbreak.

“They expired within three days,” Fair said in a telephone interview. “It was a very rapid killer.”

A few days later a male nurse who cared for the two teenagers developed the same symptoms and survived. Samples from the lucky nurse have been tested and it turned out a completely new virus had infected him, Fair and other researchers report in the Public Library of Science journal PLoS pathogens.

The genetic sequences went to Dr. Charles Chiu, of the University of California, San Francisco.

“We were astounded that this patient had sequences in his blood from a completely unknown and unidentified virus,” Chiu said. They weren’t expecting that.

“Congo is very much known for having Ebola and Marburg outbreaks. Yet about 20 percent of the time we have hemorrhagic fever outbreaks that are completely negative, which means unknown causes and they are not Ebola.”

The sequencing puts this new virus on its own branch of the bad virus family tree -- somewhat related to Ebola and the virus that causes Lassa fever, another horrific killer, and most closely related to the rhabdoviruses. This family usually only infects animals with one notable exception -- rabies.

But rabies is not known to cause hemorrhaging. It’s plenty horrible on its own, of course, killing virtually all patients if they aren’t vaccinated soon after infection. 

A nurse who took care of the first infected nurse had antibodies to the new virus. It doesn’t look like the teenagers infected one another, says Fair, but they probably infected the first nurse, who probably infected the second. Tests of other villagers have found no more evidence of the virus, however, which is good news.

“Although the source of the virus remains unclear, study findings suggest that Bas-Congo virus may be spread by human-to-human contact and is an emerging pathogen associated with acute hemorrhagic fever in Africa,” the researchers wrote.

Africa is loaded with nasty viruses. Lassa fever virus comes from a family known as arenaviruses and causes 500,000 cases of hemorrhagic fever a year. Crimean-Congo hemorrhagic fever and Rift Valley Fever viruses are in another family called bunyaviruses; Ebola and Marburg viruses are filoviruses that kill anywhere between 30 percent and 90 percent of victims. They’re also helping wipe out great apes such as gorillas in Central Africa. This adds a new one to the list.

It worries Chiu because its closest relative is spread by biting flies in Australia. “We think that is potentially a valuable clue. This virus may have come from an insect vector,” Chiu says. “What is scary about this virus is if it does happen to be spread by insects, it has the potential to be something like West Nile."

West Nile showed up in the United States for the first time in 1999, having never been seen here before. It causes regular outbreaks in Africa and parts of Europe, however, and some experts think a mosquito or an infected person carried it on a flight to New York. It’s killed 147 people in an especially bad U.S. outbreak this year, although more than 90 percent of people infected with West Nile never even know it.

New viruses often cause disease -- there was severe acute respiratory syndrome or SARS, which killed 800 people and infected 8,000 in 2003 before it was stopped. Scientists are now watching a similar virus that has emerged in the Middle east.

Chiu says there is not enough information to know how deadly the new Bas-Congo virus is.

“It  has probably been lurking out there in remote areas and causing sporadic cases of hemorrhagic fever and no one had the resources to discover it,” Chiu said. “This is probably the tip of the iceberg. I believe there are many, many more of these emerging viruses that have yet to be discovered,” he added.

“This points to the importance of being vigilant, especially these remote areas of Africa and Asia. This is the area that I believe the next generation of emerging viruses will come from.”

Fair agrees, and says his team will be looking. They’ll also be checking to see if bats or insects can spread it. “It is a frightening prospect. That is why the next step in this process is to look for the vector,” Fair said.

That’s not so easy. Fair’s team and hundreds of other scientists have been looking for the reservoir -- the animal or insect source --of Ebola. That would be a bat or other creature that can carry it without getting sick itself. So far they have had no luck, although fruit bats are a major suspect.

And for the new Bas-Congo virus, the trail is now three years old. “Everything we do will be as a forensic investigation,” Fair said. “We really have to go look for a needle in a sack of needles.”

And in the meantime, there’s an outbreak of Ebola to cope with. Fair says a coordinated effort is going on, although this isn’t the worst outbreak he has seen. It’s killing about 30 percent to 40 percent of patients -- not nearly as bad as some strains, which killed up to 90 percent of victims.

“If you had to get Ebola, this is the strain to get,” he said.

Related stories:

• West Nile cases jump
• New virus related to SARS
• Rabid animals on the rise as human vaccine supply tightens
• Health teams face real-life horror in Ebola battle
• Ebola out of control in Congo, WHO says

Jim Gathany / Centers for Disesase Control via EPA

A Culex quinquefaciatus female mosquito feeds on human blood. This species is a known vector for West Nile Virus.

DALLAS —When Dr. Robert Haley spotted a dead blue jay lying in his neighbor's driveway early this summer he became suspicious. When he saw another blue jay dead in the birdbath at his Dallas home the next morning, he knew it was a bad omen of disease.

What he could not predict at the time was that the bird corpses heralded one of the worst U.S. outbreaks of West Nile virus on record, with nearly 40 percent of cases in Texas alone.

"It's unusual to see dead birds lying in the open," said Haley, chief of epidemiology at the University of Texas Southwestern Medical Center. "Typically, birds die in some out-of-sight place or they are carried off by animals if they die out in the open."

West Nile is transmitted from sick birds to humans and other mammals by mosquitoes and was first detected in the United States 13 years ago, in New York City. Texas declared a state of emergency last month after seeing the worst toll from West Nile this year, which has reached 3,545 total cases and 147 deaths nationwide, according to the U.S. Centers for Disease Control and Prevention. Other states with large outbreaks include Mississippi, Michigan, South Dakota, Louisiana, Oklahoma and California.

"From the beginning, I thought it could be a bad year," said Haley, who spent 10 years working for the CDC and now lives in the epicenter of the outbreak. "But it turned out to be much worse than anyone imagined. It was a public health disaster."

More than 400 new U.S. cases of West Nile virus emerged in the last week, in an outbreak that remains one of the worst of record but has begun to show signs of slowing down. Experts hope the outbreak has peaked as cooler weather sets in and widespread pesticide spraying takes effect. Now is the time to learn the lessons for the future.

Good weather for mosquitoes
Five counties within the Dallas-Fort Worth area — the fourth largest metropolitan area in the country — recorded 28 of the 63 deaths and 869 of the 1,429 cases reported to the Texas Department of State Health Services by Tuesday. Dallas County alone has recorded 16 deaths and 371 cases, according to county authorities.

CDC and state officials believe a year's worth of record high temperatures and intermittent rainfall this past spring contributed to the severity of the epidemic by affecting bird and mosquito populations. Following a record hot summer and drought conditions in 2011, Dallas-Fort Worth had a warm winter with fewer than normal freezes followed by bouts of rain in the spring, officials said.

"One of the things we are closely looking at is the effect of weather on this year's outbreak," said Lyle Petersen, director of the CDC's Division of Vector-Borne Infectious Diseases. "West Nile outbreaks tend to be difficult to predict. Why it occurred in Dallas more than other areas is a matter of speculation at this point, and it's something that we're going to be looking at very carefully."

In addition, health officials in the region have witnessed an especially large number of neuroinvasive cases, the more severe form of the disease that often leads to meningitis and encephalitis.

Haley estimates that about 25,000 people likely were infected with West Nile in Dallas County this summer. Of those, about 80 percent showed no symptoms at all, while many of the remaining residents came down with West Nile Fever, a mild form of the disease that is largely under-reported and only sporadically tested. State figures show only that Dallas County recorded 168 West Nile fever cases and 154 neuroinvasive cases.

Death toll may rise
Haley and others say the worst should be over in terms of new infections, but more cases are expected to be reported due to the lag time between infection, testing for the virus and reporting to state agencies and the CDC. The death toll is also likely to rise as it can take weeks to months for patients to deteriorate.

In the meantime, Dallas residents are still coming to terms with the ravages of the outbreak, which dwarfs the four deaths seen in the region in 2006.

With no vaccine to prevent West Nile in humans, the only defense is prevention — wearing insecticide outdoors and pesticide spraying by ground and air.

Dr. Don Read, a surgeon in Dallas who was infected by neuroinvasive West Nile in 2005 while walking in his Dallas neighborhood and now runs a support group for survivors, said people tend to think they are invincible. "I didn't think I would get it until I did. It only takes one mosquito bite."

Read, who was infected at age 63, spent almost five weeks in the intensive care unit. He now wears braces on his legs due to polio-like paralysis, but considers himself lucky to be alive.

The sound of a plane buzzing overhead spreading insecticide in the suburban community of Southlake was a welcome sound to Ann Dachniwsky, 47, who spent much of the summer so fatigued from neuroinvasive West Nile that her only activity was "going from the bed to the couch back to the bed."

At the height of her illness, her husband and three children took turns waking her up every few hours to force her to drink.

"My balance and sight were affected so I could barely work or see. I was flat on my back for weeks," she said. "I was a healthy, active person. I'm getting better but I can barely manage one activity without needing to lay down afterwards."

Dachniwsky's family had pleaded with the Southlake City Council to allow aerial spraying of pesticide for the first time since an encephalitis outbreak nearly 50 years ago. Dallas and nearby Denton counties conducted aerial spraying missions in August. Southlake, which is partially in Denton and Tarrant counties, was sprayed. Officials in Tarrant County, home of Fort Worth, chose to spray only by ground.

As the outbreak has slowed, Dallas County health officials continue to be criticized both for not moving fast enough to start spraying and also for going too far in the breadth of the aerial spraying program once it started.

"We had a protocol in place and we followed it," said Zach Thompson, director of the Dallas County Department of Health and Human Services. "We started with public education, followed by localized ground spraying, then enhanced ground spraying and finally aerial spraying."

As research and analysis of the outbreak continue, Thompson said officials will work diligently to avoid a repeat of history.

"We were surprised by the magnitude of the outbreak this year but we feel that our response was appropriate," Thompson said. "Hindsight is 20/20." 

More stories in Vitals

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LM Otero / AP file

Mosquitoes were sorted at a laboratory in Dallas County, Texas, as an outbreak of West Nile virus mounted last month.

A spike in West Nile virus infections in the U.S. this summer has strained the nation’s laboratory testing capabilities, creating brief shortages of diagnostic test kits and forcing lab staffers in some states to work extra shifts or rely on temporary hires for help.

The outbreak has surged to at least 2,636 cases and has caused 118 deaths, officials with the Centers for Disease Control and Prevention said Wednesday. Of those, 1,405 cases have been serious neuroinvasive disease infections. That ranks it as the most serious outbreak to date since the mosquito-borne virus was detected in the U.S. in 1999., said Dr. Lyle Petersen, director of the CDC's division of vector-borne infectious diseases.

The epidemic also has highlighted gaps in the system used to track and treat illness outbreaks and epidemics, experts say.

“West Nile just brought it back to us,” said Irina Lutinger, senior administrative director for NYU’s Langone Medical Center and a spokeswoman for the American Society for Clinical Pathology. “We only have limited resources to accommodate such an increase.”

In Oklahoma, two primary in-state reference labs had to halt West Nile tests briefly in late August because they ran out of kits, even as the nation’s leading commercial labs had to suspend or delay tests -- or prioritize samples of those suspected of having the most serious illnesses.

“We had a complete pause in testing,” said Kristy Bradley, the Oklahoma state health epidemiologist. “What I was surprised about is that it reached such a critical level before we were aware of it.”

In Texas, where latest figures show West Nile has sickened at least 1,127 people and caused 50 deaths, state public health laboratory staffers have been working extended shifts, including weekends and holidays. Four temporary lab workers were recently hired to help meet demand, said Grace Kubin, director of the Texas Department of State Health Services laboratory.

Neither health officials nor the makers of the tests would release figures about the number of West Nile tests performed this year compared with previous years.

However, everyone agrees that it is way up.

“I can say it’s probably doubled, easily, for the human testing,” said Kubin. Tests of environmental samples from mosquito pools, for instance, have tripled over normal levels, she said.

West Nile virus is a mosquito-borne disease that has been detected this year in 48 states. Most infected people, about 80 percent, never know they have it. About 20 percent, however, develop West Nile fever, which causes flu-like symptoms, including headache, fever, body aches, swollen lymph glands and, sometimes, a rash. Less than 1 percent of infected people develop severe West Nile disease, the neuroinvasive form of the infection, which can lead to meningitis or encephalitis. 

The outbreak is still short of the worst-ever West Nile season in the U.S.; in 2003, there were 9,862 total cases, 2,866 neuroinvasive cases and 264 deaths. But those figures included artificially high numbers inflated by one state's posting of cases of West Nile fever, Petersen said. This year's high number of neuroinvasive cases for the second week of September make it the worst outbreak to date, Petersen said.

However, he said that West Nile cases typically peak in mid-to-late August and that officials believe the outbreak is beginning to wane, especially as cooler weather approaches.

"Based on historical data, we’ve turned the corner on the epidemic," Petersen said. "We’re hopeful that the worst of the outbreak is behind us."

The shortage of testing kits manufactured by the nation’s leading provider, Focus Diagnostics, was caused largely because unanticipated demand outstripped supply, said Wendy Bost, spokeswoman for Quest Diagnostics, the commercial lab that runs Focus. The backlog is adding as much as three to five days to the turnaround times on West Nile virus tests, Bost told NBC News.

“The magnitude of the current outbreak of West Nile virus in the United States was not widely anticipated by public health authorities,” Bost said.

But Food and Drug Administration officials said they became concerned about a potential shortage of test kits in late August, when they contacted Focus and found the company had backorders of the West Nile virus IgM antibody test kit. That test determines whether a patient is acutely infected with the West Nile virus.

At the urging of the FDA, the CDC and other health officials, Focus ramped up production of the test kits, which can take weeks to produce and have a shelf life of about 24 months.

That helped avert a larger problem, said Kelly Wroblewski, director of infectious disease programs for the Association of Public Health Laboratories.

“At this point, there isn’t really a shortage,” she said. “In most cases, if stoppage had to happen at all, it only had to happen for a maximum of two days. I think it’s a supply and demand situation like anything else.”

In some states hit hard with West Nile, such as South Dakota and Mississippi, officials said they’ve managed to keep up with testing demand.

“We are not falling behind,” said Lon Kightlinger, the South Dakota state epidemiologist.

In Texas, lab director Kubin said she actually heard early that there might be a shortage of test kits and asked her supervisor to stock up.

Because there's no treatment for West Nile virus in humans, no one was denied care because of the lab test delays. With a different virus, however, any lag in testing could have slowed vital treatment, experts said. 

However, the shortage of lab techs to handle West Nile demand highlights a larger issue, said Lutinger, of the ASCP.

In the U.S., about 11,000 new lab workers are needed every year, but only 6,000 are graduating, Lutinger said. As older workers, called laboratorians, retire and fewer newcomers fill the slots, the vacancy rates in the field are growing.

“By 2018, if more students and second careerists are not recruited to become laboratory professionals, the shortage could be as high as 18 percent in areas such as blood banking,” she said.

About 70 percent of doctors’ decisions are based on the outcome of lab analysis, Lutinger noted.

“At a time of critical importance of providing diagnosis and test results to arm these physicians with these results, we frequently face the challenges of being unable to have trained technologists,” she said.

Too few lab workers may mean delays in diagnosis and treatment of disease – and slower response to large-scale public health crises, such as the 2009 H1N1 flu pandemic.

“It certainly has the potential to hinder our ability to respond in emergencies,” said Wroblewski.

Part of the problem may be that it’s a low-profile profession, so not many students consider becoming laboratory technologists, she said. The starting pay for public labs can be moderate, about $40,000 to $50,000 a year, and commercial labs may steal workers away by paying up to 50 percent more.

That presents a challenge for public health labs, as the current West Nile outbreak demonstrates.

“If the projections of the ASCP are correct and this continues to be a problem for the next several years, the situation of public health labs being in a good position could change,” Wroblewski said.

Related stories: 

• Is spraying for West Nile safe?
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CDC

West Nile virus cases, which are spread by mosquitoes, continue to rise in the U.S., health officials said.

West Nile cases in the U.S. continue to climb, jumping 25 percent in a week, with 1,993 cases nationwide and 87 deaths in the country’s worst outbreak for this time of year since the virus was detected here in 1999, health officials said Wednesday.

Texas continues to log the most cases, with at least 888 reported illnesses and 35 deaths, according to figures from the Centers for Disease Control and Prevention. But Dr. David L. Lakey, commissioner of the Texas Department of State Health Services, said his latest figures actually show that Texas has reported 1,013 cases and 40 deaths.

"Officially, this is the worst week ever for West Nile in Texas," Lakey said.

The nationwide numbers were up from 1,590 cases and 65 deaths reported Aug. 28, said Dr. Lyle R. Petersen, director of the CDC's division of vector-borne infectious diseases. 

"We expect this increase to continue for the next several weeks, probably until October," said Petersen, who added that he was infected with the virus himself a few years ago. Many cases have not yet been logged because of the lag in reporting time, he said.

More than half the cases -- 54 percent -- are the serious neuroinvasive variety, which can lead to encephalitis or meningitis. The vast majority of West Nile infections, some 80 percent, are so mild that people don't know they're infected. About 20 percent develop symptoms and about 1 percent may develop serious, neuro-invasive illnesses.

West Nile has been detected in 48 states, with human cases reported in 44 states, Petersen said. Five other states -- Missississippi, South Dakota, Oklahoma, Louisiana and California -- also continue to report high numbers of cases.

The outbreak is still far shy of the worst year ever for West Nile virus infections and deaths in the U.S. In 2003, 9,862 cases were reported and 264 people died, according to the CDC's records.

The disease is spread by infected mosquitoes, which breed in water. Officials have said a mild spring, hot weather and heavy rains may have contributed to the epidemic this year, but they're not sure. Other factors include the type and number of infected mosquitoes, the number of birds that become infected and other factors, he said. 

“There’s a very complicated ecology to the transmission of these viruses in nature,” Petersen said. Why the outbreak is so bad in Texas is a matter of speculation, he added. 

The outbreak may have begun to peak, said health officials, who hope they're seeing a decline in cases -- and in mosquitoes that test positive for the virus, Lakey said. Sprayed pesticides appear to have reduced the population of mosquitoes, he added.

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• West Nile spreads across the US, but don't expect a hurricane effect
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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:

Is spraying for West Nile safe?

Cluster of hantavirus cases worries officials

Five things you need to know about West Nile virus

Texas state health officials, alarmed by the worst outbreak of West Nile virus since the infection first hit the United States in 1999, started spraying insecticide from airplanes Thursday night.

Years of research show the poisons being used in the spray are safe for humans — and certainly safer than the virus, health officials said. But blogs and social media lit up with concerned comments from people afraid the insecticide might hurt them, their children or other creatures in the environment. 

“This is not science. It is ignorance, fueled by chemical corporations seizing the opportunity to poison everyone,” Judith Winchester, a Dallas-area designer, posted on the North Texas Poison Center 's Facebook page.

The poison center and state health officials, as well as the Centers for Disease Control and Prevention and the Environmental Protection Agency, say the spray being used is about as safe as an insecticide can be. Called Duet, it’s made up of two products, both synthetic versions of a chemical made by chrysanthemum flowers.

AP Photo/courtesy the Northwestern Mosquito Abatement District

Culex pipiens, left, is the primary mosquito that can transmit West Nile virus to humans, birds and other animals. The bite of this mosquito is very gentle and usually unnoticed by people. At right is an Aedes vexans. It is a very aggressive biting mosquito but not an important transmitter of disease.

“Risks with aerial spraying are very, very low, especially compared with the risk of disease,” said Carrie Williams, a spokeswoman for the Texas State Department of Health Services. “We believe it is a safe and very effective approach for Dallas.”

Insecticides first got a bad name back in the 1960s, when it became clear that DDT was killing birds by thinning their eggshells. Then it turned out it was a likely cause of human cancer, too, and it was banned in the U.S. in 1972.  And because so many insecticides are nerve agents, they have worried doctors, environmentalists and the public. There’s one class of pesticides called organophosphates that do appear to damage the nervous systems of people who get exposed to high amounts — farm workers, for example.

But the pyrethrins in Duet — that’s their chemical name – are formulated differently and work through a different mechanism. They kill mosquitos in very low doses. And they don’t affect mammals, humans included, in the same way they affect insects.

According to the National Pesticide Information Center (NPIC), a joint venture between Oregon State University and the EPA, pyrethrins work by blocking the nerve activity of insects and they kill adult mosquitos by direct contact. But they’re not as dangerous to bigger creatures, even birds.

“Pyrethrins are one of the least poisonous insecticides to mammals," the center says on its website. They break down quickly into inactive forms in the body and don’t build up in the soil. They do, however, kill honeybees and can poison fish and other aquatic life. That’s one reason that Texas is spraying at night – to minimize the effects on bees, butterflies and other beneficial insects.

They are used in pet flea and tick preparations and in lice control shampoo for humans. The EPA says they don’t cause cancer and don’t hurt pregnant women or their unborn babies.

Compare this to West Nile, which has made 30,000 Americans sick since it arrived in the New York City borough of Queens in 1999. “The 693 cases reported thus far in 2012 is the highest number of West Nile virus disease cases reported to CDC through the second week in August since West Nile virus was first detected in the United States in 1999,” the CDC says.

“Over 80 percent of the cases have been reported from six states (Texas, Mississippi, Louisiana, Oklahoma, South Dakota, and California) and almost half of all cases have been reported from Texas.”

West Nile itself isn’t especially deadly — only about 20 percent of infected people even know they have something, other than perhaps a mild cold. And most who develop symptoms get what’s called West Nile fever, which has the typical symptoms of many viruses — fever, headache, tiredness and, sometimes, a rash.

But 1 percent of cases get severe disease, usually meningitis, an inflammation of the spinal cord, or encephalitis, which is inflammation of the brain. These are usually older people are patients with suppressed immune systems. These symptoms can kill quickly or leave people with nerve damage such as paralyzed limbs.

There is no human vaccine for West Nile — there’s one for horses — and no real treatment. People with severe infections get supportive care such as IV fluids to make sure they don’t die of dehydration and breathing support if the nerve damage affects their ability to breathe.

The other chemical used as a defense against West Nile is DEET, an ingredient in the most effective insect repellents. It seems to stop mosquitoes from smelling their human victims. The NPIC says it’s very safe for people. “Nearly all of the DEET that is taken in through the skin is eliminated by the body within 24 hours of applying it,” the center’s website says. It also does not cause cancer and has been used safely by pregnant women.

Williams says the state health department is trying to address fears. “People have kids, they have pets,and we completely understand that people may be feeling apprehensive about this,” she says. “We have relied on the science to help us make the decision.”

And the department says spraying is only part of the answer to the problem. People need to cover up, use insecticide and,most of all, control standing water to stop mosquitoes from breeding. Even a small amount of water in a potted plant can provide a breeding ground for mosquitoes.

Related links:

• Texas sprayed as West Nile virus spreads
• Tenth West Nile death confirmed in Dallas
• West Nile virus on the rise