It's back -- and it can be very bad. But just how bad will the 2004 return of West Nile virus be?
In 2002, West Nile virus caused the largest encephalitis epidemic in history. Last years' epidemic was arguably twice as large -- possibly because new tests made it possible to diagnose mild cases of West Nile fever.
The number of serious cases was about the same: 284 deaths in 2002 and 262 deaths in 2003. Thousands of people suffered dangerous brain infections with lingering effects. Many of them -- especially those with what's come to be called West Nile polio -- may never fully recover.
That was one of last year's surprises, says Grant L. Campbell, MD, PhD. Campbell, based in Ft. Collins, Colo., heads the branch of the CDC that keeps track of West Nile cases.
"Last year we saw quite a few cases of acute flaccid paralysis -- the so-called West Nile polio," Campbell tells WebMD. "We have more than 30 such patients here in Colorado. Usually they don't have fever, then -- boom -- they get paralyzed in one limb or another. It often leads to respiratory failure and death. We saw some cases as far back as 1999, but we became more aware of it last year."
West Nile Lessons from 2003
The good news, Campbell says, is that the West Nile virus hasn't mutated into a more dangerous form. Although there have been minor changes in the bug's genetic makeup, the virus seen today is still the same virus that made its 1999 American debut in New York City.
Spread by migrating birds -- and perhaps by infected mosquitoes that find their way onto trucks and airplanes -- the virus moved south and then relentlessly marched westward. Last year, the hardest-hit states were Colorado, Nebraska, South Dakota, Texas, and North Dakota.
The virus did move west of the Rocky Mountains, but there were few human infections. Oregon and Washington reported no cases at all. California reported only three.
For reasons nobody can entirely explain, there tends to be a lot more West Nile disease when the virus finds its feet in a new area -- usually the second year after it first appears. It never, ever, goes away. But the number of cases tends to drop off. New York, for example, was where West Nile virus first landed in the U.S. But last year, that state reported only 71 human infections. Colorado, on the other hand, zoomed from 14 cases in 2002 to 2,326 cases in 2003.
"Nobody really understands the flattening out. We've seen it with St. Louis encephalitis virus in years before," Campbell says. "Part of it is bird immunity."
Most researchers think that people once infected with West Nile virus get very long-lasting immunity. That happens with similar viruses, such as the yellow fever virus. For every person who comes down with West Nile symptoms, four more never have any noticeable symptoms at all. But Campbell says that even in intensely populated areas with lots of infected birds and mosquitoes, no more than 4% of people show signs of having been infected with the virus. An unreleased study of people in Slidell, La., Campbell says, found that West Nile-infected mosquitoes bit only about 2% of the population.
Hot Spots: California, Colorado
This pattern bodes ill for California. Already this year, a dead crow and two live house finches in Southern California tested positive for West Nile virus.
"We worry about Southern California. By that theory, California should light up this year -- but it all depends on climate and mosquito control and luck," Campbell says. "If California is going to be a problem, you might see human cases very early. Cases in May or June or early July are very unusual and could be a sign of something big. Just a lot of dying birds and horses could suggest something big coming."
The pattern also suggests that Colorado -- last year's hardest hit state -- may see fewer cases this year.
"The big question is, what will happen in Colorado this summer," Campbell says. "If we continue to have a hot, dry summer, we could have activity again this year. If you start seeing cases in early July rather than late July in Colorado, it might portend a big epidemic."
It's too soon to tell whether this year's first probable human case of West Nile virus -- seen in a 79-year-old man from southern Ohio with severe viral encephalitis -- is an unusual aberration or a sign of things to come. Ohio reported 108 cases of West Nile virus infection in 2003 -- down from 441 cases in 2002.
"We know very little about treatment of West Nile virus disease," says Carlos del Rio, MD, chief of medicine at Atlanta's Grady Memorial Hospital. "Once it's diagnosed, treatment usually consists of just managing symptoms and preventing other diseases from happening to people in a weakened condition. With the encephalitis and meningitis, it is a very slow recovery. Physical therapy and rehab are all-important for recovery."
Researchers are racing to come up with treatments that are more active.
"There's a lot of interest from the National Institutes of Health in clinical trials of antiviral drugs, such as ribavirin and interferons, to see if we can decrease the symptoms of severe disease," del Rio says. "Passive immunity [treating patients with serum from people who have recovered from infection] is an area of significant interest."
Also being tried is an unusual treatment from a St. Louis firm called GenoMed Inc. The patent-pending protocol, developed by GenoMed CEO David W. Moskowitz, MD, involves the use of a common blood-pressure lowering drug: either an ACE inhibitor or one of a class of drugs called angiotensin-receptor blockers, such as Cozaar and Avapro. The idea is to slow down overreactive immune responses and speed recovery.
So far, 10 patients -- including this year's first case, the 79-year-old encephalitis patient from Ohio -- have received the experimental treatment. Nine of them, GenoMed says in a news release, got better. He thinks the same treatment may work for people with autoimmune disease, SARS, severe flu, and even the common cold.
Work is under way on a West Nile vaccine. The vaccine farthest along uses the backbone of the existing live-virus yellow fever vaccine. That, however, may be a problem.
"The yellow fever vaccine backbone has been associated with severe adverse events in elderly patients -- multisystem organ failure," Campbell says. "That will be a thorny issue. Because now you are talking about taking that backbone and putting it into thousands of elderly Americans."
Other types of West Nile vaccine are in the early stages of development.
Meanwhile, there's a good way to make sure you don't get the West Nile virus: Avoid mosquito bites. When the blood-sucking varmints appear, limit your out-of-doors time in the early evening. When you do go out, wear long sleeves and use a DEET-containing mosquito repellent on exposed skin.
And keep mosquito populations down. Search your house and yard for places where water pools: clogged gutters, flowerpots, discarded tires, and so on. Be sure birdbaths get frequent changes of water. And keep the grass low in yards and empty lots.
It is possible to get West Nile virus from a blood transfusion, from an organ donation, or from breast milk. But these types of transmission will be vanishingly small this year. Tests of donated blood and organs keep the risk of transfusion and transplant infections very low. And the benefits of breastfeeding far outweigh the slight risk of West Nile transmission.
Published April 15, 2004.