The ins and outs of West Nile

As Colorado experts warn of the potential outbreak of West Nile virus this year, they're also saying the disease is here to stay.

The Centers for Disease Control call West Nile virus a seasonal epidemic. Despite efforts to study and control the virus' spread, experts are finding it's as unpredictable as the weather. And the public, one expert says, has a false sense of security about immunity. After four years, relatively few people have been infected, about 100,000 according to the Colorado Department of Public Health and Environment. This means about 4.6 million Coloradans are still susceptible to a virus that affects individuals differently. Sometimes it comes with no effect at all, sometimes it can kill.

Human infection

For many people infected with West Nile, the virus can pass through the blood stream without symptoms. But for a small number of people, the gamble of not wearing bug spray results in encephalitis, meningitis, lifetime paralysis and even death.

For Greeley resident Jim Diehl, 44, West Nile meant weeks of suffering from meningitis and possible lifetime partial-paralysis in his left arm.

Diehl, who also suffers from multiple sclerosis, was the first case of West Nile meningitis reported in Weld County in 2003. After a bad headache turned incapacitating, Diehl spent a few weeks in the hospital and many months on physical and speech therapy.

"I had to learn how to cook breakfast again," Diehl said. "... I just didn't have any energy or strength. I just couldn't do anything."

Usually age and health are determining factors in those who suffer from meningitis or encephalitis after becoming infected, but John Pape, epidemiologist for the Colorado Department of Public Health and Environment pointed out that there have been cases among 15-year-old residents. And there is no treatment for West Nile, only prevention.

"Our prevention is, if you want to avoid exposure to this virus, you want to avoid mosquito bites," Pape said.

Only .02 of a percent of Colorado's population has been infected in the past four years. And a survey estimates that for every 150 people infected, 20 to 25 suffer West Nile fever and one suffers meningitis or encephalitis, according to the CDC.

That means about 80 percent of people infected with West Nile have no symptoms of infection at all.

This, coupled with doctors not testing those who suffer only flu-like symptoms, has created a false sense of security in the community, Pape said. He said many people think they've probably already been infected and built up immunity. But very few people have actually been infected.

Weather or not

The rate at which West Nile spreads each year, infecting birds, animals and humans alike, is as unpredictable as rain, Pape said. In fact, West Nile's spread is closely tied to precipitation and temperature.

The Culex mosquito, the genus that carries and transmits West Nile, thrives in hot, wet conditions. Female mosquitoes lay eggs in standing water, and the warmer that water is, the faster the offspring grow up to become procreating adults. Depending on the temperature, a mosquito can go from egg to a bug in a few weeks or just a few days. The faster the insect grows up to lay more eggs, the quicker the population grows.

"The cycle keeps continuing and becomes an exponential growth curve. One mosquito lays 100 eggs, and those lay 100 more eggs. The population goes into rapid climb and infection rates climb," Pape said.

Temperature also plays a role in how fast a Culex mosquito becomes infectious once it has bitten a bird carrying West Nile.

The virus enters the mosquito's blood stream, where it then must grow and travel into the saliva before the insect can infect another bird. Since mosquitoes are cold-blooded, their metabolism operates faster when it's warmer and the virus replicates faster as well.

Female mosquitos transmit the West Nile virus when they bite birds in order to get enough protein to lay eggs. The longer a female lives, the more eggs she lays and the greater number of birds she infects with her pre-pregnancy blood meals.

A man-made problem

There's little historical record of the Culex mosquito in Colorado, said Dr. Michael Weissmann, an entomologist for Colorado Mosquito Control in Brighton. In old writings, he's seen stories about springtime bursts in the pesky insect population, but few samples of Culex exist from times when the rivers flowed only during snowmelt.

Once people built towns in Colorado with irrigation ditches, reservoirs and nonnative shade trees, the conditions for the Culex genus, a lover of still waters, became ideal.

"We create our own problems a lot of the time," Weissmann said.

Until 1999, when West Nile first entered the United States in New York City, no one paid much attention to the increased Culex population, which was just the cause of irksome, itchy bites, Weissmann said. But now that they're the carriers of a sometimes-deadly disease, people -- and scientists -- are paying more attention.

Pape said the scientific community still doesn't fully understand what affects the Culex breeding cycle and, along with that, the spread of West Nile. But the Centers for Disease Control and Prevention is working on improving mosquito control techniques, updating surveillance protocol and sharing information between states.

The CDC, however, also cut funding for West Nile this year, said Trevor Jiricek, director of environmental health services for the Weld County Department of Public Health and Environment. Weld received only about $27,000 for education, outreach and surveillance this year, compared to about $44,000 in 2006.

As a result, the county will test mosquitoes only in peak infection months this year. And if funding isn't restored, Jiricek said his department will face a tough challenge in creating a viable surveillance system.

Human immunity

Using knowledge of viruses similar to West Nile, scientists believe that those infected develop a lifetime immunity to the virus, Pape said. But that immunity doesn't stop the spread of West Nile, since the virus is continually being kept alive in birds and mosquitoes that pass it between each other.

Human infection, in fact, is an unhappy accident for the virus. We are "dead-end hosts" that will never build up enough virus in our blood to pass along the infection to a new mosquito, Pape said.

Humans also won't likely ever build up an immunity to West Nile simply from being infected naturally, said Dr. Mark Wallace, director of the Colorado Department of Public Health and Environment.

While several drug companies are developing a West Nile vaccine for humans, Pape questions whether there will be a big enough demand to release it.

Weld County Deaths in 2003

Total Deaths -- 1,228

Cardiovascular disease -- 396

Heart disease -- 300

Malignant neoplasms -- 257

Alcohol-induced deaths -- 22

Suicide -- 31

Injury by firearm -- 19

West Nile Virus (encephalitis/meningitis) -- 6

Source: Colorado Department of Public Health and Environment

FAQs about West Nile

Q: What Is West Nile virus?

A: West Nile virus is a potentially serious illness. Experts believe the virus is a seasonal epidemic in North America, reaching its peak in summer.

Q: What are the symptoms of West Nile virus?

A: A few people develop serious symptoms. About one in 150 people infected with the virus will develop severe illness. Severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks and neurological effects may be permanent.

Milder symptoms affect up to 20 percent of the people who are infected with West Nile. These symptoms can include fever, headache and body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms can last for as little as a few days, though even healthy people have become sick for several weeks.

Approximately 80 percent of people (about 4 out of 5) who are infected with the virus will not show any symptoms at all.

Q: Are older people at risk?

A: People age 50 and older are at higher risk to have severe illness associated with the West Nile virus. This age group is more likely to develop serious symptoms if they do get sick and should take special care to avoid mosquito bites.

Source: Centers for Disease Control

Guarding against West Nile virus

There is no vaccine to protect against the West Nile virus. But you can take precautions to lower your risk of being bitten by the type of mosquito that carries the virus.