West Nile virus down but not out

The West Nile virus hit Nebraska hard in 2002-03.

Prior to 2002, when Nebraska had 152 West Nile-related cases and seven deaths, no one in the state had come down with West Nile. The mosquito-borne virus had been in the United States only since 1999, when it showed up along the East Coast.

But in 2003, the state's number of West Nile cases exploded: 1,942 cases, 29 deaths. (Iowa's West Nile numbers never have been as high as Nebraska's; the Hawkeye state recorded 147 cases and six deaths in '03.)

In subsequent years, the annual number of Nebraska cases hasn't hit 270. In fact, in each of the past two years, the number of cases has been below 50, and only one Nebraskan has died from West Nile in that time.

The steep decline in cases doesn't mean the virus is gone, however. Officials say it's just waiting for the right conditions to return in full force.

Humans weren't the only victims of the virus. In 2002, the state had 1,100 confirmed cases of West Nile in equines — horses, donkeys and mules. Primarily because scientists developed a highly effective vaccination, hardly any horses in the state now contract West Nile, said Dr. Dennis Hughes, the state veterinarian. This year, he said, the state has had one reported case in a horse.

Weather conditions played a major role in the spread of the virus in 2002-03, said Roger Nasci, chief of the Arboviral Diseases Branch of the Centers for Disease Control and Prevention's Division of Vector-Borne Infectious Diseases.

“In many parts of the country, they had just the perfect weather conditions: High temperatures and rainfall patterns that were conducive to producing large mosquito populations. We also had these exquisitely susceptible bird populations” that had no prior exposure to West Nile.

That was important, because birds are the primary hosts: Infected mosquitoes bite a bird, the virus infects that bird and other, uninfected mosquitoes feed off the bird and spread it to other birds and humans.

Bird populations still are recovering from the hit they took from West Nile in the early part of this decade. Nebraska saw a dramatic drop in the number of black-capped chickadees and lost large numbers of crows and blue jays, said Joel Jorgensen, the non-game bird program manager for the Nebraska Game and Parks Commission.

Dr. Alison Freifeld, a professor of infectious diseases at the University of Nebraska Medical Center, said that “since wild birds are a main reservoir of West Nile virus, if you were to decrease that reservoir by killing them off, you're going to have less circulation of the virus.”

In 2002, Raptor Recovery Nebraska took in 240 West Nile-infected raptors — mostly great horned owls and red-tailed hawks. The annual number of infected raptors being brought to raptor centers has fallen to between 30 and 40, said Betsy Finch, the group's rehabilitation coordinator.

Recent weather conditions around the country haven't favored the development of the mosquitoes that carry West Nile, Nasci said.

“Nationwide, it's been relatively cool,” he said. The heat that helps mosquitoes thrive and drives virus replication has been lacking.

“The degree to which we have (West Nile) cases is influenced to a great extent by weather patterns,” Nasci said.

Many Americans have been exposed to West Nile since it showed up in the U.S. 10 years ago, but it's still a small percentage of the population. According to a recent study of blood and plasma donors in the U.S., about 3 million people — roughly 1 percent of the U.S. population — have had West Nile exposure, leaving the other 99 percent susceptible.

The good news with West Nile is that in 80 percent of cases, people display no symptoms. Up to 20 percent of people who are infected have mild symptoms such as fever, headaches and body aches. Less than 1 percent develop more severe symptoms, such as encephalitis, meningitis or other neurological signs.

Humans don't pass on West Nile to other humans, though instances of West Nile transmission through blood transfusions were reported early this decade. Screening implemented at blood banks has significantly reduced the number of those cases.

Nasci said he would like to think that public-health actions, such as mosquito-control and -monitoring programs and educational campaigns stressing the importance of wearing mosquito repellent, have helped to reduce the number of West Nile cases.

Dr. Thomas Safranek, Nebraska's state epidemiologist, said it's important for officials to make sure they provide the public with relevant information on rainfall amounts and mosquito numbers so people can know the level of risk they face in a given year.

“We can't say, ‘The sky is falling' year in and year out,” he said.

Nasci said he expects to see major West Nile outbreaks again, but nothing like the Plains states saw in 2002 and 2003.

The number of cases, he said, “is going to be influenced by the things involved in the transmission cycle: The number of susceptible birds. The density of mosquitoes. The species of mosquitoes. And then those weather patterns.”

Contact the writer:

444-1109, bob.glissmann@owh.com