RALEIGH — A second person has died of West Nile virus, and three other new human cases have been reported in North Carolina, state health officials said Wednesday.
The new cases are part of a national epidemic that’s on pace to be the worst in the 13-year history of the disease in the United States. The Centers for Disease Control and Prevention reported Wednesday that the number of cases nationally had jumped nearly 40 percent in a week, to almost 1,600 confirmed human infections. Nearly 70 people have died.
The outbreak centers around northern Texas, with hundreds of cases reported in the Dallas area alone. But people have been infected in 43 states and the District of Columbia.
Cases in North Carolina have been confirmed in Cabarrus, Forsyth, Mecklenburg, Scotland and Wayne counties. The first death was reported in Wayne County on Aug. 14; the latest was in the western Piedmont.
The five cases make this already one of the worst years on record in the state. Still, the comparatively small number seems to underline a question that puzzles West Nile experts: Why North Carolina has so few human cases of the disease.
West Nile virus is carried by birds and spread to humans by mosquito bites. Its effects are often mild; about 80 percent of infected people have no symptoms at all. Others have symptoms that can include body aches, fever, headaches, nausea, vomiting, and sometimes swollen lymph glands.
About 1 in 150 people who are infected will develop a severe form of the disease that can bring high fever, headache, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. Some effects may be permanent.
First case in New York
West Nile was first found in humans in New York in 1999 and has since been confirmed in humans in every state except Alaska and Hawaii. It has infected more than 1,000 people in the Northeast over the years, and thousands more in the Deep South and Midwest.
In North Carolina, though, there have been only 52 cases, nearly half of them in 2003 during another national epidemic. Even the District of Columbia has had almost twice as many confirmed cases.
Experts aren’t sure why, any more than they know the cause of the savage outbreak in Texas this year.
It’s always hard to predict outbreaks of mosquito-borne illnesses, or to say precisely why they happen, Lyle Petersen, director of the CDC’s Division of Vector-borne Infectious Diseases, said in a telephone news conference Wednesday.
The ecology of the disease is complex, with a huge number of intersecting factors. In addition, there often isn’t good data on factors such as the makeup of the bird population or how many of them might have immunity to the disease.
Other factors include weather, climate, a host of issues related to mosquito populations and how much time people spend outdoors. One theory about the current national outbreak is that the warm winter, early spring and hot summer in many parts of the country played a role, Petersen said.
Still, no one really knows all the reasons for the outbreak, let alone why the disease remains so scarce among people in North Carolina.
“It’s always harder to study the absence of something than its presence,” said Michael Reiskind, an assistant professor at N.C. State University who specializes in public health entomology.
The virus is likely present among birds in every one of the state’s 100 counties, say state health officials. Reiskind said his best guess why the virus doesn’t often move from those birds to humans is that there is something about the mosquito population – there are 57 kinds in North Carolina – that prevents it.
Some types of mosquitoes are efficient at spreading the virus among birds, while others are “bridge vectors” that are good at moving it between birds and humans. Both actions can affect how likely it is for humans to become infected, and it’s possible the state lacks a combination of mosquito varieties that make the jump to humans more common, Reiskind said.
That theory sounds good, said Dr. Erin Staples, a CDC epidemiologist who studies diseases spread by biting insects. Still, the broader reasons also may include how the mix of mosquito species meshes with other factors such as weather, the size and mix of the bird population and how cases are diagnosed and reported.
One clue, though, might be the relatively low incidence of a disease called St. Louis encephalitis. Some Southern and Midwestern states recorded hundreds of human cases of the disease in outbreaks from 1964 through 2010, but North Carolina had just six.
Bird, mosquito mix
Because it’s spread the same way and often by the same mosquitoes as West Nile, and shows a similar pattern of incidence, that could mean the mix of birds and mosquitoes here is a significant reason for the modest number of West Nile cases, Staples said.
North Carolinians still should take precautions against mosquito bites, Staples said, noting that not only do we have West Nile and the rarer, but also the more deadly eastern equine encephalitis and La Crosse encephalitis.
The complexities that keep scientists from fully understanding what causes outbreaks or why there aren’t more cases in some areas also means North Carolinians shouldn’t put too much faith in past experience.
“Just because traditionally you haven’t seen it, I can’t predict that you won’t get large numbers in years to come,” Staples said. “We just can’t know for sure.”
News researcher David Raynor contributed to this story.