Following the death of a 6-year-old Fayette County, W. Va., girl from a rare disease transmitted by mosquitoes, three states are focusing on a puzzling phenomenon.

The first grader, infected in late September with La Crosse encephalitis, passed away after about a week in the hospital. The rare mosquito-borne illness is usually recorded about 75 times a year nationwide, but is most likely to be found in only a few counties in West Virginia, Ohio and North Carolina.

Since 2001, between one half and two-thirds of all cases have been recorded in these states, even though the disease has been logged in 28 states.

Health officials aren’t sure why the disease is so common in the three states. It appears less frequently in the upper Midwest, in states like Wisconsin, where it was first isolated in 1963 and where it got its name.One explanation is that doctors in these areas are more familiar with the disease, and more likely to have patients tested for it. If that’s the case, it’s likely that La Crosse encephalitis is more common than recorded, and is being underreported.

“The reason these states have more cases is because there’s been a stronger effort to look for the disease,” said Richard Gary, public health entomologist with the Ohio Department of Health. “Doctors have to know what they’re looking for to begin with. And especially if the illness is mild, there’s less motivation to get the tests done.”Since 1963, Ohio has logged 875 confirmed cases of La Crosse encephalitis — more than any other state, according to the federal Centers for Disease Control and Prevention.

Gary said Ohio records more cases during times of public concern over similar, more familiar viruses, like West Nile. That indicates La Crosse cases are found most often when doctors are looking for diseases of its type, known as arboviruses.Symptoms usually include aches and a fever, and are more severe in children and the elderly, said Rachel Long, public health epidemiologist with Mission Hospitals in Asheville, N.C. Many healthy adults probably dismiss an onset as a mild summer cold.

“It’s probably being underreported,” she said. “Most of us have probably had it.”In West Virginia, Fayette County and neighboring Raleigh County are the prime areas for La Crosse encephalitis. Between 1999 and 2005, there were 211 confirmed cases in the state, 83 of which were in these counties — 42 in Fayette and 41 in Raleigh. Most other counties with cases had only a handful, according to the West Virginia Department of Health and Human Resources.

“I really don’t have any idea why it’s so common here,” said Dr. Donald Newell of the Fayette Clinic in Lochgelly. “We have a lot of people that live in wooded areas, but there are a lot of places like that in West Virginia.”La Crosse encephalitis is spread by the Eastern treehole mosquito, which generally makes its home in holes in hardwood trees along with old tires and other receptacles for rainwater. A common explanation is that the disease is found often in West Virginia because of its abundance of hardwood forests.

But it may be because doctors in Fayette and Raleigh counties are more attuned to the disease, Gary said.La Crosse encephalitis has a low fatality rate, less than 1 percent of cases, according to the CDC. Ohio, with its 875 cases, has confirmed only six deaths from the disease.But that disguises other serious, long-term problems, said Dr. Jim McJunkin, a pediatrics professor at West Virginia University’s Charleston division.

McJunkin conducted a study that found high rates of behavioral problems in children who contracted the disease. In severe cases, the disease can even lead to epilepsy.“There’s a tendency in the literature to underestimate the severity of this disease,” he said. “It is a concern that we have these kinds of outcomes, and in the scheme of things, it’s a significant concern.”

— Associated Press