BLACKSBURG, Va., May 30, 2003 – Virginia Tech entomologists have documented for the first time the presence of mosquitoes infected with La Crosse virus in four Southwest Virginia Counties.
Sally Paulson, an associate professor of entomology, said the find from samples taken last year and recently analyzed was surprising because no human cases of the disease had been reported in the counties before.
"It surprised me to get so many positives from so few samples from a place I wouldn't have expected any," Paulson said.
The counties in which the positive samples were collected for the first time were Montgomery, Giles, Pulaski, and Floyd. The disease had previously been found in the counties of Wise, Dickenson, Buchanan, and Tazewell. About 15 human cases of La Crosse encephalitis have been reported in Virginians in the past 10 years.
The positive results were returned from approximately 135 "pools," samples containing a number of mosquitoes captured at individual locations. Because the laboratory report only reports a positive result from a pool, Paulson said it is impossible to say exactly how many individual mosquitoes were infected.
"We had seven positives reported, and our collecting methods did not target the species that carry La Crosse virus," she said. Paulson and her graduate students collected the samples as part of a project to check for West Nile virus. Different mosquito species are implicated in the two diseases.
The mosquitoes that spread La Crosse virus are members of the Aedes and Ochlerotatus genera. The Eastern Tree Hole Mosquito is the primary vector for the disease, though the Asian Tiger Mosquito has also been implicated in its spread. Both live in woodland habitats but can be abundant in urban and suburban areas.
According to a fact sheet on the disease from the Centers for Disease Control, about 70 people are infected with La Crosse encephalitis nationwide each year, though the agency considers the number of cases to be considerably under-reported. Most infections are mild, but can result in encephalitis, or swelling of the brain, progressing to seizures and coma. The CDC fact sheet also warns of potential adverse effects on IQ tests and school performance following infection.
"There's been a lot of talk about West Nile virus, and people are fairly well aware of it," Paulson said. "This is something that isn't as well known. While older people are more at risk for severe infection with West Nile virus, it is the young, those under 15, who are most at risk for La Crosse encephalitis."
Another difference is that the mosquitoes that transmit La Crosse virus are active during the day, while those that carry West Nile virus are most active at night and in twilight conditions.
Measures communities and homeowners can take include getting rid of or make unusable breeding areas, which consist of areas of standing water. Around homes, that can be as simple as tipping the water out of an old tire or changing the water in a birdbath every few days. Communities may use chemical or biological measures to control mosquito populations.
"The first step is to understand the problem, and that is what We're working on now," said Paulson. "After that, it is a matter of raising awareness that a problem exists and that there are things people can do to counter that problem."