Reported West Nile infections represent just a fraction of cases because many never get confirmed, said Roger Nasci, a research entomologist at the CDC's Arboviral Diseases Branch in Fort Collins. "For every person diagnosed with meningitis or encephalitis with West Nile, there's approximately 140 people infected. The vast majority will be asymptomatic," he said.
West Nile, carried by Culex pipiens and Culex restuans mosquitoes, can produce fever and neurological problems without causing brain inflammation. Often, the symptoms are so vague that doctors will just prescribe fever reducers like acetaminophen, without seeing the need to perform blood tests that could detect the virus. Nasci estimated that doctors diagnose only 3 percent to 5 percent of the West Nile cases that cause symptoms short of meningitis, encephalitis and muscle weakness.
The most devastating of the mosquito-linked illnesses is Eastern equine encephalitis, which is rare, but fatal in about a third of cases. There is no treatment. Survivors often have brain damage. EEE is carried by Culiseta melanura mosquitoes, which live in marshes, swamps and other bodies of still water. So far, New York has reported the only human case for 2011. Seven other states have detected the disease in mosquitoes, birds and other animals.
As of Aug. 30, Arkansas reported a probable human case of St. Louis encephalitis, typically found in Eastern and Central states, and most dangerous to older patients. Florida and Nevada have detected it in insects and animals, the CDC's latest tally showed.