Cooling temperatures usually mean the end of the road for mosquitoes, but travelers must remain vigilant to reduce their risk of mosquito-borne West Nile virus, especially if they are visiting states that have been hard hit, such as Colorado, Nebraska and South Dakota.
West Nile virus, named for the district in Uganda where it was first isolated in 1937, can be transmitted when a mosquito, infected when it feeds on infected birds, bites a human. The virus also can be transmitted by transplanted organs and transfused blood (U.S. blood donations are now routinely screened for the virus), but it is not spread by casual contact, according to the federal Centers for Disease Control and Prevention.
As of Oct. 1, the CDC had received reports of 5,861 human cases of West Nile virus in the United States this year. Colorado had reported 1,991 cases and 36 deaths; Nebraska, 999 cases and 15 deaths; and South Dakota, 840 cases and eight deaths. California has not yet reported a human case.
But CDC officials say the state is expected to be the next hotbed of West Nile activity. An increase in West Nile virus-infected mosquitoes is predicted within two to three years, says John Roehrig, a lab scientist and chief of the arbovirus diseases branch of the CDC in Fort Collins, Colo. The prediction is something to be aware of but not alarmed about, he says. Until temperatures are consistently cold or there is a frost, travelers should protect themselves from mosquitoes, says Dr. Brian Terry, a travel medicine specialist in Pasadena.
Precautions include applying insect repellent containing DEET to exposed skin, wearing long sleeves outdoors and spraying thin clothing with DEET repellent. Also be aware of peak mosquito hours, dusk to dawn.
In many cases, travelers can update themselves before departing by checking out the CDC's site, , or the state or county health department of the place they're visiting.
The World Health Organization site, www.who.int, also has West Nile virus activity information, although much of it concentrates on the U.S. and Canada.
About 80% of those bitten by an infected mosquito won't get sick, the CDC says, but the remaining 20% will, and 1 in 150 will develop severe symptoms. Those generally appear three to 14 days after the bite. Mild symptoms include fever, headache, nausea, vomiting and a rash.
More severe symptoms, which can indicate more serious illness, include stiff neck, high fever, severe headache and coma.
Those most severely affected by the virus can develop serious conditions such as West Nile encephalitis, a brain inflammation, or West Nile meningitis, an inflammation of the membrane around the brain, or both.
Milder symptoms usually clear up on their own. For more severe symptoms, immediate medical attention is advised.