Mosquito-Borne Diseases

Mosquitoes can carry an array of serious, sometimes deadly diseases-among them West Nile virus, malaria, encephalitis, dengue fever and yellow fever.

According to the Centers for Disease Control and Prevention, the number of mosquito-borne disease cases is growing in this country.

The rate of reported encephalitis cases has risen to more than 2,500 annually. There are no approved human vaccines to prevent many forms of encephalitis. Active strains of encephalitis include La Crosse, St. Louis, eastern equine and western equine.

West Nile virus infections tend to be more serious in older adults but the disease can greatly impact even young, vigorous individuals. The CDC received reports of 2470 human cases of West Nile last year, 88 resulting in death, and 900 individuals in all age groups experiencing the most severe form of the infection resulting in neuroinvasive disease. Many cases of West Nile are not diagnosed because, according to the CDC, about 80 percent of people infected with the virus show no symptoms.

West Nile first emerged in the United States in New York in 1999, with 62 reported cases. Since then, the virus has progressed westward and southward. Last year WNV was detected for the first time in several Western states where it had not been seen previously, meaning that the disease now been detected in every state in the continental United States.

This year officials predict California may see its first real WNV epidemic—the first infected bird was found in San Jose in February—and say human infections will occur across the country, though experts are reluctant to offer predictions about the severity of outbreaks in specific regions.