BALTIMORE – Two imported mosquito-borne illnesses continue to push into the United States, threatening to take up residence in areas with a favorable environment.
Neither dengue fever nor West Nile virus are naturally endemic to any part of the United States, Dr. Larry Davis said at the annual meeting of the American Neurological Association. But both diseases show some indications of establishing a local infective reservoir.
Dr. Larry Davis
Dengue remains a rare disease here, with about 650 cases reported in 2013. The majority of those occurred in patients who had just returned from endemic countries. But since 2009, local transmission appears to be occurring in three hot spots: Hawaii, Texas, and south Florida, including Key West and Miami, said Dr. Davis, a professor of neurology at the University of New Mexico, Albuquerque.
Locally acquired disease first popped up during 2009-2010 in Key West, Fla. During that time, 29 residents developed clinical dengue. Many more probably carried the virus, however, since a robust immune response usually tamps down about 80% of first infections. But those asymptomatic carriers provide a potent reservoir for hungry mosquitoes, and dengue is one virus that doesn’t need an intermediate host. Instead, Dr. Davis said, any mosquito that bites an infected person passes the agent directly into the bloodstream of his next meal.
During the Key West outbreak, the Centers for Disease Control and Prevention (CDC) estimated that about 5% of those living on the small island had been infected at some point.
Last year, 53 clinical cases were identified in south Texas. About half of these were locally acquired, which is another sign that the virus may be taking up residence, Dr. Davis said. And, so far this year, four residents of Miami-Dade County, Fla. have been diagnosed with locally acquired disease. The outbreak recently prompted local health officials to issue an alert for mosquito-borne disease, and remind residents to take precautions against being bitten.
In addition to its direct transmissibility, dengue has another neat infective trick, Dr. Davis noted. There are four serotypes, each with their own unique immunogenic profile. Thus, infection with any one confers no protection against any of the others. Any vaccine, therefore, would have to protect against all four serotypes.