Migrant mosquito harbors mysterious virus.

Migrant mosquito harbors mysterious virus

An Asian mosquito species now spreading through the United States carries a potentially dangerous virus, new research indicates. Scientists say they have no evidence that the mosquito, which has migrated into 20 states since it arrived in Texas five years ago, has caused illness in any of those states. But the virus discovery raises the ante in a debate about the danger posed by this imported, aggressively biting insect.

The Asian tiger mosquito, Aedes albopictus, hit U.S. shores in 1985, lurking in a batch of used tires imported from Japan. Since that landfall, the black-and-white-striped immigrant has colonized the U.S. South and West with extraordinary rapidity (SN: 9/23/89, p. 202).

Some researchers fear that the mosquito, which in the Eastern Hemisphere transmits debilitating and sometimes fatal tropical diseases such as dengue fever and Japanese encephalitis, might serve as a carrier for these or other infectious diseases in the United States. Indeed, laboratory tests with deliberately infected mosquitoes have shown that Ae. albopictus can transmit several New World pathogens, including LaCrosse encephalitis virus, a mosquito-borne organism that already infects tens of thousands of U.S. individuals annually, causing brain inflammation in about 100 each year. But until now, scientists had no evidence that free-living Ae. albopictus in the United States actually harbored any viruses.

D. Bruce Francy, an entomologist with the U.S. Department of Agriculture in Fort Collins, Colo., presented the first such evidence last week in New Orleans at the annual meeting of the American Society of Tropical Medicine and Hygiene. Francy and his colleagues collected more than 14,000 Ae. albopictus specimens from sites in Indiana, Illinois and Missouri, then subjected batches of them to sensitive virus-culture techniques. To their surprise, they found that mosquitoes collected near Potosi, Mo., harbored a previously unidentified member of the Bunyamwera virus group.

It remains unclear whether this microbe, dubbed Potosi virus, can cause human illness. Some Bunyamwera viruses appear innocuous; others cause encephalitis in Panama and Africa, Francy says. In recent experiments, he found that laboratory mice whose brains were infected with the Potosi virus died in about nine days.

Francy notes that a preliminary search for Potosi virus antibodies in Missouri residents came up negative, suggesting the mosquitoes have not transmitted the virus to significant numbers of people. However, he adds, "this recovery of naturally infected mosquitoes clearly establishes the potential for this mosquito species to become a vector ... of public health importance in the United States." Even if Potosi virus proves harmless, he says, "we think it's quite possible that albopictus may become involved in the transmission of LaCrosse virus and other viruses with human pathogenic potential."

Details on the Potosi findings will appear soon in SCIENCE, says co-worker George Craig, an entomologist at the University of Notre Dame in South Bend, Ind. He and others have expressed increasing alarm as Ae. albopictus -- in defiance of state and federal mosquito control efforts -- has almost completely displaced the more placid and easily controlled Ae. aegypti from many southern states. Richard L. Berry of the Ohio Department of Health in Columbus likens the spread of Ae. albopictus to a wholesale distribution of empty guns. "Sooner or later," he says, "Mother Nature is going to pass out the ammunition."

Not everyone agrees that the Asian mosquito's spread seriously threatens U.S. public health. "Could an [Ae. albopictus-generated] epidemic happen? Sure," says entomologist Robert W. Gwadz of the National Institute of Allergy and Infectious Diseases in Bethesda, Md. "But has it happened? No. And do I lose sleep over it? No."

Besides, he and others say, there's no love lost over the displacement of Ae. aegypti, which can transmit yellow fever.

Nonetheless, many entomologists voice concern that nearly every U.S. effort to eradicate the resilient Asian intruder -- whether by burying used-tire piles, eliminating sources of standing water or applying insecticides to surrounding areas -- has failed.

"It's just a matter of time before albopictus becomes a vector for a human pathogen in this country," says Thomas Monath of the Army Medical Institute of Infectious Diseases in Frederick, Md. "But just how important it will be remains to be seen."