Hitching a ride with imported insects.

Hitching a ride with imported insects

Not all emerging viral diseases result from human intrusion into viral turf. Viruses, too, have ways of getting around -- although even then, humans often provide the free transportation.

Perhaps the best example is one close to home, with its roots in the growing worldwide trade in used tires. In the years since World War II, the United States has imported millions of used tires for retreading. Many of these come from Asia--home of the mosquito Aedes albopicturs, whose bite can inject the microbes responsible for yellow fever, dengue fever and other tropical diseases. In its native habitat, A. albopictus typically matures in rainwater that collects in tree stumps and other hollows. Lately, it has taken a liking to water-filled tires. And as shiploads of these tires have arrived in the United States, so has A. albopictus.

The mosquito, which can survive exposure to most commonly used pesticides, has now infiltrated 17 southern U.S. states and several regions of South and Central America. Although no A. albopictus-related epidemics have yet occurred in the United States, the immigrant mosquito brings with it the very real threat of a U.S. dengue fever outbreak, say officials from the Centers for Disease Control in Atlanta. This disease, and its more severe form called dengue hemorrhagic fever, already strikes about 100 million people each year through much of Asia and the Southern Hemisphere, where the mosquito is endemic. Mostly affecting children, the hemorrhagic fever can cause internal bleeding, shock and death. To date, all attempts to develop a dengue vaccine have failed.

Elsewhere, human-induced changes in insect ranges have already led to severe viral epidemics. In 1980, for example, Brazil's Amazon valley saw its worst outbreak of a recurring viral disease called Oropouche. While not fatal, the disease debilitates its victims for several days with severe nausea, muscle pains and in 5 to 10 percent of cases progresses to an inflammation of tissues around the brain. Health officials have linked the 1980 outbreak and other Oropouche epidemics to huge piles of cacao husks that have accumulated as the crop's cultivation there has increased. Filled with rainwater, the husks serve as breeding grounds for tiny insects, called midges, that spread the disease.