It goes without saying that the more airplanes there are, the easier it is for people to get from one place to another. The same logic applies to epidemics. Substitute mosquitoes for airplanes and diseases for people and you can begin to appreciate why scientists are concerned about the recent resurgence of dengue fever.
Dengue, at least in its classic form, is a flu-like disease, though more severe. It is caused by a virus that is transmitted by the Aedes aegypti mosquito and, to a lesser extent, its cousin Aedes albopictus. At one time, dengue was considered an uncommon and relatively benign disease. In recent decades, however, the picture has changed dramatically. Today, epidemics of dengue are more frequent and virulent—even potentially fatal—and they affect larger numbers of people than ever. Each year there are an estimated 50 to 100 million cases globally and several hundred thousand cases of a severe manifestation called dengue hemorrhagic fever. The Centers for Disease Control and Prevention (CDC) calls dengue "the most important mosquito-borne viral disease affecting humans." Although dengue does not pose a serious threat to tourism—at least not yet—it has benefited enormously from travelers and the mosquitoes that prey on them.
The earliest reported epidemics of dengue occurred virtually simultaneously in 1779 and 1780 in Asia, Africa, and North America. For the next 150 years, epidemics were sporadic, occurring every 10 to 40 years.
The years after World War II, however, marked the beginning of what has become a global pandemic of dengue. Epidemiologists now identify four serotypes of the virus—dengue-1 through dengue-4. (Once you've been exposed to one, you gain a lifelong immunity to that particular variety of the virus—but not to the others.) Dengue-3, absent from Central America for almost two decades, reappeared in 1994, causing waves of epidemics the following year; experts believe it came from Asia. In 1998, some 600,000 cases of dengue were reported in the Americas and the Caribbean—more than double the number just three years earlier.
Dengue hemorrhagic fever first emerged as a significant threat in the Philippines in 1954, and soon spread to Thailand; since then at least 12 Asian and Pacific countries have experienced epidemics. DHF reached Cuba in 1981; by 1998, at least 18 countries in the Americas had reports of the disease, totaling some 11,000 cases.
The same virus causes both classic dengue and DHF. Doctors don't know exactly why one disease is worse than the other; some evidence suggests that exposure to more than one strain of dengue predisposes you to the more severe reaction.
The spread of the dengue virus is directly attributable to the spread of the mosquitoes that carry it; the aedes, in turn, owe much of their success to us. A. aegypti is a resourceful species. It lays its eggs in water-filled receptacles that tend to be situated near people, from flowerpots and birdbaths to scraps of plastic. In the 16th and 17th centuries, aedes found a niche in the wooden water casks aboard sailing ships and was inadvertently transported to ports around the world. In 1983, A. albopictus, the Asian tiger mosquito, was discovered in the United States. It lays its eggs in the water that accumulates in automobile tires and is thought to have arrived in one of the millions of used tires shipped regularly from Asia to the United States for retreading. The mosquito is now found in nearly two dozen states.
The primary vessel of the dengue virus isn't the aedes mosquito, however, it's Homo sapiens. People are moving around the world in greater numbers and at faster speeds than ever. More of them are catching dengue, and more of them are reaching their destinations while still actively infected—whether the migrant is an impoverished Peruvian looking for work in Argentina or a Canadian backpacker flying from Quito to Caracas. If there is a dense population of aedes mosquitoes at the port of arrival, an epidemic could ensue. "The mobility of humans is an incredible factor in the movement of viruses," says Paul Reiter, a medical entomologist at the CDC.
Epidemiologists are keeping a particularly close eye on the Texas-Mexico border, where there are roughly 70 million northbound crossings each year. Northern Mexico reports several thousand cases of dengue annually; Texas, by contrast, has only a handful of cases, typically from returning travelers. In 1999, however, at least 17 cases were acquired in Texas proper—evidence that the disease may be gaining a foothold in the state. But even though the aedes mosquito is well established here, health officials don't envision dengue epidemics sweeping through the States. The reason is simple, and behavioral: Americans, especially those in the warm, Southern states, prefer the air-conditioned seclusion of their homes. South of the border, by contrast, as in most tropical countries, residents spend more time outdoors, exposed to mosquitoes.
Of course, many of those tropical countries are favored by travelers. On the whole, though, the risk of catching dengue while traveling abroad is fairly low. Each year, between 100 and 200 U.S. travelers contract the disease while outside the country or come down with it shortly after returning. Your risk, of course, depends on where you're going and how. Are you paddling up the Congo in a canoe or relaxing in a Four Seasons villa?Dengue is prevalent in the Indian subcontinent, Southeast Asia, southern China, Africa, Latin America, and the Caribbean. Dengue epidemics tend to be seasonal, occurring during and shortly after the rainy season. Before setting out, check the CDC Web site (www.cdc.gov) for specifics on your destination.
Should you actually come down with dengue, you will almost certainly survive. Symptoms appear 3 to 14 days after a mosquito bite, and they can be dramatic: sudden high fever, throbbing headache, nausea, vomiting, an unsightly rash, and ferocious joint and muscle pain. The illness can take 7 to 10 days to play itself out. (Physicians suggest you drink fluids and get plenty of bed rest, which is all you'll feel like doing anyway.) Typically, dengue abates with no lasting effects.
Dengue hemorrhagic fever is more serious, though less common. Initially, the symptoms are like those of classic dengue; then they worsen. Victims often suffer from acute stomach pains, and bleeding from the gums, fingernails, or nose. Untreated, the illness can lead to shock, even death. DHF is fatal in about 5 percent of cases—typically in children and young adults living in less-developed countries.
The good news is that health officials consider dengue among the most preventable of modern diseases. Prevention starts in the back yard: Regularly empty water from containers around the house, and get rid of any junk that collects it. Travelers are encouraged to practice evasive maneuvers: mosquito repellent, long pants and long sleeves, the usual common sense. But don't expect modern science to rid us of aedes mosquitoes anytime soon: DDT-spraying efforts were largely abandoned in the 1960's, when it became clear they were ineffective.
"A highly successful species like Aedes aegypti can colonize virtually anything that collects water, even an old shoe," says Reiter. "It's unimaginable that we could eliminate it entirely."
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