Enrico Fabian for The New York TimesA worker employed by the local authorities in Delhi spraying insecticides into an open sewage drain.The best way to avoid getting sick from dengue during epidemics may be to avoid visiting the homes of people from northern climates or homes with children.
Since much of the adult population in South Asia has previously been infected and is thus immune to at least one or more of dengue's four strains, mosquitoes that live or breed near susceptible children or recent arrivals from areas where dengue is not endemic are among the most dangerous, said Dr. Thomas W. Scott, a profe ssor of entomology at the University of California at Davis.
By contrast, mosquitoes that feed solely on immune adults cannot become infectious.
The problem is compounded, several experts said, because India's refusal to acknowledge the scope of the dengue epidemic leads many recent arrivals and tourists to fail to take adequate precautions â€" like diligently wearing clothing that covers their ankles and wrists and using insect repellent.
Sure enough, the expatriate community in Delhi has been devastated by the disease this fall.
“Both my wife and I got dengue at the same time, and we have been out of commission for more than two weeks,†said Marco Carrano, an architect who moved from New York City to Delhi six years ago. “Several close friends are sick, too. It's incredibly debilitating.â€
Dengue would barely exist without its pesky propagator, the Aedes aegypti â€" a kind of house cat of mosquitoes. Unlike other mosquitoes, Aedes aegypti feed almost exclusively on humans during daylight hours and rarely venture more than 100 meters from their birthplace. It is a largely urban insect that lives in and around homes and apartments and breeds in small quantities of clean water, like clogged drains and flowerpots.
Instead of feeding just once before laying eggs on a three- to four-day cycle like other mosquitoes, Aedes aegypti feed daily, which multiples their infectiousness. Mosquitoes are attracted to sweat and generally prefer biting young adults and men, studies show. Half of those infected by dengue are bitten in their own homes, studies show, so eradication efforts that only spray insecticides in public spaces and not inside homes â€" as many efforts in India do â€" are rarely successful.
India's yearly dengue epidemic generally follows the monsoon, possibly because the nation's poor infrastructure and ubiquitous trash offer myriad places for mosquitoes to breed a fter rainfall. But the threat of dengue infection never entirely recedes, and outbreaks have been reported in dry regions and seasons. Indeed, dengue's seasonality may be linked more with how temperature changes affect the gut of mosquitoes than with wet weather, Dr. Scott said.
The reason is that mosquitoes do not become instantly infectious after feeding on a person who has the dengue virus. Instead, the virus takes 10 to 14 days to move from the mosquito's gut to its salivary glands, which then inject the virus during feedings. But that transfer between the gut and mouth becomes longer as nighttime temperatures drop, Dr. Scott said. Since mosquitoes only live about six weeks, many more mosquitoes die before becoming infectious as the difference between day and nighttime temperatures grows.
Dengue's history is fairly murky, but many researchers believe it is among the continuing curses of the African slave trade that spread the disease around the world. In 1780 , Dr. Benjamin Rush, one of the signers of the Declaration of Independence, recorded a major outbreak in Philadelphia, Pennsylvania, and a pandemic swept through the Caribbean, Mexico and the United States in 1827 and 1828.
Concerted eradication campaigns and widespread use of the insecticide DDT eliminated dengue from much of the Western Hemisphere by the mid-1950s, but it has slowly crept back.
