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It was a hot, sticky day when patient zero arrived at a local hospital in Brownsville, Texas, in June 2005. Her body was racked with chills, she couldn’t stop vomiting, her blood pressure was perilously low, and she was passing blood in her urine. Clueless as to the cause, doctors pumped her up with fluids to treat dehydration, dosed her with antibiotics, and sent her home. But when blood tests and clinical evaluation were done with the help of the regional Texas Border Infectious Disease Surveillance project, a surprising culprit was unmasked: dengue hemorrhagic fever, a deadly viral disease usually regarded as a risk only in the tropics.

Long thought eradicated in the United States, dengue is roaring back. There had been prior cases of the disease’s milder cousin, classic dengue fever, in Brownsville, a bustling metropolis of about 140,000 people at the southernmost tip of Texas on the Gulf coast. But this was the first well-documented case of the more serious form of dengue infection, hemorrhagic fever, in a Texas resident infected in the continental United States. It is unlikely to be the last. From 1995 to 2005, some 10,000 cases were reported in the United States and the Texas-Mexico border region. The Centers for Disease Control and Prevention (CDC) believes that many cases are never counted, so these figures may be a vast underestimate.

A range of factors influence the spread of the dengue virus, but rising global temperatures may be the most important of all. Like many tropical diseases, dengue is spread by mosquito bites, and mosquitoes are exquisitely sensitive to climate. Frost kills both adults and larvae, which is why the disease hadn’t previously been able to get a foothold in the United States. With the advent of warmer winters, there is nothing holding the insects back. As a result, the two species of mosquito capable of transmitting dengue fever—Aedes aegypti and Aedes albopictus, also called the Asian tiger mosquito—have substantially expanded their habitat range since the middle of the 20th century. They are spreading into temperate areas in at least 28 states in this country, even as far north as New York and New Hampshire, according to a 2009 report by the Natural Resources Defense Council (NRDC).


Hotter, more humid weather shortens mosquito breeding cycles. Heat speeds up the incubation of the dengue virus, making it infectious much sooner and for more of the insect’s life span. Female mosquitoes bite more frequently when the thermostat rises. Climate change is likely to usher in an era of more extreme weather, including the heavy rains and flooding that create ideal mosquito breeding grounds. And dengue is far from the only risk. Along with mosquitoes, ticks, mice, and other carriers are likewise surviving milder winters and fanning out across the country, spreading an encyclopedia of pathogens: Lyme disease, Rocky Mountain spotted fever, equine encephalitis, St. Louis encephalitis, anaplasmosis, and babesiosis, a once uncommon malaria-like infection in the United States. “Vector-borne diseases are rapidly changing their distribution and frequency,” says medical entomologist Lyle Petersen, director of the Division of Vector-borne Diseases for the CDC.

In the 1950s and 1960s, military-style insect-control campaigns in the United States carpet bombed mosquito breeding grounds with DDT. Curtailing use of DDT certainly has contributed to some of the current resurgence, but the problem is far too broad to explain (or solve) with pesticides alone. “Climate change will cause a worsening of the common health problems we already see,” says John Balbus, senior adviser for public health at the National Institute for Environmental Health Sciences. “There will be incremental changes in the next 5 or 10 years, but that might not compare to what we’re going to see in a matter of decades. In trying to not be alarmist, scientists have systematically underestimated the threat.”

Infectious disease is only one component of that threat. Rising temperatures—an average increase in the United States of 2 degrees Fahrenheit over the past 50 years—are exacerbating a whole range of modern ills, including pollution, urban crowding, and inadequate medical facilities. Asthma, allergies, and heart disease get worse under the stress of heat and smog. Cities are particularly affected due to air pollution and what experts now call “urban heat islands,” created as asphalt, pavement, and buildings concentrate heat. In developing countries, extreme weather could drive even more people away from farms and into overcrowded shantytowns. “People are moving to these giant urban slums without adequate sanitation, which are perfect mosquito breeding zones,” Petersen says. At the same time, violent hurricanes, floods, and fires—likely symptoms of global warming—could stretch the limits of many countries’ health-care systems. Look at Pakistan, which was ravaged by the worst monsoon-induced floods in its history last August. The flooding displaced more than 17 million people and triggered outbreaks of cholera.