“Climate change is the biggest global health threat of the 21st century,” a team of British scientists and academics wrote in The Lancet last year. The threat has been completely neglected, marginalized, and ignored by the global health community and by policymakers, according to pediatrician Anthony Costello, director of University College London’s Institute for Global Health and lead author of the report. “Yet in terms of our well-being, in terms of our survival over the next 100 years, it is absolutely the top political issue involving the ecosystem that we should be talking about,” he adds.
Patient zero recovered. But Brownsville’s public-health officials were alarmed at the possibility of a deadly dengue epidemic because there is no vaccine to protect against it or medicine to cure it. The grave worry was that “if dengue moves into a population that hasn’t experienced it and has no natural immunity, we are going to see more severe disease,” says Susan Fisher-Hoch, an epidemiologist at the University of Texas School of Public Health in Brownsville who was involved in studies of dengue in her state?. “The way we live hermetically sealed in our houses—with screens on the windows and air-conditioning—protects us enormously. But what about those who don’t live well—the people in trailer parks sitting on their porches surrounded by mosquitoes?”
The potential risk was great. Brownsville is becoming a test lab for how this country will respond to the surge of new and renewed infectious diseases. As soon as it became clear that patient zero was a case of indigenous dengue hemorrhagic fever, American health officials began collaborating with counterparts in Matamoros (a Mexican city just across the Rio Grande with a population of nearly three-quarters of a million), dispatching teams to do a blood-sampling survey to uncover the overall extent of dengue infections. They knocked on doors throughout Brownsville, in Matamoros, and in the colonias, the squalid shantytowns that line the border. There, residents live in cramped quarters, with poor sanitation, no running water, and no paved roads. The old tires, rusty buckets, and plastic containers that litter the encampments collect stagnant water, making them ideal breeding grounds for mosquito larvae.
Health officials discovered that nearly 1,300 people were infected with dengue fever as of the end of August 2005. The results of a random survey were even more startling: The officials found that 76 percent of the residents surveyed in Matamoros had dengue antibodies, indicating prior exposure to the virus. They also found evidence of past dengue infection in nearly 40 percent of those tested in Brownsville. Of the 24 Brownsville residents who had never traveled outside the United States, 25 percent tested positive for dengue—which meant the illness was now firmly entrenched here. Patient zero was merely the visible manifestation of a tropical disease that had already put down roots in the United States.
Despite the thousands who have been stricken, most Americans are not aware of dengue—yet. But epidemic outbreaks throughout Latin America—in Brazil, Mexico, Honduras, Paraguay, Costa Rica, Bolivia, and Cuba—now hit nearly a million people annually. Inexorably, the disease has been extending its reach farther north. Cases are being reported with increasing frequency in Puerto Rico and Florida and now number in the thousands there. Dengue cases have been confirmed in almost every state and as far up as Maine, Minnesota, and Washington. The numbers cast a foreboding light on the range of the aggressive Asian tiger mosquito, which transmits not only dengue but Chikungunya fever, a particularly nasty infection that causes excruciating joint pain.
Over the past three decades, the World Health Organization has identified at least 30 new or resurgent diseases around the world. Confronting these diseases will require greatly expanded surveillance networks; Brownsville amply illustrates the inadequacies of the existing system. In the United States, some of the most alarming outbreaks are occurring in the suburbs, where rising temperatures may help explain the spike in such tick-borne illnesses as babesiosis; hga (human granulocytic anaplasmosis), a potentially lethal flulike infection; and Lyme disease, the most common vector-borne disease in this country. The black-legged ticks that spread Lyme-causing bacteria are hardy survivors found on parts of the West Coast, in the Midwest, and on the East Coast from at least South Carolina right up into Canada. Lone star ticks, abundant in the South, transmit infections like tularemia, ehrlichiosis, and a Lyme-like illness not yet identified. They have been spotted as far west as Texas and Oklahoma and along the eastern seaboard as far north as Maine.
Louis Magnarelli, a medical entomologist and director of the New Haven–based Connecticut Agricultural Experiment Station, suspects that subtle changes in the climate may be behind the spread. Warmer winters, wetter and earlier springs (which expand the time during which ticks can pick up the disease), increased humidity, and greener environments can all contribute to the increased incidence of ticks and the growing populations of hosts, including large mammals like white-tailed deer; smaller ones such as white-footed mice, the principal carrier of Lyme disease; and many species of birds. “We’re seeing more disease—the number of cases being reported is very conservative—and ticks are expanding their geographic range,” Magnarelli says.
The increased mobility of the modern world makes the situation worse. Scientists say that the most virulent version of Lyme disease may have reached our shores from Europe by ship. Infected individuals or parasitic hitchhikers can now go anywhere in the world in less than 24 hours and deliver reservoirs of malaria, dengue, or Chikungunya fever to newly temperate ecosystems. “When you have this combination of tremendous global transport of people and goods, changing ecosystems, and changing climate,” Balbus says, “it raises the possibility of emerging diseases.”
The synergy of warmer temperatures, extreme weather (both wet and dry), and increased mobility is what enabled the West Nile virus to become entrenched in North America. The deadly pathogen first emerged in the West Nile district of Uganda in 1937, then lay dormant for about 20 years before it cropped up again in Israel in the 1950s and reappeared in Romania in 1996. Each of these outbreaks occurred after an unusually dry, hot spell, according to research by Israeli scientists, creating the perfect incubator for Culex pupiens, a common house mosquito that transmits West Nile.
In 1999 the virus was identified in New York City, possibly reaching our shores inside a mosquito stowaway on an international flight, but more likely carried in the bloodstream of someone already infected. Once again, that summer was unusually hot and dry in New York, and the high temperatures helped West Nile gain a foothold. “By mid-August there were 10 days over 100 degrees that were followed by heavy rains and flooding, which made the mosquito populations explode,” recalls Tracey McNamara, a veterinary pathologist who was head of pathology for the Wildlife Conservation Society at the Bronx Zoo during the initial West Nile outbreak.
Stagnant pools of water teeming with mosquitoes lured thirsty birds, and they became infected, transmitting the West Nile virus. “The virus just swept through the bird population at the zoo, killing crows, flamingos, even bald eagles,” McNamara says. Within a year, 62 people were stricken with West Nile disease and seven were dead. When another hot, dry summer hit in 2002, the disease spread like wildfire across the United States and into Canada. Since then, more than 30,000 cases of West Nile disease have been reported in the United States. Some have been left severely disabled by a pathogen that attacks the nervous system, and more than 1,000 have died.
“We think we’re protected by the Atlantic and the Pacific oceans, but the reality is that we’re an eight-hour plane ride away from someone who has just flown into London from Africa,” says medical toxicologist James Diaz, program director of environmental and occupational health sciences at Louisiana State University School of Public Health in New Orleans. “And don’t forget we lost millions of birds as West Nile marched across the United States, and we lost untold numbers of horses and other animals, too.”
Could malaria and other exotic diseases that today rarely occur in the industrialized nations of North America and Europe follow a similar path? Diaz thinks so. He points out that competent mosquito vectors for dengue and malaria are already present in the United States. “They’re simply awaiting an opportunity to transmit these diseases locally between arriving infected airline travelers and nearby residents,” he says.