The New Diseases on Our Doorstep
Tiny, wizened, and sinewy, Maribel stepped on the leash of her snarling pit bull to keep it from attacking a group of uniformed men and women as they poked through the nooks and crannies of her home. It took all the strength the old woman could muster to hold the furious dog at bay, but she herself showed no sign of indignation at the invasion of her privacy. If anything, she looked resigned and slightly bemused. This lifelong resident of one of the poorest districts of Matamoros, the Mexican town just across the border from Brownsville, Texas, on the Gulf of Mexico, seemed to understand that the operation was, well, for her own good. The técnicos, a vector surveillance team organized and dispatched by the Matamoros Department of Health, were on a seek-and-destroy mission. Their target: mosquito larvae.
In recent days a fresh cluster of dengue fever cases had been confirmed in Maribel's neighborhood. Dengue is endemic in Matamoros; in 2004 a blood-sampling survey found dengue antibodies in 78 percent of the city's residents, which means all who tested positive had been infected with the virus at some point in their lives, though it may have gone undiagnosed. According to José Luís Robles López, the medical services coordinator for the city, dengue's grip has only tightened in the years since. It used to surge from August through October, in the wake of the summer rains, with cases leveling off throughout the rest of the year. But more and more, Robles López says, dengue is diagnosed steadily all year round.
The reasons are many, and Matamoros is not alone. Around the world, the incidence of dengue fever has risen thirtyfold in the last 50 years, with increases across all key indicators: the number of cases, the frequency of epidemics, the severity of the disease, and the geographic range over which outbreaks occur. Each year the virus is responsible for 50 million to 100 million infections, a half-million hospitalizations, and 22,000 deaths in more than 100 countries. Many experts believe that dengue is now the most worrisome arthropod-borne virus, or arbovirus, in the world (arthropods, which include insects, spiders, and crustaceans, have segmented body parts and an exoskeleton). And now it's making its way into the United States.
For most Americans, dengue remains an under-the-radar threat. But as the virus creeps northward, we need to be much more alert. The context for the disease has changed significantly and will continue to do so. Understanding more about dengue can also arm us with perspective on the evolving face of other vector-borne and infectious diseases (a vector is an agent, usually an arthropod, that carries a pathogen from one animal, or host, to another). Dengue's proliferation can be attributed to several interconnected factors: rising temperatures and altered rainfall patterns due to climate change, population booms in impoverished urban areas with inadequate municipal services, increased international travel and trade, compromised or dismantled mosquito-reduction programs, and the glut of man-made containers (especially those made of plastic or rubber) that serve as ideal mosquito breeding sites.
Some or all of these factors affect the transmission patterns of most arboviruses as well as other vector-borne diseases, such as Rocky Mountain spotted fever and Lyme disease, both of which are transmitted by ticks. Mosquito-borne arboviruses include West Nile virus, St. Louis encephalitis, equine encephalitis, chikungunya -- the list goes on. Scientists have documented hundreds of arboviruses. None is as entrenched as dengue, but many are spreading well beyond the areas in which they were originally identified and for which they're often named. Some, such as Lyme disease and West Nile virus, have already exacted a heavy toll on human health in the United States.
Zoonoses -- infections or diseases that are transmissible from vertebrate animals to humans -- add another level of complexity. Many, though far from all, are carried by a vector. But all zoonoses and arboviruses are influenced by a uniquely modern set of ecological disruptions: climate change, deforestation, human migration, urbanization, the intrusion of humans and domestic animals into areas where arthropods flourish, industrialized farming and animal husbandry, and so on.
What's clear is that effective control measures will require coordination across multiple scientific disciplines. "We need to cast a broader net," says Howard Frumkin, director of the National Center for Environmental Health at the Centers for Disease Control and Prevention (CDC). "Some of the health problems we face are new, others are reemergent, and the causes are ever more complex." Frumkin sees the need for communication channels to open among nontraditional partners, such as climate scientists and urban planners, park rangers and health departments, eco-database modelers and sewage engineers. The Brownsville-Matamoros border area, a dengue hot spot, focuses a lens on these issues. It demonstrates how pressing and complex the needs are, while at the same time suggesting ways in which collaborative efforts can begin to meet those needs before our health-care defenses are overwhelmed.