Dengue Reborn: Widespread Resurgence of a Resilient Vector

Abstract

Dengue—a viral disease that can refer to both dengue fever and the more severe dengue hemorrhagic fever (DHF)—swept away records again this past spring as it raged across Brazil, infecting more than 160,000 people and killing more than 100. The reports were similar to those out of Southeast Asia in the summer of 2007, South America the previous spring, and India the fall before that. Although it may not be the most devastating of the mosquito-borne diseases—malaria strikes 10 times more people and yellow fever kills more of its victims—dengue has become a major public health concern for two reasons: the speed with which it is spreading and the escalating seriousness of its complications. In the nineteenth century, dengue fever was a mild illness found in the tropics. Deaths were rare, and years passed between major epidemics. But since the mid-twentieth century, the range of the dengue virus has steadily broadened. In the last 50 years, its worldwide incidence has increased 30-fold, and various estimates posit that anywhere from one-third to nearly one-half of the world’s population are now at risk of becoming infected. Moreover, today’s dengue infection is not what it once was. DHF, a complication of dengue infection that was not recognized until the 1950s (although cases probably occurred as early as 1870 in India), now appears in many dengue epidemics. In addition to the fever, rash, headache, and muscle and joint pain of classic dengue fever (which earned dengue its nickname of “breakbone fever”), DHF sometimes causes hemorrhaging that can lead to shock and even death. Epidemic DHF is now a leading cause of hospitalization and death among children in several Southeast Asian countries. Worldwide, of the 50 million dengue infections estimated by the World Health Organization (WHO) each year, there are 500,000 cases of DHF and 22,000 deaths, mainly among children. Once considered mainly an Asian disease, dengue fever and DHF now also permeate the tropical Americas. Between 1995 and 2001, the number of dengue cases in the Americas doubled, according to the WHO. By 2007, the annual incidence there reached nearly 900,000 cases, with more than 25,000 people suffering DHF. The dengue virus comes in four distinct serotypes. Individuals who become infected with one serotype obtain lifelong immunity against that serotype but not against the other three—and there is good evidence that a previous dengue infection increases the odds of developing DHF upon infection with a different serotype. “Somehow, having that prior infection enhances invasion of target cells by new dengue [serotypes],” explains Laura Harrington, a medical entomologist at Cornell University. Dengue experts agree on many of the causes of the disease’s spread, including demographic changes and interruptions in vector control efforts. But some controversy has surfaced over whether climate change—often cited as a factor in broadening disease vector habitats—has had or will have anything to do with the virus’s expansion. “It’s too early to predict what effects global warming will have, if any,” says David Morens, senior scientific advisor at the National Institute of Allergy and Infectious Diseases (NIAID) in Bethesda, Maryland. “But it’s certainly something to be concerned about.”