Climate Change Spurring Dengue Rise, Experts Say

Eliza Barclay in Mexico City
for National Geographic News
September 21, 2007
Climate change is accelerating the spread of dengue fever throughout the Americas and in tropical regions worldwide, researchers say.

More rainfall in certain areas and warmer temperatures overall are providing optimal conditions for mosquitos—which spread the virus that causes dengue—to breed and expand into new territories.

By 2085 climate change will put an estimated 3.5 billion people at risk of dengue fever, the United Nations's Intergovernmental Panel on Climate Change (IPCC) said in March. (Related news: "Dengue Fever: Growing Threat Rivals Malaria, Ebola, Experts Say" [October 18, 2006].)

"Climate change is incurring lots of unintended consequences for health around the world," said Paul Epstein, associate director of the Center for Health and the Global Environment at the Harvard Medical School. Epstein also worked on the IPCC report.

The upsurge in dengue, the world's most widespread vector-borne virus, is part of this wider trend.

For instance, heat waves and heat-related illnesses and death, an increase in incidence of tropical diseases, and a rise in tick-borne Lyme disease are all becoming a reality, Epstein said.

Dengue in the Americas

Dengue—which is usually not fatal—is most commonly spread by the Aedes aegypti mosquito, a domestic, day-biting insect that favors human blood.

Dengue transmission is largely confined to tropical and subtropical regions, since freezing temperatures kill the mosquito's larvae and eggs.

Dengue's burden may be most serious in the American continents, according to the U.S. Centers for Disease Control and Prevention.

The number of cases reported in the Americas increased from 66,000 in 1980 to 552,000 in 2006, according to the Pan American Health Organization. Brazil, Paraguay, and the Dominican Republic have all had serious epidemics in recent years.

And in Mexico dengue cases have increased by more than 600 percent since 2001, according to Mexico's National Center for Epidemiology and Disease Control.

Mexican health officials have begun to acknowledge that climate change may be a factor in the uptick of dengue cases.

Pablo Kuri, director of the center of epidemiology, told reporters last week that he is seeking 1.8 million U.S. dollars (20 million Mexican pesos) to aid seven Mexican states with the highest incidence of dengue.

Andres Flores Montalvo is director of climate change studies at the National Institute of Ecology in Coyoacán.

"There are many factors that could explain the growth in the number of cases of dengue in Mexico, but surely increases in temperature and precipitation are influential," Flores Montalvo said.

For example, a 2006 study published by both the National Institute of Ecology and the National Institute of Public Health found an increase in the incidence of vector-transmitted diseases—such as dengue and malaria—that were associated with rising temperatures and rainfall patterns. (Related news: "Warming May Spur Extinctions, Shortages, Conflicts, World Experts Warn" [April 6, 2007].)

New Niches

One of the most notable changes is that cases of dengue are now appearing in Mexican states outside the traditional range of the dengue-carrying mosquito, including the northern state of Chihuahua. (See a map of the region.)

Horacio Riojas is the head of the environmental health unit at the National Institute of Public Health and the lead author of the 2006 National Institute of Ecology study.

The combination of higher temperatures and greater humidity is allowing the dengue mosquito to flourish in its native habitat as well as in new regions of Mexico, he said.

"The vector [mosquito] ... is generating more and better niches where it can thrive," Riojas said.

The rise in natural disasters such as hurricanes in Mexico has also indirectly contributed to the spread of dengue, Riojas added.

After Hurricane Stan hit southern Mexico in 2005, public health researchers detected changes in the hydrology of rivers caused by the hurricane's force—which created new reservoirs for the dengue mosquito to breed.

Both Riojas and Flores Montalvo agree that more research is needed to understand the relationship between dengue and climate change in Latin America.

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