Rising global temperatures may be helping to spark a population boom in insects and disease-carrying animals, creating unexpected threats to human populations, a number of scientific reports say.
Earlier this year a report by the United Nations Intergovernmental Panel on Climate Change found that heat waves typified by stagnant masses of warm air that fails to cool at night will intensify throughout North America in the coming decades. A 2005 report by Harvard scientists carried similar findings.
"There is no question that insects do better where it is warm," David Pimentel, a professor of entomology at Cornell University in Ithaca, said Friday.
West Nile disease, which appeared eight years ago in Queens, now has spread across the United States and Canada. Before 1999, it had not existed on the continent. Epidemiologists are uncertain exactly when -- and how -- West Nile, which is spread by birds that carry the virus, reached the United States. Mosquitoes that feed on fowl pick up the virus and transmit it to people.
The Centers for Disease Control and Prevention estimates that 23,975 people have been sickened by West Nile since it first appeared in the United States; 962 have died.
CDC scientists have noted that in 1999 only two subspecies of mosquitoes were carriers of the disease: Ades vexans and Culex pipiens. Now, entomologists have identified 62 subspecies of mosquitoes that carry the virus.
The saga of West Nile, Pimentel and other experts say, does not differ much from stories of other diseases that are borne by vectors and are now being spread in regions where they didn't exist before.
World Health Organization scientists have identified more than two dozen diseases that are flourishing in regions that once were characterized by icy winters. As winters warm, niches are created for disease vectors, such as mosquitoes. Lyme disease-carrying ticks have been detected on penguins in Antarctica, some experts say.
Some scientists speculate that climate change has already triggered epidemics in regions where people are not braced for the onslaught: Malaria-carrying mosquitoes, for example, are flying to higher elevations in Africa and South America because they are no longer limited by what were once colder temperatures at higher altitudes.
"Mosquitoes thrive in warm, moist conditions," Pimentel said. "Ticks also do better under warm conditions. They are going through their life cycle faster. For example, rather than going through a life cycle in three weeks, it's now estimated that some mosquitoes go through it in two weeks.
"That means there are more generations per year and of course that adds to the overall population of mosquitoes."
Dr. Richard Horowitz, president of the International Lyme and Associated Diseases Society in Bethesda, Md., said global warming may be driving the proliferation of the Ixodes scapularis tick, or deer tick, which carries the bacterium Borrelia burgdorferi.
He also said global warming appears to be aiding a population boom in mice, especially the white-footed mouse, which carries Lyme disease ticks.
"Global warming is kind of a paradox when it comes to ticks," Horowitz said. "Ticks don't generally do well in direct sunlight. They like moisture, which is why they exist at the edges of lawns," usually where it is shady, especially when there is a canopy of trees and shrubs.
