 |
 cientists using computers to simulate
the general circulation of the earth's climate have predicted that rising
global temperatures will increase the potential transmission of the dengue
fever virus. Dengue fever is now considered the most widespread viral infection
transmitted in man by insects, whether measured in terms of the number of
human infections or the number of deaths. Their report appeared in the March
1998 issue of Environmental Health Perspectives, the monthly journal of
the National Institute of Environmental Health Sciences.
 Most of the new areas of increased
potential risk were predicted to be temperate regions that currently border
on endemic zones. These fringe areas represent places where humans and the
primary carrier, the mosquito Aedes aegypti, often coexist, but where
lower temperatures now limit disease transmission. Lead author Jonathan
Patz, MD, MPH, from the Johns Hopkins School of Public Health said, "Since
inhabitants of these border regions would lack immunity from past exposures,
dengue fever transmission among these new populations could be extensive."
Unlike the yellow fever virus, carried by the same mosquito, the dengue
virus is not vulnerable to any vaccine or drug. Major epidemics of dengue
have occurred in the southeast United States, the largest in Galveston,
Texas, in 1922, when over 500,000 people were stricken. The last outbreak
in Texas occurred as recently as 1995, during an unseasonably hot year.
The researchers used three different
general circulation models to predict the patterns of global climate change;
all three showed that dengue's epidemic potential increases with a relatively
small temperature rise. The higher a virus's epidemic potential, the fewer
mosquitoes are necessary to maintain or spread dengue in a vulnerable population.
The geographic range of Ae. aegypti
is limited by freezing temperatures that kill overwintering larvae and eggs,
so that dengue virus transmission is limited to tropical and subtropical
regions. Global warming COULD increase the range of the mosquito. In addition,
the time the virus must spend incubating inside the mosquito is shortened
at higher temperatures.
An estimated 2.5 billion people are
currently at risk from dengue infection, and since the late 1970s dengue
has reemerged in the Americas. In 1997, 240,587 cases of dengue were reported
in Brazil alone, according to the ministry of health. This past January,
responding to weather generated by El Niño, dengue transmission rates
in southeastern Brazil increased nearly sixfold from 1997 levels. Outbreaks
in urban areas infested with Ae. Aegypti, can be explosive, involving up
to 70-80% of a population.
While the accuracy of long-term climate
forecasting by computers will continue to be questioned, the global warming
scenarios predicted by the various different computer models are increasingly
coming to resemble one another. Climatologists are projecting that global
climate will change at an unprecedented rate over the next century. Dr.
Patz said, "Our study makes no claim that climate factors are the most
important determinants of dengue fever. However, our computer models illustrate
that climate change may have a substantial global impact on the spread of
dengue fever."
The study was funded in part by the
Climate Policy and Assessment Division of the U.S. Environmental Protection
Agency, the National Institute of Public Health and Environment (RIVM) (The
Netherlands), and the Center for Medical, Agricultural, and Veterinary Entomology
of the U.S. Department of Agriculture.  |
jhsph.edu.