Baltimore,
MD (3/11/98)- The combined climatologic cataclysms of global warming
and El Niño could bring more than warm winters and floods to the
Northern hemisphere, they also could be bringing dengue fever, warn public
health authorities.
Dengue fever is the most widespread viral infection transmitted to man
by insects. Dengue fever is an acute infectious disease caused by the flavivirus
(a small RNA virus related to yellow fever, tick-borne encephalitis, St
Louis encephalitis, and Japanese encephalitis) which is transmitted by
infected mosquitoes. The disease causes headache, fever, muscle pain and
rash, but is rarely fatal. However, a more severe form of the disease seen
mostly in children, dengue hemorrhagic fever, causes severe symptoms including
fever, shock, hemorrhaging from the nose and mouth, respiratory distress
and death.
A recent computer simulation of the general circulation of the
earth's climate is the basis for growing concerns about the spread of the
disease to temperate regions from it current base in the south. The
computer study suggests that most of the new areas of increased potential
risk are the temperate regions bordering on current endemic dengue zones.
These fringe areas represent places where humans and the primary carrier,
the mosquito Aedes aegypti, often co-exist, but where lower temperatures
now limit disease transmission.
"Since inhabitants of these border regions would lack immunity
from past exposures, dengue fever transmission among these new populations
could be extensive," says Jonathan Patz, MD, MPH, from the Johns Hopkins
School of Public Health said,
Unlike the yellow fever virus, carried by the same mosquito (Ae.
Aegypytpi) , 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.
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. The new study suggests
that 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.
Dengue has been on the increase in the Americas for the past 20 years.
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 six-fold 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 report appeared in the March 1998 issue of Environmental Health
Perspectives, the monthly journal of the National Institute of Environmental
Health Sciences.
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