- by Dr. Kim Knowlton

Understanding the impact of global warming on the spread of dengue

Dengue fever is expanding into new geographic areas. The number of people at risk from the disease is growing faster than ever before. In the US – a place you might not associate with dengue – the estimated number of people hospitalized with dengue more than tripled from 81 in 2000 to 299 in 20072.

Global warming is one of a number of factors accelerating dengue’s spread.


Summarized from the 2009 NRDC Fever Pitch report1 http://www.nrdc.org/health/dengue/files/dengue.pdf

The growing risk of dengue fever across the US 

A 2009 study by the Natural Resources Defense Council (NRDC) revealed the extent to which global warming had increased dengue vulnerability across temperate areas of the United States.


“’Fever Pitch’ was our deepest look into the then current state of dengue fever across the US,” reveals Dr. Kim Knowlton, study author and senior scientist with NRDC’s Science Center in New York City.
Dr. Knowlton and her team began their research by mapping dengue cases reported in the US between 1995 and 2005. They gathered case reports from the Centers for Disease Control and Prevention (CDC), ArboNET, the Pan American Health Organization (PAHO), the World Health Organization’s DengueNet, and other sources. The team overlaid this map with a map showing the location of vector species using data provided CDC’s Division of Vector-Borne Diseases and census information about how many people live in each area.

The researchers were surprised by what they found.

A real cause for concern 

Their analysis found 173 million Americans lived in areas vulnerable to dengue.
Mosquito species that can transmit the disease had already become established in at least 28 states. This meant disease outbreaks were highly possible far beyond the U.S-Mexico border and Hawaii regions – the only regions to see outbreaks before that time.


Image courtesy of NRDC

Rising international travel increases outbreak risk  

With so many Americans now living in areas where the vector is established, one of the main concerns was – and still is – that someone returning from abroad could start the chain reaction that would lead to local dengue transmission within the US.

“A person with an active infection could transmit their infection to a mosquito that can eventually infect a non-infected person in a chain of local transmission,” explains Dr. Knowlton. “Most Americans don’t even think about the symptoms even if they are traveling to an area where it’s endemic or there’s an outbreak. They may come back with symptoms, but don’t visit their doctors.”


NRDC Fever Pitch report1 Download pdf

A lasting impact 

Less than a year after the publication of the study’s findings, the CDC made dengue a notifiable disease in the US.
Dr. Knowlton is confident that awareness around dengue is continuing to grow in the US: “More clinicians are thinking about dengue. They’re questioning their patients about potential dengue symptoms and may ask about their recent travel history.”
In the years since Fever Pitch1 was published, Dr. Knowlton’s department has been getting calls about dengue every summer.
Dr. Knowlton is, however, conscious that there is more to be done: “We don’t yet fully understand the complexities of infectious disease transmission cycles. We don’t understand fully the climate change component and it would be really smart to give ourselves the benefit of knowledge in that area.”

 A fight on a global scale 

We all have a role to play in the fight against dengue.
After all, dengue really is a worldwide public health issue – and not just an issue for those living in tropical or sub-tropical regions.
Dr. Knowlton believes strongly that more must be done to provide people with municipal water and sanitation, with homes and with shelter. “These things are enormously important in dealing with the global dengue crisis. We’re all living in a global community. Helping others to have a better standard of living does matter – not just because it’s the right thing to do… but because it affects everyone’s health globally.”


Summarized from the 2009 NRDC Fever Pitch report1

Learn more about how researchers are combating growing insecticide resistance  


Image courtesy of NRDC

About Dr. Kim Knowlton

Dr. Kim Knowlton is a senior scientist with NRDC’s Science Center in New York City. Earth science was always her first passion; her earliest memories are of reading Jacques Cousteau. After studying earth and physical science for many years, Dr. Knowlton became interested in how environmental changes are affecting people’s health – how our activities affect the environment and how the environment, in turn, affects us, via climate change. Since joining NRDC in 2007, Dr. Knowlton has branched out to study infectious disease, coastal flooding, sea level rises, and aeroallergens. Today she watches and shares the waterfront on climate and health. Connecting the dots between climate change and health is far more than a professional mission: they are human rights issues, economic development issues, and environmental justice issues as well.

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