Climate Change and Infectious Disease: Is the Future Here?
Climate Change and Infectious Disease: Is the Future Here?
Considering climate change and extreme weather events when analyzing the spread of disease is a fairly new idea. In one of the first papers to call attention to the potential connection, published in 1989, author Alexander Leaf listed immune system depression, health care and sanitation deficiencies, pollution, population shifts, malnutrition, vector shifts, and contaminated water supplies as factors that could spur a rise in infectious diseases in a warming climate. Studies published as part of a series in The Lancet in the fall of 1993 first began to link increased cases of infectious disease to longer seasons, hotter temperatures, and increased rainfall. Studies published since then have coupled aspects of climate change to increased outbreaks of viral illnesses such as West Nile virus (WNV) and dengue fever, and to outbreaks of bacterial illnesses such as cholera and salmonellosis. In 1996 the Intergovernmental Panel on Climate Change (IPCC) for the first time included a chapter on public health effects in its report.
To make a firmer determination about links between climate change and infectious disease, researchers need high-quality data collected over long periods detailing changes in the numerous factors that go into the spread of disease. But those data haven't been collected. "That is one reason some of us look at cases where [there are] unseasonable conditions, like in an El Niño where there are unseasonal rain or drought conditions or unusually high temperatures," Patz says. "We look at that and say, as scientists, okay what did that extreme weather pattern do to disease? If we can study what happens to disease in extreme weather events, it gives us a window into the future [where such extremes are expected to become more common]."
Most public health officials contacted for this article agree that climate change and extreme weather events will move more infectious diseases northward. Warmer winters and high-latitude warming—occurring twice as fast as overall warming—are already contributing to shifts and expansions of vector ranges. In addition, extreme weather events—occurring with greater frequency and intensity—are often associated with outbreaks of water-, mosquito-, and rodentborne diseases, says Paul Epstein, associate director of Harvard's Center for Health and the Global Environment.
But predicting how the interaction of factors will play out is not always straightforward. According to the World Health Organization (WHO) Scientific Working Group, dengue is the most rapidly spreading vectorborne disease in the world, with the average annual number of reported cases increasing by more than 7.5 times between 1970–1979 and 2000–2005. Malaria, on the other hand, is decreasing in all regions in response to highly targeted control efforts—in 2009 there were an estimated 225 million cases and 781,000 deaths worldwide, down from 233 million cases and 985,000 deaths in 2000. In the World Malaria Report 2010 the WHO stated, "A realistic view of what would have happened without control activities … cannot be established from the data currently available," but suggested that, absent control activities, short-term climate variations could be expected to affect disease trends.
Lyme disease, spread by ticks carrying the bacterium Borrelia burgdorferi, has been expanding for decades in the United States, but it is difficult to know how much of that expansion is a result of infected ticks expanding their geographic range versus growing awareness and better detection of the disease. It is clearly both, says Richard S. Ostfeld, a disease ecologist with the Carey Institute of Ecosystem Studies in Millbrook, New York. "We know [Lyme disease] has spread into the mid-Atlantic states, into Maryland and Virginia and north into New Jersey and New York and southern Canada," he says. "It is possible that the spread northward is influenced by a warming climate, but that wouldn't explain the spread southward. We just don't know what the other factors are."
Other diseases are predicted to become more prevalent as a result of climate-related changes in water and food sources. Water contamination from flooding can cause shortages of clean water that lead to the spread of diarrheal diseases such as cholera as well as enteric diseases such as typhoid. Not having enough water for cleaning and bathing can cause infections such as scabies and trachoma, whereas drinking too little water can lead to harmful infections of the bladder and kidneys. Warm temperatures and rainfall have also been tied to the spread of foodborne contaminants. For example, contamination of crops with aflatoxins—potent mycotoxins produced by Aspergillus flavus fungi that can cause developmental and immune system suppression, cancer, and death—is linked both to increased rainfall and to drought.
One thing that seems fairly clear is that not all areas will see uniform shifts in infectious diseases; these, like other climate-related changes, will be highly dependent on local factors. "In some areas [in the United States] we expect to see a disease increase, and in others areas we expect to see a decrease. How climate change will affect that is really a wildcard at this time," says Ben Beard, associate director for climate change at the CDC's National Center for Emerging and Zoonotic Infectious Diseases.
Diseases of Interest
Considering climate change and extreme weather events when analyzing the spread of disease is a fairly new idea. In one of the first papers to call attention to the potential connection, published in 1989, author Alexander Leaf listed immune system depression, health care and sanitation deficiencies, pollution, population shifts, malnutrition, vector shifts, and contaminated water supplies as factors that could spur a rise in infectious diseases in a warming climate. Studies published as part of a series in The Lancet in the fall of 1993 first began to link increased cases of infectious disease to longer seasons, hotter temperatures, and increased rainfall. Studies published since then have coupled aspects of climate change to increased outbreaks of viral illnesses such as West Nile virus (WNV) and dengue fever, and to outbreaks of bacterial illnesses such as cholera and salmonellosis. In 1996 the Intergovernmental Panel on Climate Change (IPCC) for the first time included a chapter on public health effects in its report.
To make a firmer determination about links between climate change and infectious disease, researchers need high-quality data collected over long periods detailing changes in the numerous factors that go into the spread of disease. But those data haven't been collected. "That is one reason some of us look at cases where [there are] unseasonable conditions, like in an El Niño where there are unseasonal rain or drought conditions or unusually high temperatures," Patz says. "We look at that and say, as scientists, okay what did that extreme weather pattern do to disease? If we can study what happens to disease in extreme weather events, it gives us a window into the future [where such extremes are expected to become more common]."
Most public health officials contacted for this article agree that climate change and extreme weather events will move more infectious diseases northward. Warmer winters and high-latitude warming—occurring twice as fast as overall warming—are already contributing to shifts and expansions of vector ranges. In addition, extreme weather events—occurring with greater frequency and intensity—are often associated with outbreaks of water-, mosquito-, and rodentborne diseases, says Paul Epstein, associate director of Harvard's Center for Health and the Global Environment.
But predicting how the interaction of factors will play out is not always straightforward. According to the World Health Organization (WHO) Scientific Working Group, dengue is the most rapidly spreading vectorborne disease in the world, with the average annual number of reported cases increasing by more than 7.5 times between 1970–1979 and 2000–2005. Malaria, on the other hand, is decreasing in all regions in response to highly targeted control efforts—in 2009 there were an estimated 225 million cases and 781,000 deaths worldwide, down from 233 million cases and 985,000 deaths in 2000. In the World Malaria Report 2010 the WHO stated, "A realistic view of what would have happened without control activities … cannot be established from the data currently available," but suggested that, absent control activities, short-term climate variations could be expected to affect disease trends.
Lyme disease, spread by ticks carrying the bacterium Borrelia burgdorferi, has been expanding for decades in the United States, but it is difficult to know how much of that expansion is a result of infected ticks expanding their geographic range versus growing awareness and better detection of the disease. It is clearly both, says Richard S. Ostfeld, a disease ecologist with the Carey Institute of Ecosystem Studies in Millbrook, New York. "We know [Lyme disease] has spread into the mid-Atlantic states, into Maryland and Virginia and north into New Jersey and New York and southern Canada," he says. "It is possible that the spread northward is influenced by a warming climate, but that wouldn't explain the spread southward. We just don't know what the other factors are."
Other diseases are predicted to become more prevalent as a result of climate-related changes in water and food sources. Water contamination from flooding can cause shortages of clean water that lead to the spread of diarrheal diseases such as cholera as well as enteric diseases such as typhoid. Not having enough water for cleaning and bathing can cause infections such as scabies and trachoma, whereas drinking too little water can lead to harmful infections of the bladder and kidneys. Warm temperatures and rainfall have also been tied to the spread of foodborne contaminants. For example, contamination of crops with aflatoxins—potent mycotoxins produced by Aspergillus flavus fungi that can cause developmental and immune system suppression, cancer, and death—is linked both to increased rainfall and to drought.
One thing that seems fairly clear is that not all areas will see uniform shifts in infectious diseases; these, like other climate-related changes, will be highly dependent on local factors. "In some areas [in the United States] we expect to see a disease increase, and in others areas we expect to see a decrease. How climate change will affect that is really a wildcard at this time," says Ben Beard, associate director for climate change at the CDC's National Center for Emerging and Zoonotic Infectious Diseases.
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