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Climate changes could increase the risk of infectious diseases
Disease models examined; experts urge continued surveillance.
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TORONTO — Rising global temperatures, the related effects on ecosystems and the impact on infectious disease spread have microbiologists and epidemiologists analyzing history, realigning scientific focus and monitoring trends of weather patterns and global disease emergence.
The merge of two disciplines – microbiology and climatology – to form infectious disease models based on weather changes and global climate changes has proven daunting in complexity, yet increasingly significant for scientists who hope to minimize health effects using novel and probability-based models.
Although global warming and the potential effects to human health are well-documented, temperature prediction and disease spread are not an exact science, according to panelists who convened to discuss the topic at a symposium during the American Society for Microbiology 107th General Meeting, held here. The panelists were infectious disease epidemiologists who focus on environmental microbiology and public health.
Microbiologists studying environmental change must examine factors that have an effect on the tiny organisms to the entire ecosystem. They must also examine the effects of both slight and severe weather events, on infectious disease.
“This represents the shift from the intense reductionism that has captured science in the past century to a holistic approach of understanding infectious disease,” said Rita R. Colwell, PhD, professor at the University of Maryland and Johns Hopkins University Bloomberg School of Public Health.
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Preparedness for rapid spread of infectious disease spurned by weather and environmental change should still be a major part of the global public health arena, the panelists said, as the global community is connected as never before.
Seasonality concept
For diseases such as cholera and malaria, scientists have been able to track, map and chart integrated patterns pertaining to seasonality, which includes peak transmission times spanning more than 20 years. Seasonality patterns also affect influenza because as the global temperature rises, peak transmission times may blur as seasons diminish.
According to a recent WHO report, the first detectable changes in human health attributable to climate change will involve the seasonality of infectious diseases, including malaria, dengue fever and foodborne infections, such as salmonella, which peak in warmer months. The interpretation of these altered patterns must include environmental change observation.
Twenty-five years ago, cholera was thought of as a simple human-to-human transmission. “Instead, we now know that it is an intricate web of bacterium in the natural environment carrying out natural functions,” Colwell said.
These natural functions — nitrogen cycling, oil degradation and chitin degradation — all affect marine creatures such as copepods, which are abundant in freshwater and seawater. The copepods are associated with cholera outbreaks from freshwater; when environmental events occur, such as tidal increases or sea surface warming, copepods can become abundant. Their abundance is also seasonal and can cause outbreaks in developing nations lacking proper water treatment.
In the case of cholera, sea surface temperatures and sea levels can be attained through satellite and then implemented in models to predict where epidemics could occur, Colwell said.
Waterborne illnesses
Waterborne illnesses, such as cholera, can be effected by climate, which is a clear driver of precipitation. Rains, winds and temperatures all influence the growth, transport and survival of bacteria causing infectious disease. About 70 million people per year are affected by flooding alone, according to the panelists. Flooding, as evidenced by the health outcomes seen after the December 2004 Indian Ocean Tsunami and Hurricane Katrina in 2005, has a strong affect on health. Flood victims can be exposed to various serious diseases including hepatitis, cryptosporidium and leptospirosis.
“When we start talking about extreme events, we must recognize that about 50% of the waterborne outbreaks occurring in communities have a climate signature,” said Joan B. Rose, PhD, environmental microbiology laboratory director and Homer Nowlin chair in water research at Michigan State University.
Flooding is of particular concern to scientists examining global temperature changes and rising sea levels.
Developed nations with adequate sanitation have problems with overflow following natural disasters such as Hurricane Katrina, and sometimes even without a weather event, as seen in the Great Lakes regions. Meanwhile, the rest of the world must expel waste into the environment without treatment.
“If we start to understand that the landscape, the climate signature and the disease are all connected, we can put in early warning and better prevention toward long-term health effects associated with events,” Rose said.
Hantavirus and El Niño
Weather extremes such as El Niño also provide patterns for prediction as shown by Hantavirus in the U.S. Southwest.
Predicting individual risk for Hantavirus a year in advance in the United States is not only possible, it is also far from novel, according to Terry Yates, PhD, vice president for research and development and professor of biology and pathology at the University of New Mexico.
Hantavirus continues to occur in Canada and Southwestern United States. The virus has been killing people for thousands of years, but only recently has it been officially classified by modern science. Zoonotic diseases are not only directly affected by environmental change, but infection can be quick and fatal.
“It is not trivial that these zoonotic diseases, the ones that occur in nature, are the ones that people turn to when they want to make biological weapons,” Yates said.
Legends from the Navajo Nation of the United States date back thousands of years and pinpoint environmental predictors of Hantavirus.
A particular Navajo sand painting depicts a deer mouse outside a medicine wheel. The legend warns that when humans are out of balance with the land they may have abundant crops that lead to wasteful behavior. If the Navajo people become wasteful, the deer mouse becomes angered and takes the youngest and strongest by invading homes and leaving droppings. The people then become sick and die.
“What they described was not only the disease but the transmission of it,” Yates said. “The only time a dry land Native American farmer is ever going to have an overabundance of food is when they have an overabundance of moisture and the right conditions for growing without the aid of irrigation, and that points to El Niños.”
Yates pointed to the 1993 Hantavirus outbreak in the U.S. Southwest and preceding El Niño, then its progression to Canada a year or two after the outbreak.
Using models formed from the 1993 infectious disease outbreak, predictions can be made about increased risk for Hantavirus following years with El Niño conditions.
“Hantavirus kills 40% of people who catch it no matter what we do. That’s a dangerous virus to not even know it existed,” Yates said.
An understanding of the biodiversity of the planet at a microbial level is necessary to map infectious disease risks related to weather and environmental changes. “We do have the techniques, tools and the ability to use this, so it’s time we started using it,” Yates said.
Environmental change
When considering the environmental impact on disease, scientists should note humans have been the greatest agents of change in world history and should realize the numerous interdependencies of the organic world humans both depend on and effect, panelists said.
“Human impact is as great as any of the glaciers or asteroids,” said Stephen S. Morse, PhD, associate professor of epidemiology at Columbia University and founding director and research scientist at the Center for Public Health Preparedness and the National Center for Disaster Preparedness. “Whether we like to think of it or not, we’ve probably driven more species into extinction, in most cases unintentionally, than most of the natural disasters over the years.”
Although scientists now better understand seasonality, the concept of seasonality and infectious diseases are still poorly understood, Morse said. Influenza, for example, is considered seasonal in some climates and is year-round in the tropics, but scientists have no clear reasons for this seasonality, he said.
“I think we have a great deal to learn about the world in which we live and about our own affects on the world, especially how we may be spreading and precipitating new diseases,” Morse said.
Surveillance and prudent efforts are necessary, he added.
The big picture
In addition to moving from microbiology to an environmental microbiology model, infectious disease analysis should be considered in time, space and probability.
“At the moment, we are looking at the past to understand the present and to predict the future. The two are different things,” said David J. Rogers, PhD, professor of ecology at the University of Oxford.
Scientists must use three different concepts: the past, the present and the future. There are no certainties in any of it, just probabilities and well-educated guesses, he said.
According to Rogers, the dilemma lays in balancing the probability of an infectious disease emergence or species crossover, such as concerns for H5N1, against the degree of potential infectious disease effect, from minimal to catastrophic.
“I’m told that the chance H5N1 will mutate to a transmissible form of flu is low probability, but its consequences are absolutely vast,” Rogers said. “It could wipe out a good proportion of people on the planet.”
Epidemiologists prioritize illness as high probability and low consequence vs. low probability but dire consequences. They also consider matters such as climate and the intricacies of an environmentally dependent ecosystem from tides to deer mice. The focus must also shift from microbiology to advanced meteorology.
Climatologists already have a difficult task predicting future climates, he said.
“If you add diseases on top of climate predictions, there are extra links in the chain of causation,” Rogers said.
For example, malaria prevalence predictions based on climate and other environmental factors would save lives if models could be derived. Time scales of each organism, from bacteria to vector to host, must join the equations.
Malaria involves a parasite that divides every 20 minutes in the body of a mosquito, which lives about 10 days. The mosquitoes reproduce every 10 days. The chain of causation tacked onto a malaria time scale plus climate estimates can only produce a best guess due to the multitude and complexity of factors.
“So we are guessing where malaria will be in 50 years,” Rogers said.
Globalization
Scientists concerned about global temperature changes and infectious diseases agree that globalization has increased epidemic risk to a point that nearly no one can use geography as a defense against infectious disease.
With an estimated 6 billion people on Earth, connected as never before, epidemiologists and climatologists have a hard task in preparedness and surveillance. SARS, H5N1, foot and mouth disease and HIV are just a few examples of diseases that would have taken much longer to spread so quickly years ago.
“Every person on this planet, as Duncan Watts emphasized, is no more than five handshakes away from every other person on this planet,” Rogers said.
To illustrate this point, Rogers noted that currently, drug-resistant tuberculosis is a major concern in South Africa. “Fifty years ago, that wouldn’t have mattered in the West, but it is now only a plane ride away from where most of us live, and that’s a rather sobering thought,” Rogers said. – by Kirsten H. Ellis
| Additional information about the effects of climate change on infectious disease can be found on these websites: |
| Information from WHO about climate change and global health: Information on the effects of climate change from the U.S. Information on the impact of climate change on infectious Information from the National Resource Defense Council on global |
For more information:
- Colwell RR. Climate change and waterborne diseases.
- Rose JB. Extreme weather events and their effects on human health.
- Morse SS. Emerging viruses and other infectious disease risks related to climate change.
- Yates T. Predictive modeling for disease epidemics and pandemics.
- Rogers DJ. Global perspectives on infectious diseases, climate and health.
- Divisional group symposium: Environmental change and infectious disease. #0009. Presented at: The American Society for Microbiology 107th General Meeting; May 21-25, 2007; Toronto.