By Kelly A. Reynolds, MSPH, Ph.D.

Many of the world’s epidemic diseases are influenced by long-term changes in climate and short-term fluctuations in weather. Weather may influence exposures to microbial pathogens due to increased transport and dissemination of the agents via rainfall and runoff and the survival and/or growth through temperature changes. Watershed protection, infrastructure and storm drainage systems affected by increased rainfall may lead to an increased risk of contamination events.

While disease outbreaks are apparent following severe storm events such as hurricanes and floods, there are several other climate-related factors affecting the emergence and prevalence of disease that are less obvious. Changes in the environment, whether due to natural phenomena or initiated by humans, can upset the delicate ecological balance of the Earth and its living populations. These microorganism populations may thrive due to the presence of a new survival niche or new vectors, hosts or transmission routes. In addition, the loss of predators or competing populations may promote growth, survival and the opportunity for greater transmission to new hosts. Climatic events may increase sea surface temperature and sea level, leading to higher incidence of waterborne infections and toxin-related illnesses such as cholera and shellfish poisoning. The effect of climate stress may be confounded by changes in human susceptibility to infectious disease, through malnutrition due to climate stress on agriculture and stress due to increased flux in ultraviolet radiation.

Areas of concern
The microbial diseases most affected by ecological changes include those with animal carriers or insect vectors, in particular parasitic diseases such as leishmaniasis, schistosomiasis, cryptosporidiosis and giardiasis. Malaria, dengue and viral encephalitides are diseases that are also very sensitive to climate. Cryptosoridia is a parasitic organism well known to the water treatment industry, causing significant levels of waterborne disease. Cattle, dairy cows and wildlife such as deer serve as reservoirs of Cryptosporidia. Runoff from agricultural areas can release the oocysts into the drinking water supply and turbidity overload can decrease the efficacy of water treatment processes. Storm events have also been blamed for serious outbreaks of pathogenic bacteria, such as E. coli, due to runoff contamination of shallow groundwater wells.

Vibrio cholerae is another example of a bacterium that’s highly sensitive to long and short-term changes in climate. These pathogenic bacteria are associated with marine plankton and are globally distributed with this host. Epidemics related to this organism have been linked to climatic events such as El Niño.

Water shortages are also problematic with regard to risks of microbial outbreaks. Universally, when water is in greater demand, the lack of safe water supplies increases and more people depend on rainwater as their water source. The result of less water is a negative association between water availability and diarrhea rates. Similarly, as temperature increases, diarrheal disease also increases. Thus, conditions of drought, global warming and desertification all contribute to increased disease.

Pathogenic dust?
Researchers are constantly attempting to predict the origin of emergent microbes. While it’s likely that some evolve as new organisms, we often inadvertently volunteer to potentially expose ourselves to exotic pathogens by traveling to regions of endemic disease. Microorganisms can travel via exported livestock or produce or even through bilge water of freighters via international ports of call. Still another likely scenario is that pathogens can travel from endemic regions to us via intercontinental dust storms.

Recently, a massive sandstorm blowing off the northwest African desert blanketed hundreds of thousands of square miles of the eastern Atlantic Ocean with a dense cloud of Saharan sand. Earlier this year, an unusually large dust cloud that originated in northwest China drifted across the continental United States and lingered over Denver and other areas, at times obscuring views of the Rocky Mountains. Such storms have been known to negatively impact the air quality in the United States, but some scientists are concerned the health effects may extend beyond the exposure to polluted air. Evidence is mounting that unique microbial species originating in Africa, Asia or other countries can be found transported by dust to the United States.

To date, no evidence has been presented proving that intercontinental dust has been responsible for the spread of microbial epidemics. On the contrary, some experts believe that dust storms are beneficial, reducing the greenhouse effect, by decreasing the amount of sunlight, and thus heat, reaching the Earth’s surface. Scientists at the Goddard Space Flight Center in Greenbelt, Md., say that airborne dust from large deserts can be a vital source of nutrients for both ocean and terrestrial ecosystems—for example, in ocean regions that are deficient in iron.

Climate and disease
Many questions remain regarding the impact of climate on public health, including: What is the current status of the nation’s health and what are current stresses on our health and how might climate variability and change affect existing or predicted problems? What are the data gaps that must be investigated to accurately assess the impact of climate variability on human health? The Climate Change and Human Health Integrated Assessment team is led by the U.S. Environmental Protection Agency (USEPA) and a multidisciplinary group of researchers from 12 academic institutions around the United States. It aims to address these questions and reduce the uncertainty in risk assessment of climate change and the impact on public health by:

  1. Analyzing key climate-sensitive diseases that have the potential to expand or contract, intensify or shift in spatial distribution;
  2. Developing an interdisciplinary, integrated approach that addresses the complexity of anticipated disease responses to climate and ecological change (i.e., utilizing quantitative methods to further improve the risk assessment and, for one case study, illustrating the costs of alternative options for reducing health risk); and
  3. Adopting risk communication strategies to ensure that findings can effectively inform policy makers on the public health risks associated with climate change.

Initial studies, based on analysis of data from 1972 to 1994, showed that a temporal and spatial correlation was evident between extreme rainfall events and waterborne disease. Many other pathogens are known to be seasonal, as represented by the fact that heavy rainfall was associated with overall disease incidence. Such analyses will help to identify high-risk watersheds by observing clustered cases in past outbreaks. This is the first step to focused improvement of watershed protection and disease prevention.

Similarly, the presence of Vibrio cholerae was found to correlate with warmer water temperature. The peak number of cases appears to lag approximately three weeks behind ambient temperature. During the warm winter temperatures of the 1997-98 El Niño, the number of children hospitalized for severe diarrhea was two-fold over expected numbers.

Overall, waterborne disease outbreak data from 1971 through 1994 have been analyzed for groundwater and surface water in 2,105 U.S. watersheds. Between 20 percent and 40 percent of those outbreaks were associated with extreme precipitation (the highest 20 percent of readings over a 20-year period). This relationship with extreme precipitation was found to be statistically significant for both surface water and groundwater, although it was more apparent with surface water outbreaks.

Monitoring the elements
While it’s not likely that we’ll find a way to control Mother Nature, we do have tools available to predict when, where and how climatic changes may affect disease incidence. The application of new monitoring techniques such as molecular fingerprinting to track the source of specific contaminants, and satellite remote sensing to evaluate coastal algal blooms and agricultural activity promise to aid in predicting future outbreaks. In addition, climate analysis, disease modeling and spatial statistical analysis may be used collectively.

Use of remote sensing with satellite imagery offers the potential of predicting conditions of microbial outbreaks or epidemics. This technology is particularly well suited for pinpointing constraining endemic factors and has become highly sophisticated and affordable. In addition, geographical information systems (GIS) and global positioning systems (GPS) can be used to record spatial information of great accuracy as well and rapidly digitalize collected data so that comparisons may be made relative to physical maps. These tools can aid in epidemic forecasting by developing global networks for surveillance and prediction.

Understanding the link between climatological and ecological change, as determinants of disease emergence and transmission, will help to optimize preventative strategies. Such analysis will require the expertise of multiple disciplines including climatologists, biologists, social scientists and physicians. In 1990, Congress initiated the Global Change Research Act, authorizing the U.S. Global Change Research Program (USGCRP) to prepare periodic assessments of the effects of global change on the natural environment, agriculture, land and water resources, human health and biological diversity. The USEPA is also committed to these assessment efforts.

An assessment of risk
Risk assessments of the health impacts of global climate change (GCC) are hindered by two factors. First, dose-response relationships between weather parameters and many of the likely health effects haven’t been developed; and second, reliable estimates of future regional climates across the United States are still beyond the scope of current modeling efforts.

Although hypothesized infectious disease effects have been widely discussed, there haven’t been thorough quantitative studies addressing the many processes at work. Increased disease surveillance and integrated modeling are needed. It’s vital to understand and correlate our knowledge of transport and fate of microbial pollutants with recorded data on snowmelt, rainfall and precipitation. Waterborne disease outbreak data are currently available at the state level and could be used to examine when and where water systems are at risk. Recent studies have begun to have this direct focus.

In addition, there’s much interest on the impact of climate variability in the marine environment combined with stresses such as introduction of new species, over-fishing and other aquatic stresses. Further monitoring and methods development over time promise to close the gap on many of the unknowns surrounding the issue of disease incidence and climate.


  1. Rose, J.B., et al., “Climate and waterborne disease outbreaks,” Journal of American Water Works Association, Vol. 92, No. 9, pp. 77-87, September 2000.
  2. Royston, R., “China’s Dust Storms Raise Fears of Impending Catastrophe,” National Geographic News. June 1, 2001, website:,
  3. The Climate Change and Human Health Integrated Assessment Web, Johns Hopkins University, Baltimore, Md., 1998-2000, website:
  4. U.S. Geological Survey of Saharan Dust events, website:
  5. Weather satellite photographs from the NASA/Goddard Space Flight Center and ORBIMAGE, website:

About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of rapid methods for detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds also has been a member of the WC&P Technical Review Committee since 1997.


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