The Effects of Global Warming and Climate Change on Infectious Disease Patterns
Climate Change
The earth’s atmosphere was first equated to that of a greenhouse in 1827 by Jean Baptiste Fourier (Khasnis & Nettleman, 2005). The ‘greenhouse effect’ describes the natural ability of the earth’s atmosphere and associated gases to retain heat radiated from the sun (Khasnis & Nettleman, 2005). Upon arrival in the earth’s atmosphere, the sun’s radiation is absorbed providing a warming effect to the earth’s climate (Khasnis & Nettleman, 2005). A large portion of the absorbed radiation is reemitted back into the earth’s atmosphere for release (Khasnis & Nettleman, 2005). Earth’s ‘greenhouse gases’, namely methane, nitrous oxide, carbon dioxide, hydrofluorocarbons, sulfur hexafluoride, and perfluorocarbons serve as a barrier to the release of much of the reemitted radiation from the earth’s surface (Khasnis & Nettleman, 2005). Although the ‘greenhouse effect’ and the associated ‘greenhouse gases’ allow for the sustainability of life on earth, an overabundance of these gases has caused the emergence of global warming (Khasnis & Nettleman, 2005). Since the turn of the twentieth century, global temperatures have increased by 0.6°C as depicted in Figure I (NASA Goddard Institute for Space Studies, 2002). Future
Figure I. Increases in the average global temperature from 1880 to 2002. Source: NASA Goddard Institute for Space Studies
predictions surrounding global temperature increases during the next century allude to a 2°C to
6°C increase (Climate Action Network Canada, n.d.). Because of its northern geography, the effects of global warming in Canada are expected to be more severe than other parts of the world with temperature increases forecasted between 6°C and 10°C (Climate Action Network Canada, n.d.). In northern Canada, birds foreign to the region, such as robins, are becoming more prevalent (Climate Action Network Canada, n.d.). Polar bears and other artic creatures are losing their habits to global warming due to the excessive melting of ice (Climate Action Network Canada, n.d.). As a result of the warming of the earth’s atmosphere, other climatic events have become more prevalent, hence the concept of climate change (Nugent, 2004). Specifically, extreme weather events and precipitation levels have risen and are expected to continue in an upward trend (Nugent, 2004). In the past decade, extreme weather events in Canada have drastically increased from less than 20 at the turn of the twentieth century to nearly 140 in 1999 (refer to Figure II) (Environment Canada, 2005). To compound on the increased prevalence of extreme weather events, by 2070, precipitation levels in Canada are expected to rise between 5
Figure II. Increases in extreme weather events over the past century as a result of global warming. Source: Environmental Canada
and 25 percent as illustrated in Figure III (Natural Resources Canada, 2007). As a result of global warming and the subsequent increases in extreme weather events and precipitation levels,
Figure IIII. Future predictions reveal substantial increases in precipitation amounts in Canada. Source: Natural Resources Canada
alterations in infectious disease patterns are also anticipated (Landrigan & Garg, 2002). Because the scope and consequences of global warming are complex and expand far beyond the increases in global temperatures, extreme weather events, and precipitation amounts, the remainder of this paper will focus on the interaction between climate change and infectious disease patterns in North America, especially Canada.
Infectious Diseases
To fully understand the connection between climate change and infectious disease patterns, a basic understanding of infectious diseases is needed. By definition, “infectious diseases are caused by invading organisms called pathogens […such as…] bacteria, viruses, parasites, fungi, and molds” (Merrill & Timmreck, 2006). For a pathogen to produce disease, it requires the presence of three elements: host, environment, and time (Merrill & Timmreck, 2006). This relationship is beautifully depicted in the epidemiologic triangle as seen in Figure IV (Center for Disease Control and Prevention, n.d.). Because the interaction between pathogens
Time
Figure IV. The epidemiologic triangle illustrates the connectedness and dependency of the host (disease-causing pathogen) on its environment and host. Source: Center for Disease Control and Prevention
and the environment is integral to the development, sustainability, and virulence of infectious diseases, climatic changes caused by global warming will alter the disease patterns of infectious pathogens (WHO, 2003). Specifically, increases in atmospheric temperatures, precipitation, humidity, and extreme weather events due to global warming will significantly change the environments in which infectious disease pathogens survive therefore altering their disease patterns (WHO, 2003).

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Infectious diseases can be grouped into four main categories based on their preferred environment and mode of transmission (Merrill & Timmreck, 2006). Waterborne, foodborne, vector-borne, and rodent-borne infectious diseases are all caused by pathogens with varying environmental preferences (Merrill & Timmreck, 2006). The prevalence and distribution of each of the four categories of infectious diseases has currently changed or is predicted to change as a result of global warming and climate change (Nugent, 2004). The next section of this paper will briefly discuss waterborne, foodborne, vector-borne, and rodent-borne infectious diseases respectively in relation to geographic spread, severity, and incidence rates.
Waterborne Diseases
Waterborne diseases are transmitted by pathogens that thrive in wet environments (Nugent, 2004). These pathogens are extremely sensitive to climate changes, namely water and temperature variations (Nugent, 2004). The climatic changes caused by global warming, specifically increases in precipitation and global temperatures will provide an ideal environment for waterborne disease pathogens to thrive (Khasnis & Nettleman, 2005). Therefore, the prevalence of waterborne diseases is predicted to rise due to the increase in the associated disease-causing pathogens and excessive levels of precipitation (Nugent, 2004).
Among the effects of global warming, larger quantities of precipitation will increase the chance of contamination in surface runoff water and groundwater (Nugent, 2004). Because of deforestation and the subsequent urbanization of many regions in North America, the amount of surface runoff water has increased (Landrigan & Garg, 2002). Traveling long distances over urbanized surfaces, such as concrete, asphalt, and tar, increases the likelihood of water contamination from chemicals present at the surface (Landrigan & Garg, 2002). Groundwater contamination is primarily a result of pesticide and herbicide use and the increased seepage into the soil due to excessive precipitation (Landrigan & Garg, 2002). Substantial amounts of contaminated surface runoff water and groundwater may cause water treatment facilities to overflow, allowing a source of tainted water to enter into a region’s drinking and recreational water supplies (Nugent, 2004). Recent Canadian outbreaks of waterborne diseases include “[…] E.coli in Walkerton, Ontario; Cryptosporidium in Collingwood, Ontario; and Toxoplasma in the greater Victoria area, British Columbia” (Natural Resources of Canada, 2007). The causes of these outbreaks can be traced back to surface water and groundwater contamination (Natural Resources of Canada, 2007). To quantify the risk posed by contaminated surface water and groundwater, the American Journal of Public Health published an alarming report, stating that “of the 548 waterborne disease outbreaks reported between 1948 and 1994 [in the United States], 133 were known to be from surface water contamination, 197 were known to be form groundwater contamination, and 218 had an unknown water contamination source” (Curriero, Patz, Rose, & Lele, 2001). As precipitation levels continue to escalate as a result of global warming, the prevalence of waterborne diseases and their associated pathogens will subsequently rise.
Foodborne Diseases
Foodborne diseases can originate from two possible sources: (1) the exposure of food products to contaminated water and (2) the growth of disease-causing pathogens within various food products (Nugent, 2004). Increased amounts of tainted water combined with warmer temperatures provide an ideal breeding ground for algae (Landrigan & Garg, 2002). Oceans and lakes contaminated with algae pose a threat to the surrounding aquatic wildlife and the creatures in the upper hierarchy of the food chain. Among other marine wildlife, fish and mussels are most at-risk for algae contamination (Nugent, 2004). Human consumption of algal contaminated fish and mussels can result in waterborne disease transmission (Nugent, 2004). Such an incident occurred 21 years ago in Prince Edward Island when “[…] 107 people were hospitalized and four died as a result of eating contaminated mussels” (Nugent, 2004). Other foods, such as fresh fruits and vegetables are also prone to contamination from tainted water used for cleaning and irrigation purposes (Nugent, 2004). An example occurred “in 1997, [when…] 150 Michigan students and teachers contracted the foodborne disease Hepatitis A after eating imported strawberries” (Nugent, 2004).
In addition to water contamination, foodborne diseases can originate from the development of disease-causing pathogens within the food itself (Nugent, 2004). Warmer temperatures caused by global warming may entice people to remain outdoors for greater periods, potentially putting themselves at-risk of consuming food that has been left without refrigeration (Nugent, 2004).
Vector-Borne Diseases
Disease-causing pathogens transmitted from insects to humans are referred to as vector-borne diseases (Merrill & Timmreck, 2006). Vector-borne disease patterns will feel the effects of global warming more than any other type of infectious disease since “the most common vectors, arthropods, are cold-blooded, meaning that their internal temperature is greatly affected by the temperature of their environment” (Khasnis & Nettleman, 2005). Climatic consequences of global warming will directly impact the vectors’ breeding and growth rates as well as the length of biting season and exposure to humans (Natural Resources Canada, 2007). Furthermore, increases in global temperatures and precipitation will stimulate the production, growth and transmission of the pathogens that vectors transmit (WHO, 2003). Therefore, vectors and the pathogens they carry are predicted to increase in prevalence in climates that have experienced or are predicted to experience an increase in average seasonal temperatures as a result of global warming (Natural Resources Canada, 2007). To further the effect of warmer temperatures on the production and growth rates of various vectors, floods and heavy rainfall produced by global warming can leave behind standing pools of water which make ideal habitats for breeding and growth (Landrigan & Garg, 2002).
The effects of global warming and climate change on vector-borne disease patterns have already become evident in Canada. Currently, the Canadian environment can sustain a variety of vector-borne diseases including West Nile virus, encephalitis, Lyme disease, and Rocky Mountain spotted fever (Natural Resources Canada, 2007). Between 2002 and 2006, Canada reported and identified approximately 2,300 human cases of West Nile Virus (Public Health Agency of Canada, 2007). Warmer Canadian winters allow mosquitoes that carry West Nile virus and encephalitis to survive through the winter months increasing their reproductive season and subsequent growth and biting rates (Natural Resources Canada, 2007). Lyme disease, carried by ticks is already present in much of the United States with sporadic occurrences in Canada (Natural Resources Canada, 2007). If the warming trends associated with global warming continue, the migration of ticks and their associated diseases to Canada will occur in the near future (Natural Resources Canada, 2007). Vector-borne diseases not present in epidemic proportions in North America, such as malaria, the plague, and yellow fever have the potential for reemergence if current global warming patterns do not subside (Nugent, 2004).
Rodent-Borne Diseases
Rodent-borne diseases are transmitted to humans by rats, chipmunks, and squirrels (Nugent, 2004). The primary effect of global warming on disease-carrying rodents is a disruption or alteration to their food supply (Nugent, 2004). Food may become scare in regions experiencing droughts forcing rodents to relocate to other regions with a greater abundance of food (Nugent, 2004). In areas experiencing increased amounts of precipitation, such as Canada, the potential for survival, reproduction, and growth will increase for rodents inhabiting the region (Nugent, 2004). A shift in geographical location of disease-carrying rodents will disrupt the natural rodent-borne disease patterns associated with an area. In Canada, a shift in the geographical range of deer mice has caused the Hantavirus Pulmonary Syndrome to spread to regions in the Yukon, where its presence has never been experienced (Natural Resources Canada, 2007).
Shift in Disease Prevalence
Over the past century, the western culture has seen a tremendous shift in prevalence from infectious diseases to chronic diseases. At the turn of the nineteenth century, tuberculosis, pneumonia, and influenza attributed to nearly 30 percent of all deaths in the United States (Merrill & Timmreck, 2006). Currently, cancer and heart disease cause almost 74 percent of all deaths in the United States (Merrill & Timmreck, 2006). The effects of global warming on infectious diseases patterns could cause a reversal in disease prevalence, from chronic diseases back to infectious diseases (Longstreth & Wiseman, 1989). Warmer temperatures and wetter climates create ideal breeding grounds for waterborne, foodborne, vector-borne, and rodent-borne disease pathogens (Nugent, 2004). Increasing the quantity of the disease-causing pathogens and the medium in which they survive could cause a reemergence of selected infected diseases, such as malaria, yellow fever, and the plague in regions experiencing dramatic temperature shifts (Longstreth & Wiseman, 1989).
Effects on the Canadian Health Care System
Because global warming involves a complex array of environmental processes and consequences, its future trends and resulting climatic effects are difficult to predict (Khasnis & Nettleman, 2005). The subsequent changes in infectious disease patterns caused by global warming will be just as difficult to predict as global warming itself (Khasnis & Nettleman, 2005). Therefore, the efforts of health officials and environmental specialists to predict and prepare for the effects of global warming on the environment and on human health will be primarily based on predictions. To best predict and prepare for the future outcomes of global warming, a wide array of specialists must be involved (Shope, 1991). Environmental specialists, such as meteorologists, biologists, and ecologists are required to predict the future climatic outcomes of global warming (Shope, 1991). Once predictions have been made by the environmental specialists, a wide array of health officials can attempt to forecast the resulting disease patterns. Epidemiologists will be involved in identifying the distribution and determinants of infectious diseases as well as the control and prevention process (Bartfay, 2008). In conjunction with the epidemiologists, public health officials can communicate the predictions and findings of the epidemiologists, various health researchers, and environmental specialists to the public and other national and internal authorities (Bartfay, 2008). This process has currently been utilized in Canada in response to the emergence of West Nile virus (Landrigan & Garg, 2002).
Subsequent to the predictions of various environmental specialists and health officials regarding the changes in climatic conditions and infectious disease patterns, clinicians and hospital officials can prepare treatments and vaccines to counteract the shift in infectious diseases. Since most of the infectious diseases that have the potential for reemergence in North America have been previous eradicated from the continent, current vaccines are nonexistent (Longstreth & Wiseman, 1989). Due to the lack of vaccines and subsequent immunity, the North American culture is at risk for contracting and transmitting a wide array of infectious diseases such as malaria, cholera, and the plague (Longstreth & Wiseman, 1989). Developing and implementing vaccines is a crucial step in protecting the public from a widespread, fatal epidemic (Longstreth & Wiseman, 1989). In the occurrence of an infectious disease outbreak, hospital personnel and health officials need to have the proper treatments and procedures available to the public to avoid widespread morbidity and mortality.
In short, the North American health care systems can expect to see a shift in disease prevalence. As a result of global warming, infectious diseases have the potential to become more prevalent. To effectively respond, infectious disease treatments and facilities need to be devised and implemented into health care systems across the continent. Furthermore, medical schools need to revise their curriculum to incorporate additional training for infectious disease treatment and prevention.
Conclusion
Global warming and the resulting climatic conditions is an issue that should have been attended to ‘yesterday’. Since consequences directly affect human health and associated ecosystems, global warming has become a worldwide crisis. Increases in temperature, extreme weather events, and precipitation create ideal breeding and growth habitats for many infectious disease-causing pathogens (Nugent, 2004). Among other diseases, infectious diseases transmitted through water, food, vector, and rodent sources are most at risk for being disrupted and altered (Nugent, 2004). Due to current and predicted global warming trends, northern climates could see a reemergence of a variety of infectious diseases including malaria, yellow fever, and West Nile virus (Nugent, 2004). In response to the shift, North American health care systems need to devise and implement associated vaccines, treatments, and facilities to effectively manage the newly reemerged infectious diseases. Because many infectious diseases have been eradicated from North America, the lack of immunity in the western culture could cause a continental or global epidemic with fatal outcomes (Longstreth & Wiseman, 1989). To avoid the spread of infectious diseases, a collaboration of efforts between environmental specialists, health officials, and the public is necessary. Without the combined efforts from the global population, global warming and its subsequent health and climatic effects will eventually destroy mankind. As put in the words of Al Gore, “we are entering a period of consequences” (Gore, 2005).
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