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Climate Change and Infectious Disease

Climate change is no longer predicated upon possibility, but is now a matter of temporality. Human activities, the most consequential being the combustion of fossil fuels, have been empirically proven to be contributing directly to the warming of the atmosphere. Greenhouse gases are produced by both industrial means and the various modes of transportation employed by humanity. These emissions include, but are not limited to, carbon dioxide, methane, sulfur dioxides and nitrous oxides. They are produced mainly as the by products of partially combusted fuel sources in the pursuits of industry and locomotion. These greenhouse gases are so called for the effect they produce of trapping heat within the earth’s atmosphere similar to the manner in which a greenhouse functions. The ambient global sea and air temperatures have been incrementally increasing since the advent of the industrial revolution and are expected to continue to rise proportionally with the release of anthropogenic sources of pollution. Various and equally emphatic voices are now imploring both government and public alike to begin to modify the means in which they conduct business and travel in an attempt to slow the seemingly exponential increase in carbon-based emissions and the related rise in global temperatures. As all scientifically minded individuals know, once the progression of an exponential ‘natural curve’ passes a certain point, all relative data tend to change very rapidly and possibly non-reversibly. Indeed, evidential data compiled by climatologists suggests that climate change may not be gradual but may involve sudden jumps between drastically different states (2). Attempting to retain a modicum of optimism, we can only hope that the current trends of industrial expansion and subjective opportunism are able to be transformed to more sustainable means of trade and social maintenance. If we plan on leaving a world worth existing in for our children, we had better concert our efforts towards this end sooner rather than later.
The consequences of climate change have many far-reaching implications for the field of public heath. The causative polluting agents responsible for climate change alter the compositions of air, water, and soil. The major contributing greenhouse gases are emitted into the atmosphere and enter the seas and the land through condensation and precipitation. Aside from heat-related illnesses directly attributable to these pollutants, there are many other factors which contribute to the morbidity and mortality of the population. Smog itself and toxins within the drinking water and food chain on which we rely combine to lower both life expectancy and quality. As industry churns on and people seem more obsessed with inefficient, gas-guzzling vehicles to complement their lifestyles, the smog in the air is becoming more intense. The smog is ubiquitous with urban centers and has been causatively linked to higher incidence of both asthma and heart disease (2). Another troubling observation made by researchers is that most of the health impacts of climate change will affect people least responsible for causing them and least able to mitigate the effects, and thus presents a “global ethical challenge” (15). As those in modern, industrialized societies are able to alleviate the effects of pollution through remedial activities and avoidance tactics, those in more impoverished situations are unable to cope and will suffer far greater casualties.
Climate change affects the health of humanity directly through heat waves, drought and smog but also affects it indirectly through the spread of infectious disease (15). One of the major consequences of climatic change is a changing in infectious disease transmission patterns (18). Changing weather patterns have very profound effects on the vectors of many human pathogens which are particularly sensitive to changes in the climate. Increases in temperature and changes in precipitation patterns can dramatically broaden the geographic distribution of many vector-borne diseases. The most important vector of human pathogens is the mosquito, responsible for transmitting many diseases such as malaria, yellow fever, dengue fever and West Nile virus, to name a few. Since 1950, ambient global nighttime temperatures have increased 1.86ºC with daytime temperatures having risen more gradually over the last 100 years by 0.88ºC (3). This slight increase in temperatures allows mosquitoes to travel further north and establish new niches for themselves in previously non-inhabited areas as no natural predators may yet be present or adapted to their presence. This amounts to a major public health hazard, for if the newly introduced arthropods (mosquitoes) are carrying or incubating any parasites or viruses to which the population has not been previously exposed, the people will not have the necessary antibodies to thwart off such infections. Such newly introduced infectious diseases have the potential to quickly turn to epidemic proportions before being recognized by the public health authorities and properly mitigated. In this instance, many physicians and environmental inspectors may not either recognize the symptoms of the infectious agent or be prepared to treat them with appropriate medication. In addition, warming global temperatures and their related effects have been shown to increase disease transmission in already endemic areas. One such effect, the El Niño Southern Oscillation (ENSO) – a periodic reversal of currents in the Pacific Ocean that disrupts the world’s climate has been linked to greater climate variability (15) and localized rainfall which can magnify epidemic risk greatly (18). Now, it is beyond the scope of this paper to discuss all of the infectious diseases that may or have been associated with migrations due to climate change. Discussion along this vein will be limited to the recognized vector(s) and disease agent of dengue fever.
Dengue fever is a disease that was previously confined to the tropics and sub-tropical regions of the world. It is characterized by a diffuse rash, fever, severe prostration and the muscle and joint pains from which it has been commonly referred to as ‘break bone fever’ (10). The causative agent of dengue fever is a blood-borne virus (family Flaviviridae) and it is transmitted from person to person via the bite of an infected mosquito. The specific vector most responsible for transmission of the virus is the Ae. aegypti mosquito who incubates the virus for a short period (1-2 weeks) after feeding on an infected individual (10). The virus replicates within the mosquitoes salivary glands and becomes infective after reaching the necessary titer through the incubatory period (10). There are four distinct serotypes of the virus that have been identified and once a person contracts the disease they develop immunity against the strain responsible for the infection but become more vulnerable to the other strains (6). If that same individual is exposed to any of the other three serotypes, they are at greater risk of developing the more severe form of the disease dengue hemorrhagic fever (DHF) and it’s severe complication due to internal bleeding, dengue shock syndrome (DSS) (6). Children are particularly susceptible to developing the more severe forms of the disease, the case fatality rate (CFR) being as high 40% if untreated (13). Children are at greater risk partly because the antibodies, passively acquired through trans-ovarian transmission from the mother while in utero increase the likelihood and severity of successive infection with another serotype (4). There is no vaccine for dengue fever and even if one were developed it would leave those inoculated with immunity to only one of the four serotypes and more prone to developing DHF/DSS if exposed to any of the other three (1). The only therapy available to those infected is fluid replacement therapy, similar to peripheral cholera treatment (13). Also, immunization with a flavivirus outside the dengue complex can also predispose one to a secondary antibody response (DHF/DSS) if exposed to the dengue virus (8).
Dengue is the most important disease caused by an arbovirus (arthropod-borne virus) of public health significance (5). In the 1950’s there were only nine countries that reported dengue activity and today the geographic distribution includes more than 100 countries world-wide (5). According to the WHO, there are an estimated 50 to 100 million cases reported annually, with 500,000 hospitalizations and 22,000 deaths, most of which are children (14). The pathogen has tended to migrate northward as the temperatures have risen, increasing its’ distribution to higher latitudes. While the hemorrhagic form of dengue has been increasing in the developing world, the problem is most dramatic in the Americas according to the Centers for Disease Control and Prevention (16). An article contained within the Journal of the American Medical Association indicated that more than 760,000 cases were reported in the Americas last year, 20,000 of which involved the more virulent form of the disease (DHF/DSS) (9). Aedes albopictus (the asian tiger mosquito), which is a capable vector of the dengue virus, was first identified in the United States in 1985 and can now be found in 36 states. Ae. aegypti, the primary vector of dengue has long been present in the southern U.S.. Recently, cases of dengue fever have been reported in Texas and, with the rising temperatures that accompany climate change, are expected to move north (1). By 2085, climate change will put an estimated 3.5 billion people at risk of contracting dengue fever, the UN’s International Panel on Climate Change (IPCC) said in March, 2007 (11).
Climate change can affect infectious emerging diseases that require cold-blooded invertebrate species as hosts in a multitude of ways (13). Mosquito maturation is affected by slight increases in temperature. As the lifecycle is shortened, the time taken to develop from egg to adult is hastened and smaller adults emerge requiring more frequent blood meals to develop eggs (1). As localized temperatures rise, there will be a proportional increase in generational turnover, resulting in more mosquitoes and greater probability of infectious disease transmission (17).
Besides affecting the distribution and magnification of the vectors (Ae. aegypti and Ae. albopictus), warming temperatures contribute significantly towards virus replication, maturation and period of infectivity. This, in turn, increases the number of mosquitoes who can become infectious at a given time by shortening the incubation period of the virus (13). This also results in longer periods of transmission as the warm seasonal periods lengthen. The increase in the possibility of transmission will result in a statistically higher seroprevalence in the population, thereby raising the number of individuals at risk of a sequential and more severe infection (DHF/DSS) (13). Of course, the elevations in temperatures that are associated with climate change will allow for new serotypes of the dengue virus to migrate within the vectors themselves. This will increase the possibility of introduction into populations lacking natural immunity and the possibility of initiating DHF/DSS, which can have a CFR as high as 10% even with prompt diagnosis, treatment and care (7).
Faced with such a dilemma of increasing prevalence of disease, our most effective means of prevention are classical public health education measures providing information on reducing the risk of exposure and transmission as well as monitoring for emerging diseases (12). Personal protection and habitat destruction are the keys to eliminating risk of dengue exposure. Wearing light colored clothing, limiting the amount of skin exposed and application of repellant are all proven preventative measures against mosquito bites. Traditional knowledge of mosquito feasting periods leaves one prone to believe that dusk and dawn are the two worst times of day for risk of bites. The primary vector of dengue (Ae. aegypti), however, is an exception to this mode of thought. Being particularly homophilic, this species is highly adapted to the habits of its prey. Ae. aegypti is primarily a day feeder and obtains up to 90% of its blood meals from human beings (13). The species is also highly domesticated, preferring urban environments to traditional breeding grounds like swamps and wetlands. It appears that the species has evolved in tandem with humans perhaps, in part, because human activity and urbanized environs tend to reduce their natural predators disproportionately (1). The mosquito is also highly adapted in its’ means of reproduction, laying eggs in practically any receptacle capable of accumulating water. Thus, key to the habitat destruction of Ae. aegypti is the removal of all trash and debris from surrounding areas. Of course, in most urban environments, trash has become a permanent part of the landscape and any attempt at eradication is a practical futility. Hence, the species ensures its continued survival on the negligence of our own, one attribute which is sure to remain consistent.
Mitigation of climate change itself is more of a complex matter, being compounded by political, social and economic factors. At a personal level, however, there are many ways in which small changes can contribute to the amelioration of greenhouse gas emissions. Most of these include reducing our dependency on fossil fuel sources which, ultimately, is the only strategy available to us in order to reduce the risk of catastrophic climate change (2). We can choose alternatives modes of transportation that reduce our personal consumption of gasoline, such as walking, biking and carpooling. Public transport is another means of reducing our ‘carbon footprint’. If we necessarily require the use of a car, we can attempt to purchase and use the most fuel efficient and low polluting vehicles (2). Hybrid vehicles, in particular, release far less greenhouse emissions than traditional gasoline powered cars. Realistically speaking, though, these are economically unfeasible purchases for most ecologically conscious individuals. Other ways in which one can reduce their burden on the environment include the composting and recycling of plastics and cardboards, which reduce greenhouse gas emissions associated with landfills, and the conscious transformation of ones use of energy in heating and lighting the home (2). Also, awareness of the proportional consequences of air travel and its related pollution burden on the environment are necessary components of a personalized ethical approach to reducing the environmental stress associated with greenhouse gases. On a governmental level, we need to put public pressure on politicians to ensure that temporary profits of industry and petroleum refining do not outweigh the consideration of our children’s futures. We can encourage local governments to invest in energy efficient communities by reducing urban sprawl and promoting more eco-friendly means of public transport (2). Above all, we must consciously and collectively move toward more sustainable means of production and utilization of natural resources before we reach a point of no return and the future becomes uninhabitable.
Climate change is the most significant challenge facing this generation and the most challenging emerging threat to global public heath (18). The UN’s IPCC has suggested that average ambient global temperatures may rise by as much as 3.5ºC by the year 2100 (13). This related rise in temperatures could increase human mortality to unexpected levels through heat related illness, extreme weather events and the exponential spread of infectious diseases to vulnerable populations. Considering the fact that global CO² emissions (the main contributor to climate change) are increasing, despite knowledge of their greenhouse capabilities, this projection by the IPCC may be much too conservative. As the climate changes, surveillance remains a critical component of public health as newly emergent diseases threaten the population, having expanded their geographic distribution and virulence. Of course, proactive policies toward reducing greenhouse gas emissions should be considered along with the more conventional disease prevention strategies (13). On an individual level these carbon reducing activities are both feasible and desirable. Through small changes in our lifestyles we can make a difference in our local sphere. Although, judging from the display of global negligence in concerted efforts to reduce greenhouse gas emissions (partly fueled by ignorance, partly fueled by emerging industrial powers and competitive short-term interests) and projections by climatologists of an unprecedented rate of change in global climate over the next century, the only tenable solution to preventing epidemic spread of infectious disease may be to launch aggressive campaigns regarding personal protection measures and habitat minimization world-wide through the World Health Organization.

References

1. Climate Change Connection (July, 2007). Health Impacts of Climate Change.
Retrieved February 10, 2008 from:
http://www.climatechangeconnection.org/Impacts/Diseases.htm

2. David Suzuki Foundation (2007). Climate Change: Impacts and Solutions.
Retrieved February 21, 2008 from:
http://www.davidsuzuki.org/Climate_Change/

3. Environmental Protection Agency (March, 1999). Emerging Public Health Threats and
the Role of Climate Change. Retrieved February 7, 2008 from:
http://yosemite.epa.gov/oar/globalwarming.nsf/uniqueKeyLookup/SHSU5BUNHY/
$file/health.pdf?OpenElement

4. Goddard, Jerome (2000). Infectious Diseases and Arthropods.
Totowa, N.J.: Humana Press.

5. Gutia-Sapir, Debarati & Schimmer, Barbara. (2005). Dengue fever: new paradigms
for a changing epidemiology. Emerging Themes in Epidemiology, 2(1), 142-47.

6. Inter Press Service (March, 2007). South America: Climate change fuels spread of
dengue fever. Retrieved February 10, 2008 from:
http://ipsnews.net/news.asp?idnews=36994

7. Kaplan, Colin (1997). Infection and environment.
Oxford: Butterworth-Heinemann.

8. Lashley, Felissa R. & Durham, Jerry D. (Eds.). (2007). Emerging infectious diseases:
trends and issues. (2nd ed.) New York: Springer Publishing Company.

9. Los Angeles Times (January, 2008). Dengue fever is not quite dead.
Retrieved February 10, 2008 from:
http://www.latimes.com/news/nationworld/nation/la-na-fever14jan14,0,6710330,full,
story

10.Majumdar, S.K., Kalkstein, L.S., Yarnal, B., Miller, E.W. & Rosenfeld, L.M. (Eds.).
(1992). Global Climate Change: Implications, challenges and mitigation measures.
Easton, PA: The Pennsylvania Academy of Science.

11.National Geographic News (September, 2007). Climate Change Spurring Dengue
Rise, Experts Say. Retrieved February 10, 2008 from:
http://news.nationalgeographic.com/news/2007/09/070921-dengue-warming.html

12.National Resources Canada (October, 2007). Health Effects of Climate Change and
Climate Variability. Retrieved February 21, 2008 from:
http://adaptation.nrcan.gc.ca/perspective/health_3_e.php

13.Patz, Jonathan A., Martens, Willem J.M., Focks, Dana A. & Jetten, Theo H. (1998).
Dengue Fever Epidemic Potential as Projected by General Circulation Models of
Climate Change. Environmental Health Perspectives, 106(3), 147-52.

14.Reuters (January, 2008). Tropical dengue fever may threaten U.S.: report.
Retrieved February 10, 2008 from:
http://www.reuters.com/article/healthNews/idUSNO847856420080108

15.Science and Development Network (November, 2005). Climate change ‘to hit health
in poorest nations hardest’. Retrieved February 10, 2008 from:
http://www.scidev.net/News/index.cfm?fuseaction=readNews&itemid=2482&
language=1

16.The Associated Press (March, 2007). Dengue fever on the rise in Mexico, Latin
America. Retrieved February 10, 2008 from:
http://www.ctv.ca/servlet/ArticleNews/story/CTVNews/20070331/dengue_fever_07
0331/20070331/

17.The Associated Press (November, 2007). Fever outbreak linked to climate change.
Retrieved February 10, 2008 from:
http://www.ctv.ca/servlet/ArticleNews/story/CTVNews/20071128/climate_outbreak
_071128?s_name=no_ads=

18.World Health Organization (July, 2003). Climate change and human health – risks
and responses. Retrieved February 21, 2008 from:
http://www.who.int/globalchange/climate/en/




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