Climate Change and Infectious Disease
Climate Change and Infectious Disease
Climate change is any change in the weather pattern over a given area that lasts longer than a single season. It may be local or worldwide; it may mean higher or lower average temperatures, higher or lower average rainfall, more or less frequent storms, or other shifts. Climate change can take place on a time scale of a few years, like the El Niño climate oscillation, which recurs every three to eight years, or long-term and non-reversing, like the global climate change now being caused by human fuel-burning and unsustainable agricultural practices. Climate change can interact in complex ways with infectious disease. It may encourage or discourage the growth of mosquitoes or other animals that spread disease, change the seasonal availability of hosts for pathogens (diseasecausing organisms) that can infect human beings, stimulate the evolution of new pathogens, or change temperatures or precipitation rates to make it more difficult to raise food or obtain clean drinking water. Scientists forecast that the global prevalence of some infectious diseases will increase in years to come.
The connection between climate and disease has long been suspected. Over two thousand years ago, Greek physician Hippocrates (c. 460-370 BC) taught that weather was related to epidemics of infectious disease. In trying to understand such epidemics, doctors should, he said, have “due regard to the seasons of the year, and the diseases which they produce, and to the states of the wind peculiar to each country and the qualities of its waters.” In the seventeenth century, English naturalist Robert Plot (1640-1696) wrote that if humans could make weather observations over widely separated parts of the world at one time, they might “in time thereby learn to be forewarned certainly of divers emergencies (such as heats, colds, deaths, plagues, and other epidemical distempers).”
Better understanding of the complicated relationships between climate, weather, and human health has been possible since the development of the germ theory of disease in the nineteenth century and of the science of ecology (the study of the relationships among communities of living things) in the twentieth century. Extreme weather events such as drought, flood, and heat waves have obvious, direct effects on human health; for example, the 2003 heat wave in Europe caused approximately 44,000 deaths. Such events can also cause death indirectly by triggering outbreaks of infectious diseases such as cholera. Long-term climate shifts can be accompanied by increased numbers of extreme weather events, but can also change the infectious disease picture in less obvious ways. Today, scientists are increasingly concerned with these subtle, long-term relationships between global climate change and infectious disease.
Global climate change is the shifting of climate and weather patterns over the whole world. Such changes are definitely happening—recently scientists have measured faster melting of glaciers and ice caps, rising sea levels, warmer winters, and hotter summers. Specific locations still experience occasional cold, but the cold is usually not as intense or does not last as long. The years 1995 to 2006 contained 11 of the 12 warmest years since 1850, when record-keeping began; from 1960 to 2003, sea levels rose at an average rate of .07 in (1.8 mm) per year. Rainfall has increased in some parts of the world and decreased in others.
Global climate change can occur naturally and has done so many times in the history of Earth. However, the phrase “global climate change” is most often used to refer to changes caused by human beings. Humans change climate by releasing gases into the atmosphere from agriculture and burning fossil fuels. These gases, especially carbon dioxide (CO2), methane (CH4), and nitrous oxide (NO), absorb infrared radiation (heat) radiated by the Earth's surface, preventing the Earth from losing heat to space. In effect, the atmosphere acts like a blanket wrapped around the Earth, and increased greenhouse gas concentrations make it a warmer blanket. The atmospheric concentration of carbon dioxide, the most significant greenhouse gas, has increased by about 35% since the beginning of the Industrial Revolution in the mid-1700s. As of 2007, the majority—estimated at 95%—of scientists who study climate agreed not only that global climate change is occurring, but that it is mostly caused by human activity.
Some of climate change's predicted effects include hotter and more frequent heat waves, more frequent and violent weather events such as hurricanes, warmer weather, and increased or decreased precipitation (rain and snow), depending on location. These changes affect the environmental pathways by which organisms contaminate food and drinking water supplies. They also affect human activities and settlement patterns (how people live and where they live). These changes, in turn, can affect the prevalence of diseases borne by water, insects, and rodents. Diseases such as acquired immunodeficiency syndrome (AIDS, also cited as acquired immune deficiency syndrome), which involve organisms that are usually transmitted directly from person to person, are usually less likely to be affected by climate change. Disease organisms that spend a significant part of their life-cycle outside the human body, such as the malaria parasite, are most likely to be affected by climate change.
That global warming might someday be caused by human-released greenhouse gases was first proposed in 1890 by Swedish scientist Svante Arrhenius (1859-1927). The idea was revived by American physicist Stephen Schneider (1945-), among others, in the mid-1970s. By the 1990s, climate scientists were in broad agreement that global warming is real and primarily human-caused. This view has been supported by on-the-ground weather and temperature observations and by satellite measurements of Earth's heat output. Many computer models of Earth's weather have been used to research global climate change; for example, the 2007 report of the International Panel on Climate Change concluded, partly on the basis of 14 different computer climate models, that there is “very high confidence” that human activity is causing Earth to warm.
WORDS TO KNOW
EPIDEMIC: Epidemic, from the Greek meaning “prevalent among the people,” is most commonly used to describe an outbreak of an illness or disease in which the number of individual cases significantly exceeds the usual or expected number of cases in any given population.
PATHOGEN: A disease-causing agent, such as a bacteria, virus, fungus, etc.
PREVALENCE: The actual number of cases of disease (or injury) that exist in a population.
RE-EMERGING DISEASE: Many diseases once thought to be controlled are reappearing to infect humans again. These are known as re-emerging diseases because they have not been common for a long period of time and are starting to appear again among large population groups.
VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.
As the reality of global climate change became clearer in the 1990s, scientists saw that it might have implications for infectious disease. Malaria, which kills between one and three million people per year, was studied intensively. Both the malaria parasite and the mosquitoes that transmit it to humans are affected by temperature; mosquito populations are also affected by rainfall. (Mosquitoes require stagnant water in which to breed.) A computer-based study reported in 1999 that climate change would likely have two primary effects on malaria. First, increasing warmth in temperate zones such as North America, Europe, and central Asia would allow mosquitoes to transmit the disease in previously unaffected areas. Second, decreased rainfall in some areas, such as the Amazon basin in South America, might shorten the infection season in those areas (a positive effect). A similar study published in 2004 confirmed the core findings of the 1999 study. It predicted that by the year 2080, about 80 million additional people would be at risk of malaria because of climate change.
It is difficult to predict accurately the impact of climate change on human health for two basic reasons. First, predicting climate change itself is uncertain, especially over specific parts of the continents: where will more rain fall, where less? How many heat waves, droughts, or floods will there be, and when and where? Such forecasts can only be made using computer models, and these predictions always carry some level of uncertainty when the system being modeled is as complex as the weather of Earth. Predictions of average, global effects (or continent-wide effects) are less uncertain but are also less useful in predicting the effects of climate change on infectious diseases.
Second, infectious disease patterns depend not only on climate but on human population size, population density, poverty, government prevention policies, and medical advances. For example, spending money to provide village water pumps in some African villages would tend to decrease disease from water-borne organisms and might offset some or all the negative effects (that is, those effects relating to water-borne disease) of decreased rainfall. Or, the development of a cheap, effective vaccine for malaria would alter predictions of malaria's future prevalence.
Certain large-scale issues, however, are not in doubt. For example, extreme weather events such as severe hurricanes are predicted by climate models to become more common, and such events can cause outbreaks of infectious disease. In 1998, for example, Hurricane Mitch dropped 6 ft (1.8 m) of rain over much of Central America. Besides the 11,000 people killed directly by flooding, there were 30,000 cases of malaria and 1,000 cases of dengue fever in Honduras in the aftermath of the rains. In 2005, torrential rain in the area of Mumbai (formerly Bombay), India, triggered epidemics of malaria, dengue fever, cholera and other forms of diarrhea, and leptospirosis (a bacterial disease spread by the urine of infected animals, particularly rats).
Some of the infectious-disease effects of climate change are likely to involve drinking water. As of 2007, lack of clean drinking water (water free of significant quantities of microbes, toxins, and parasites) was already one of the worst health problems in the world. At that time, over one billion people had no access to clean drinking and washing water, while some 2.6 billion lacked adequate sanitation. Water-borne infectious diseases kill approximately 3.2 million people per year; about two million of those deaths are children. Diarrhea, which is generally caused by food- and water-borne pathogens such as cholera and Escherichia coli, already kills 2.2 million people per year, the majority under five years old. The World Health Organization (WHO) predicts that the number of cases of diarrhea in third-world countries will have increased by 2–5% by 2020 as a result of climate change.
Some infectious diseases are already apparently increasing in prevalence or range because of climate change, and more quickly than has been predicted. Physician Paul Epstein of Harvard University has said, “things we projected to occur in 2080 are happening in 2006.” In 2005, a group of scientists including Epstein reported that because of warming climate, organisms that act as vectors (that is, as carriers of disease to humans), including mice, ticks, and mosquitoes, were already spreading to larger areas around the world.
Malaria, West Nile Virus, Lyme Disease
Forty percent of the world's population is vulnerable to infection by malaria, and malaria is already a worsening problem due to movements of population into malarial areas, destruction of forests, evolution of resistance to pesticides by mosquitoes and to antimalarial drugs by malaria parasites, and the breakdown of public-health facilities in some poor countries. Global warming is also contributing to the increasing prevalence of malaria and is likely to become a more important factor over the next few decades. Warmer temperatures can cause mosquitoes to mature more rapidly, breed over a longer season, bite more often, and speed up the growth of malaria parasites in the insect's digestive system. In Africa and Latin America, malaria is already spreading to higher elevations in mountainous regions as the climate at those altitudes warms. In 2005, the Harvard group projected that the percentage of the area of Zimbabwe that is climatically suitable for malaria would grow from less than one fourth today to about 90% by 2100. As a consequence, the percentage of the Zimbabwean population at risk for malaria would grow from about 45% to nearly 100%. Major climate-driven spread of malarial areas is also expected in other African countries, including Ethiopia and South Africa, and in highland regions of Latin America and Asia. On the other hand, it has been shown by projects such as the Lubombo Spatial Development Initiative in South Africa, Mozambique, and Swaziland that house-to-house insecticide spraying, systematic surveillance to detect malaria outbreaks, and improved medical care can greatly reduce malaria infection and death rates.
As noted above, AIDS is sometimes cited as typical of those diseases unlikely to be affected by climate change. However, in 2007 researchers reported that infection with malaria tends to increase the amount of HIV (human immunodeficiency) virus in a person with AIDS and to make HIV more easily transmitted to a sexual partner. Not only does malaria help AIDS spread, but AIDS helps malaria spread: AIDS weakens the immune system, making it more likely that a person will catch malaria. As malaria (probably) becomes more widespread because of climate change, the AIDS pandemic may thus be amplified along with it.
West Nile virus claims fewer lives than many other infectious diseases but has received intense publicity in North America due to its sudden re-emergence in 1999 and rapid spread since that time. The virus probably evolved about 1,000 years ago and was first identified in 1937. Outbreaks of West Nile have occurred since 1990 in Eastern Europe, Africa, and North America. Infection with the virus is most often asymptomatic (that is, without signs), but in a minority of cases it causes a debilitating or fatal infection of the central nervous system. In 2003 and 2004, West Nile cases in North America were concentrated in Colorado, Texas, Arizona, and California—regions that had undergone spring droughts. The southwestern and central parts of the United States are predicted by climate forecast models to experience more drought in the years to come because of global climate change, which increases the likelihood that West Nile will be a chronic and growing problem in these and similar states.
Lyme disease is a bacterial disease transmitted to humans by the bites of ticks (a type of blood-sucking insect). Lyme disease is found in North America, Europe, China, and Japan. Although rarely fatal, it can be severely debilitating. Ticks require wild populations of deer and mice in order to thrive and to pass Lyme disease to human beings: colder temperatures limit tick survival away from the mammal host (over 90% of the tick's life cycle), so warmer climates will allow larger tick populations and tend to spread Lyme disease to areas formerly protected by cold winters. In the United States, regrowth of forests in formerly agricultural areas has been the primary culprit so far in the increase of tick populations and the spread of Lyme disease, but scientists predict that climate change will play an increasing role in spreading Lyme disease. In the northeast and central United States and southeastern Canada, a 213% increase in tick habitat area by 2080 is predicted.
There is on going controversy over how to respond to climate change. Since change is already occurring, adaptation— changes in human practices that respond to the effects of changing climate, including new infectious disease challenges—is also already occurring. Many countries have agreed, at least in principle, to mitigate (lessen) climate change by stabilizing the amount of greenhouse gases in the atmosphere. This would require burning less fossil fuel or using new technologies to isolate the carbon dioxide released during such burning (e.g., injecting CO2 from burning coal deep into the ground, where it cannot affect the climate). Other nations, including China, among the largest producers of greenhouse gases, have opposed mandating new industrial practices or energy efficiency in the home or on the road, because many of these changes would be costly. Nevertheless, in 2006, the WHO estimated that each year at least 150,000 deaths are already attributable to climate change.
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