Climate Change and Infectious Disease

Nov 28, 2011 | Katharina Schwan | Outbreak News

Durban, South Africa will host the most recent round of UN climate change discussions from November 28 to December 6. The United Nations Framework Convention on Climate Change (UNFCCC) is an international treaty that was created almost two decades ago to discuss possible measures to mitigate and cope with climate change. 195 countries will be represented at the conference this year. Negotiations revolve around the future of the climate change response, as the first commitment of the Kyoto Protocol ends in 2012.  Additionally, the ultimate aim of this years’ meeting is to stop temperatures from rising more than 2°C by decreasing greenhouse gas emissions by 50 percent by 2050.

 

The Kyoto Protocol

The Kyoto Protocol is an international agreement that sets targets for 37 industrialized nations to reduce their greenhouse gas emissions by 5.2 percent below 1990 levels by 2012. By setting these targets, the Kyoto Protocol is an important first step in reducing gas emissions on a global scale and hopefully in mitigating the adverse effects caused by climate change.  The agreement acknowledges that developed countries contribute almost entirely to climate change. While many developing countries have signed the treaty, they do not have to commit to specific targets. The protocol has been ratified by over 187 nations, though notably not by the United States, and went into effect on February 16, 2005.

 

Understanding Climate Change

Since 1950, the average temperature of the world has started to increase gradually. Much of this warming can be attributed to human actions, such as the burning of fossil fuels and forests. Processes that lead to global warming occur within the Earth’s atmosphere. The atmosphere is a layer of gases that surrounds the planet and is essential to maintain life on Earth. One of its main functions is to warm the surface of the Earth through heat retention, also known as the greenhouse effect. When humans burn fossil fuels, excess gases, such as carbon dioxide and methane, are released into the atmosphere. These greenhouse gases strengthen the greenhouse effect, thereby warming the climate even further. Additionally, by razing forests, less carbon dioxide is removed from the atmosphere, which normally would have helped to maintain a stable temperature. Increasing temperatures will alter the hydrologic cycle (or water cycle), which describes the movement of water above and below the Earth’s surface. This will result in changing rainfall patterns worldwide. Flooding, drought, and extreme weather events, such as heat waves and storms, will become more common and severe. 

Rising temperatures and changing rainfall patterns will have significant effects on the global infectious disease burden, specifically vector-borne and waterborne diseases. 

 

Malaria

Malaria parasites and the mosquitos that carry them thrive in tropical climates characterized by heat and humidity. The changing climate favors malaria transmission over an increased range and to higher elevations. This has been seen with the introduction of malaria into the previously uninfected Kenyan Highlands, which has contributed significantly to morbidity and mortality in the region. In the past, these mountainous areas were too cool to facilitate the spread of malaria. However in recent years, health officials noticed a spike in malaria epidemics in the highland regions. Resource constraints and limited access to health facilities have made malaria a serious public health threat in these non-endemic high altitude areas.

In non-endemic areas, outbreaks of malaria are particularly dangerous, as locals have not built-up immunity to the disease. Populations living in countries and regions where malaria is pervasive acquire immunity over long-term exposure. However, this immune response can be lost if the individual moves away from the endemic area.

 

Dengue

As seen with malaria, dengue is largely confined to the tropical and subtropical regions of the world, as the mosquito that carries the virus breeds more easily and bites more frequently in a warm and humid climate. The risk of sudden and severe epidemics of dengue is increased because the virus multiplies and the mosquito develops more rapidly in high temperatures.

Although dengue was eliminated from large areas of the Americas in the 1950s and 1960s, the disease reinvaded these areas in later decades. It is currently the most widespread vector-borne viral disease affecting humans. Presently, there is no curative treatment for dengue and protective measures such as disease nets and insecticide sprays have had only a nominal effect.

The dengue burden is felt most strongly by populations in the Americas. Brazil, Paraguay, and the Dominican Republic all reported recent dengue epidemics.  In Mexico, the number of cases has increased by more than 600 percent since 2001. Severe weather conditions brought on by climate change further facilitate the spread of dengue. In 2005, Hurricane Stan hit southern Mexico and created new water reservoirs for the dengue mosquito to breed and complete its life cycle.

 

Cholera

Cholera is a waterborne bacterial disease caused by ingesting contaminated water and food and spreads easily in places of poor sanitation and crowding. Erratic weather conditions that characterize climate change foster the spread of cholera. Heavy rainstorms and flooding may hinder access to safe drinking water, thus forcing resource limited populations to resort to contaminated water. High temperatures and severe drought have the same effect. In these extreme cases, polluted water may be the only option.

Additionally, a study conducted in Zimbabwe determined that a spike in temperature shortly before the rainy season leads to a 4.9 percent increase in cholera cases within the study population. This is the first time that distinct empirical evidence has shown a direct relationship between climate change and cholera.

Recently, studies confirmed that cholera bacteria also exist in bodies of water completely untouched by human waste. Ocean surface temperatures and currents, as well as weather changes can explain the abundance and distribution of the bacteria in these waters. This evidence is substantiated by a rise in cholera outbreaks in the Bay of Bengal as sea surface water temperatures increase. The risk of cholera increased two to four times after water temperatures increased by five degrees Celsius.

 

Who is affected?

Although climate change has a serious impact on the world’s entire population, the poor in developing countries are disproportionally affected and endure the brunt of the consequences. Although the 50 poorest countries attribute only 1 percent of global gas emissions (while the United States and China emit 64,166 million tons and 45,301 million tons of carbon respectively), 99 percent of casualties occur in developing countries.

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