Researchers have found that there is a close link between local climate and the occurrence or severity of some diseases and other threats to human health. It is estimated that climate change contributes to 150,000 deaths and 5 million illnesses each year, and the World Health Organization estimates that a quarter of the world's disease burden is due to the contamination of air, water, soil and food.

In the last quarter of the 20th century, the average atmospheric temperature rose by about 1 degree Fahrenheit. By 2000, that increase was responsible for the annual loss of about 160,000 lives and the loss of 5.5 million years of healthy life, according to estimates by the World Health Organization. The toll is expected to double to about 300,000 lives and 11 million years of healthy life by 2020.

The biggest tolls were in Africa, on the Indian subcontinent, and in Southeast Asia. Most of the increased burden of death and disease were from malnutrition, diarrhea, malaria, heat waves, and floods. But those diseases will play a minor role, at best, in many regions that nevertheless will feel the effects of global warming.

Some of climate change’s impacts on health include: Increased frequencies of heat waves; more variable precipitation patterns compromising the supply of freshwater, higher  risks of water-borne diseases; and a rise in coastal flooding due to rising sea levels, etc.

Global Warming and Human Health

But even more subtle, gradual climatic changes can damage human health. During the past two decades, the prevalence of asthma in the United States has quadrupled, in part because of climate-related factors. For Caribbean islanders, respiratory irritants come in dust clouds that emanate from Africa's expanding deserts and are then swept across the Atlantic by trade winds, which have accelerated due to warmer ocean temperatures. Increased levels of plant pollen and soil fungi may also be involved. When ragweed is grown in conditions with twice the ambient level of carbon dioxide, the stalks sprout 10% taller than controls and produce 60% more pollen.

Some of the health effects may lie ahead if the increase in very extreme weather events continues. Abrupt change of temperatures leading to heat waves or cold spells have become widespread, causing indirectly fatal illnesses, such as heat stress or hypothermia, as well as increasing death rates from heart and respiratory diseases. Statistics on mortality and hospital admissions show that death rates increase during extremely hot days, particularly among very old and very young people living in cities.

Examples:

• The 2003 European heat wave — involving temperatures that were 18°F (10°C) above the 30-year average, with no relief at night — killed 21,000 to 35,000 people in five countries. Starting in August 2003, it caused more than 14,800 deaths in France. Belgium, the Czech Republic, Germany, Italy, Portugal, Spain, Switzerland, the Netherlands, and the UK all reported excess mortality during the same period, with total deaths in the range of 35,000. In France, deaths were massively reported for people aged 75 and over (60%).

• Heat waves were also reported in 2003 in Andhra Pradesh, India, and caused the deaths of 3,000 people
• In July 1995, a heat wave killed more than 700 people in the Chicago area alone. Studies based on these types of statistics estimate that in Atlanta, for example, even a warming of about two degrees (F) would increase heat-related deaths from 78 annually now to anywhere from 96 to 247 people per year.

Impacts of natural disasters

Over the period of 1995-2004, a total of 2,500 million people were affected by disasters, with losses of 890,000 dead and costs of US$ 570 billion. Most disasters (75%) are related to weather extremes that climate change is expected to exacerbate.

A massive increase in the frequency of occurrence of natural disasters such as floods, earthquakes, tsunamis, forest fires have been observed in last decades and have a direct impact in human health. Approximately 600, 000 deaths occurred worldwide as a result of weather-related natural disasters in the 1990s; some 95% of these were in poor countries. According to the Oxfam report (November 2007), the average number of natural disasters per year during early 1980s was about 120. Now, the number has increased to nearly 500.
Some examples:

• In October 1999, a cyclone in Orissa, India, caused 10,000 deaths. The total number of people affected was estimated at 10-15 million.
• In December 1999, floods in and around Caracas, Venezuela, killed approximately 30,000 people, many in shanty towns on exposed slopes.
• In 1998, Hurricane Mitch stalled over Central America and released six feet of rain, causing massive mudslides and claiming 11,000 lives
• In 2005, Hurricane Katrina left most of New Orleans under water.
• In 2006, Sichuan Province, China experienced its worst drought in modern times, with nearly 8 million people and over 7 million cattle facing water shortages

Resurgence of Infectious Diseases

The Intergovernmental Panel on Climate Change (IPCC) projections of increased temperature and precipitation suggest the emergence of more disease-friendly conditions in regions that did not previously host diseases or disease carriers.  Climate change accelerates the spread of disease primarily because warmer global temperatures enlarge the geographic range in which disease-carrying animals, insects and microorganisms--as well as the germs and viruses they carry--can survive. In addition to changing weather patterns, climatic conditions affect diseases transmitted via vectors such as mosquitoes (vector-borne disease) or through rodents (rodent-borne disease).

Climate-sensitive diseases are among the largest global killers. Diarrhea, malaria and protein-energy malnutrition alone caused more than 3.3 million deaths globally in 2002, with 29% of these deaths occurring in the Region of Africa.  Deadly diseases often associated with hot weather, like the West Nile virus, Cholera and Lyme disease, are spreading rapidly throughout North America and Europe because increased temperatures in these areas allow disease carriers like mosquitoes, ticks, and mice to thrive.

Extreme events--floods, storms, droughts, and uncontained fires--can be devastating for health. Floods spread bacteria, viruses, and chemical contaminants, foster the growth of fungi, and contribute to the breeding of insects. Prolonged droughts interrupted by heavy rains, favor population explosions of both insects and rodents. Extreme weather events have been accompanied by new appearances of harmful algal blooms in Asia and North America, and--in Latin America and Asia by outbreaks of malaria and various water-borne diseases, such as typhoid, hepatitis A, bacillary dysentery, and cholera.

Vector–borne Diseases:

Vector-borne diseases (VBD) are infections transmitted by the bite of infected arthropod species, such as mosquitoes, ticks, triatomine bugs, sandflies and blackflies.

Mosquitoes, which can carry many diseases, are very sensitive to temperature changes. Warming of their environment — within their viable range — boosts their rates of reproduction and the number of blood meals they take, prolongs their breeding season, and shortens the maturation period for the microbes they disperse. Mosquitoes and the diseases they carry—including malaria, dengue fever, Ross River virus, and West Nile virus—are especially sensitive to temperature changes and land elevation. Rates of insect biting and the maturation of microorganisms within them are temperature-dependent, and both rates increase when the air warms, enhancing the chances for disease transmission.

In highland regions, as permafrost thaws and glaciers retreat, mosquitoes and plant communities are migrating to higher ground. Both insects and insect-borne diseases (including malaria and dengue fever) are today being reported at higher elevations in Africa, Asia, and Latin America. Highland malaria is becoming a problem for rural areas in Papua New Guinea and for the highlands of Central Africa. In 1998, a malaria outbreak in Tuntunani occurred at 2,300m (Mapstone, 2009), although villages at those elevations historically weren’t threatened by malaria. Vectors have even been reported up to 3000m, but so far, endemic malaria does not exist from 1,800-2000m above sea level (Reiter, 2009). Mosquitoes that can carry dengue fever viruses were previously limited to elevations of 3,300 feet but recently appeared at 7,200 feet in the Andes Mountains of Colombia.  Disease-carrying mosquitoes are spreading as climate shifts allow them to survive in formerly inhospitable areas.

Mosquito resistance to insecticides and parasite resistance to many drugs are widespread, and there are no operational vaccines, nor any foreseen in the near future. Ecological changes, along with increased weather variability and a warming trend, appear to be playing increasing roles in the spread of this disease.

Mosquito populations, for example, are naturally controlled by reptiles, birds, spiders, ladybugs, and bats--as well as by pond fish that feed on mosquito larvae. Mosquitoes provide nourishment for these animals, but some carry malaria, yellow fever, dengue fever, and several types of encephalitis.

In the marine environment, fish, shellfish, and sea mammals consume algae that form the base of the marine food web. A reduction in these plankton feeders as a result of overfishing or disease may thus contribute to blooms of harmful algae. Plankton blooms can also harbor cholera and other bacteria, and threaten the health of swimmers, or those who consume affected fish and shellfish.

Dengue Fever

Like malaria and West Nile virus, dengue fever, which comes in four strains, is also spread by mosquitoes. However, unlike malaria, dengue fever is spread by mosquitoes that thrive in urban areas (Nelson, 2009). An infection by one of four strains will create immunity to only that strain, and will unfortunately increase the chances of infection by another strain (Ibid). The most deadly strain causes Dengue Hemorrhagic Fever (DHF), and although it is rarely fatal if diagnosed early, it severely damages the circulatory system and internal organs (Nelson, 2009).

Dengue fever originated in Africa and is transmitted by more than 130 species of mosquitoes in tropical and subtropical regions (Tseng et al., 2008). Cases have been recorded in every season and are widely distributed in many countries in South and Southeast Asia, Central America, and the Western Pacific (Tseng et al., 2008).  The number of months with average temperatures higher than 18C and the degree of urbanization were found to correlate with increasing risk of dengue fever (Wu et al, 2009). Temperature affects insect survival time and habitats as well as maturation and infective periods, and higher temperatures shorten the incubation period and viral development rate (Ibid). Ae. Aegypti, the mosquito responsible for dengue, used to breed in small natural water bodies like tree holes or rock pools (Phillips, 2008). Now, it also breeds in water that has accumulated in trash (bottles, plastics, tires) (Ibid). Furthermore, Ae. Aegypti prefer to live inside buildings rather than outside, and prefer to feed on humans instead of animals (Ibid). Therefore, these mosquitoes are considered to have adapted to the urban environment (Ibid).

Researchers studied the links between microclimate and ENSO-related weather forcing on dengue prevalence in Matamoros, Tamaulipas, Mexico over a decade of dengue observations (Brunkard et al, 2008). They found that dengue increased by 2.6% one week after every 1°C increase in the weekly maximum temperature (Ibid). They also found that dengue increased 1.9% with every one centimeter increase in weekly precipitation (Ibid). Every 1°C increase in sea surface temperature was also followed by a 19.4% increase in dengue (Ibid). These results point to climatic factors being involved dengue incidences; however most evidence still suggests that non-climatic factors play the biggest role in mosquito-borne incidences (Ibid).

For example, in Australia, the reduced rainfall and projected continuation of this climate trend leaves this region dry (Beebe et al, 2009). The government is encouraging the installation of large domestic water tanks to lessen the water supply stress; however, these tanks are ideal for mosquito breeding and scholars fear that the region can be reintroduced to dengue from Queensland where it is endemic (Beebe et al, 2009). Dengue risks won’t be directly due to warmer temperatures or rainfall patterns but due to the public’s responses to climate changes and the resulting impacts.

However, it is important to note that discounting the role of climate in disease emergence would not make sense (Brunkard, 2008). Eyewitness accounts from regions of recent disease emergence can show a relationship between disease and climate (Ibid). Along the US-Mexico border, residents and officials all associate dengue with climate (Ibid). Public health officials note that the onset of the rainy season and high temperatures brings more dengue cases (Ibid). When asked how dengue is transmitted, many residents along the US-Mexico border respond with quotes such as the following: “It’s warm all the time here, and the mosquitoes don’t die off here,” or “I don’t know. The mosquito, I think. All of a sudden, the rain comes and the dengue is here” (Ibid).

Examples of other vector-borne diseases:

• Cholera: its transmission occurs through the process of ingesting contaminated water or food. The last outbreak was reported in Iraq (August 2007)
• Chikungunya: Discovered in India in 1963, the virus caused the death of 237 people in la Reunion island where 1/3 of the island's population were infected
• Urban yellow fever: Found only in part of Africa and South America

Rodent-borne Diseases

Rodent-borne disease are carried by rats, mice, bats, or other rodents. There is evidence that diseases transmitted by rodents sometimes increase during heavy rainfall and flooding because of altered patterns of human–pathogen–rodent contact. Floods are frequently followed by disease clusters: downpours can drive rodents from burrows, deposit mosquito-breeding sites, foster fungus growth in houses, and flush pathogens, nutrients, and chemicals into waterways.

Hantaviruses

Various strains of hantavirus have become worrisome for Europe, Spain, and Portugal, most of Italy, Greece, and western Russia. In 1997, over 9,000 people contracted the virus and 34 cases were fatal (Clement, et al. 2009). Nephropathia epidemica (NE), a disease caused by hantavirus, has increased from a handful of cases annually in the 1980s to an average of 300 cases per year within the past three years (Weinhold, 2009). Before investigations could begin, researchers needed to understand why NE rose abruptly in Belgium, Germany, France, Luxembourg, and Netherlands in 2005 (Dixon, 2009). Several studies associated the outbreaks with bank voles, but the precise link was uncertain (Dixon, 2009). Clement et al. (2009) examined the possible link between temperature and precipitation with observed NE in Belgium.

As mast seed is the staple food of the bank vole, an abundance of mast would mean a large supply of food for the rodent, thereby increasing the survival rate and encouraging earlier breeding throughout the winter (Clement, 2009). Seed production had already been linked to outbreaks of the rodent population. So what was causing high mast production? Mild winters (Ibid). Clement et al. (2009) discovered a pattern where mild winters caused incidences of high mast production, which would equate an increase in bank vole population for that winter and the next spring. Peaks of NE in human populations can be seen in the year following one with high mast production (Ibid).

From 1985 to 2007, there were a total of 2,048 registered NE cases. Of the 1,678 cases within a 12 year period from 1996-2007, 828 or 49.34% occurred in the last three years (Clement, 2009). According to data from before 1990, low incidences of NE were most likely due to low medical awareness, but after 1990, 3 year peaks of NE would show, and since 1999, 2 year peaks.

Other factors that cause a higher rate of NE include an increase in human outdoor activity that results in closer contact with bank voles (Clement, 2009). For those who live or work in the forest or cut and handle firewood, risk of contact is also increased (Ibid).

The trends of incidences of NE demonstrate that infectious diseases are affected as climate change alters conditions for disease-carrying organisms, such as the bank vole.

Tick-borne Encephalitis (TBE)

Cases of TBE increased by 400% in the last 30 years (Rizzoli, 2009). Transmitted by hard ticks of the Ixodes ricinus species, the disease requires a relative humidity of 80% to avoid desiccation (Grey, 2008). In areas of good cover and vegetation, the soil surface still remains moist throughout the dry periods, and promotes the survival of the disease (Ibid). In Sweden, tick abundance correlates with mild winters and extended spring and autumn seasons (Ibid).

The altitude distribution of the disease has changed in past decades (Grey, 2008). In 1957 and 1979-80, ticks were prevalent up to 700 meters above sea level. But in 2001 and 2002, ticks were found as high as 1100 meters above sea level (Ibid). From 1957-1983, researchers found that ticks simply couldn’t complete their life cycles at higher altitudes (Ibid). The distribution previously ranged from France and southwest England to central Asia and central Europe. Its limits were northern Germany, Poland, and Lithuania, and its southern limit was the Mediterranean shore (Ibid). In 1976, cases of TBE were reported in 4 out of 3000 sites (Ibid). In 2003, 26 sites were reported, but they were all previously known for the tick (Ibid). In 2004, 14 sites had reports, but only 2 of the 14 were previously known for ticks (Ibid). These results suggest an expanded tick habitat range in new areas of Germany, Hungary, Switzerland, and the Netherlands (Ibid).

Theories about the increase in TBE point to various factors, such as deer abundance, climate change, and human behavior. Warmer temperatures caused by climate change allow adult ticks to survive cold winters, and the tick life cycle is also accelerated in southern parts of the distribution range (Grey, 2008). Changes in tick abundance have also been found to follow rainfall patterns (Ibid). Because climate change affects regions differently, some regions experience increases of tick abundances while others see decreases. Abundance is most likely not due to a permanent shift in the population, rather to long-term climate change that allows for tick survival in more favorable areas during different parts of the climate cycle.

In northern Europe, forest and wildlife management has changed; climate conditions have changed, and there has also been a 2000% and 5000% increase in Roe and Red Deer populations respectively (Rizzoli, 2009). The study done by Rizzoli et al. (2009) includes several hypotheses of factors in this staggering population boom, such as the conversion of coppices (cut woodlands) to high stand forests and the density of deer to population of ticks. However, the correlation between deer density and forest composition was insignificant; instead, it may be that small mammals are benefiting from the forest composition change (Ibid).

Alternative views include the observation that the rise in TBE was actually due to the fall of communist rule at the end of the 20th century as there was a breakdown in public health measures, economic development and land use, international travel and trade, technology and industry, human demographics and behavior, and microbial adaptation and change (Randolph, 2007).

In the Baltic region specifically, although conditions are similar, there were still differences in the patterns of changes in TBE incidences since the early 1990’s (Randolph, 2007). Agriculture shifted from large scale to small scale, which meant that conditions were more habitable for rodents. There was also less pesticide use.

Like most diseases, tick-borne encephalitis is caused by multiple factors. More studies need to be conducted to determine the specific links between the spread or distribution of TBE and the surrounding habitat and hosts as related to climate change.

Other examples of rodent-borne diseases:

• Weil's syndrome
• Plague
• Lyme disease

Examples of how diverse environmental changes affect the occurrence of various infectious diseases in humans.

Source: Climate change and infectious diseases, World Health Organization

References

Beebe, N.W. et al. (2009).Australia’s Dengue Risk Driven by Human Adaptation to Climate Change. PLoS: Neglected Tropical Diseases. 3, e429.

Brunkard, J.M. et al. (2008).Assessing the roles of temperature, precipitation, and ENSO in dengue re-emergence on the Texas-Mexico border region. salud pública de méxico. 50:3, 227-234

Clement, J., & Vercauteren, J. (2009). Relating increasing hantavirus incidences to the changing climate: the mast connection. International Journal of Health Geographics. 8

Dixon, Bernard (2009). Beware Fat Bank Voles. The Lancet: Cross-talk. 9

Grey, J.S. et al.(2008). Effects of Climate Change on Ticks and Tick-Borne Diseases in Europe. Interdisciplinary Prespectives on Infectious Diseases. 2009, 1-12.

Mapstone, Naomi South America: Climate change takes tropical diseases up the mountain. (2009, April 23). The Financial Times Limited 2009.

Nelson, Brian (2009, May 18). Dengue Fever Outbreak Far Worse Than Swine Flu. Eco Worldly, Retrieved June 04, 2009.

Phillips, M. L. (2008).Dengue Reborn: Widespread Resurgence of a Resilient Vector. Environmental Health Perspectives. 116, 382-388

Randolph, S. E. et al. (2007).Tick-borne encephalitis incidence in Central and Eastern Europe: consequences of political transition. Microbes and Infection. 10, 209-216.

Reiter, Paul (2008).Global Warming and malaria: knowing the horse before hitching the cart. Malaria Journal. 7

Reiter, P. (2008).Climate change and mosquito-borne disease: knowing the horse before hitching the cart. Rev. sci. tech. Off. Int. Epiz.. 27, 383-398.

Rizzoli, A. (2009). Forest Structure and Roe Deer Abundance Predict Tick- Borne Encephalitis Risk in Italy. PLoS ONE. 4:2

Tseng, Wei-Chun (2008).Estimating the economic impacts of climate change on infectious diseases: a case study on dengue fever in Taiwan. Climatic Change. 92, 123-140.

Weinhold, Robert (2009, Februrary 18). Hantavirus on the prowl in Europe. Environmental Science and Technology, Retrieved May 06, 2009.

Wu, P. et al. (2009).Higher temperature and urbanization affect the spatial patterns of dengue fever transmission in subtropical Taiwan. Science of the Total Environment. 407, 2224-2233.

Additional Resources

Climate Change and Human health

Environmental Health Perspectives: Driven to extremes

IPCC Fourth Assessment (2007) Hemon and Jougla, 2004; Martinez-Navarro et al., 2004; Michelozzi et al., 2004; Vandentorren et al., 2004; Conti et al., 2005; Grize et al., 2005; Johnson et al., 2005.