Tuesday, August 14, 2012

Climate change is increasing diseases.

They will be widespread and unpredictable.
Dr. Nitish Priyadarshi

An outbreak of the Ebola virus has killed 14 people in western Uganda last month. There is no treatment and no vaccine against Ebola, which is transmitted by close personal contact and, depending on the strain, kills up to 90 per cent of those who contract the virus. In recent years, Uganda has been hit with three Ebola outbreaks, the worst of which was in 2000, when more than half of the 425 people infected died.

Cases of Japanese Encephalitis (JE) has gone up to 50 in the Assam State in Eastern India. The areas mostly affected by Japanese Encephalitis are Kamrup, Sivasagar, Dhubri, Morigaon, Darrang and Nalbari. More than 400 people in northern India have died last year from encephalitis, a rare condition that causes inflammation of the brain. Around 347 people have died in Uttar Pradesh, while 54 children have died in the neighbouring state of Bihar. Cases of malaria is increasing every year in the state of Jharkhand, Assam, Orissa, Maharashtra etc.

With over 2,50,000 people testing positive for malaria last year, Orissa topped the chart for reporting the highest number of malaria cases. This was followed by 95,000 cases reported from Chhattisgarh and over 61,000 registered in Madhya Pradesh.

A 1996 report from the London School of Hygiene and Tropical Medicine calculated that, of ten of the world’s most dangerous vector-borne diseases (malaria, schistomiasis, dengue fever, lymphatic filariasis, sleeping sickness, guinea worm, leishmaniasis, river blindness, chagas’ disease and yellow fever), all but one were likely to increase, or in some way change their range as a result of climate change.

In recent years, vector-borne diseases (VBD) have emerged as a serious public health problem in countries of the South-East Asia Region, including India. Many of these, particularly dengue fever, Japanese Encephalitis (JE) and malaria now occur in epidemic form almost on an annual basis causing considerable morbidity and mortality. Dengue is spreading rapidly to newer areas, with outbreaks occurring more frequently and explosively. Chikungunya has re-emerged in India after a gap of more than three decades affecting many states.

Asia spans tropical and temperate regions. Plasmodium falciparum and P. vivax malaria, dengue fever, dengue haemorrhagic fever, and schistosomiasis are endemic in parts of tropical Asia. In the past 100 years, mean surface temperatures have increased by 0.3–0.8 °C across the continent and are projected to rise by 0.4–4.5 °C by 2070.

An increase in temperature, rainfall and humidity in some months in the Northwest Frontier Province of Pakistan has been associated with an increase in the incidence of  P. falciparum malaria. In north-east Punjab, malaria epidemics increase fivefold in the year following an El Niño event, while in Sri Lanka the risk of malaria epidemics increases fourfold during an El Niño year. In Punjab, epidemics are associated with above-normal precipitation, and in Sri Lanka, with below-normal precipitation.

According to WHO, many countries in Asia experienced unusually high levels of dengue and/or dengue haemorrhagic fever in 1998, the activity being higher than in any other year. Changes in weather patterns, such as El Niño events, may be major contributing factors, since laboratory experiments have demonstrated that the incubation period of dengue 2 virus could be reduced from 12 days at 30 °C to 7 days at 32–35 °C in Aedes aegypti .

Public health officials often use the term tropical diseases to refer collectively to a list of infectious diseases that are found primarily in developing countries. These include malaria, schistosomiasis, dengue, trypanosomiasis, leprosy, cholera, and leishmaniasis, among others. Many of these diseases are spread by insect vectors, and all of them disproportionately affect the world's poor. Malaria is the most severe of these, with the World Health Organization estimating that the disease causes about 250 million episodes of acute illness and perhaps 880,000 deaths annually.

The most widespread and severe climate-sensitive vector-borne disease in South America is malaria. Studies have shown that unusually dry conditions (for example, those caused by weather related to the El Niño–Southern Oscillation phenomenon in the northern part of the continent) are accompanied or followed by increases in the incidence of the disease. This has been documented in Colombia and Venezuela.

In Asia, dengue fever  and malaria  have been associated with positive temperature and rainfall anomalies, while in Australia arboviral disease outbreaks are most frequently associated with flooding. Urban developments in Asia and the surrounding regions may have a substantial impact on trends in the transmission of dengue fever. In some areas, such as Viet Nam, effects of past civil instability and slow economic growth may also be implicated.

Climate change would directly affect disease transmission by shifting the vector's geographic range and increasing reproductive and biting rates and by shortening the pathogen incubation period. Climate-related increases in sea surface temperature and sea level can lead to higher incidence of water-borne infectious and toxin-related illnesses, such as cholera and shellfish poisoning. Human migration and damage to health infrastructures from the projected increase in climate variability could indirectly contribute to disease transmission. Human susceptibility to infections might be further compounded by malnutrition due to climate stress on agriculture and potential alterations in the human immune system caused by increased flux of ultraviolet radiation.

Of the many scientists who have projected, predicted and warned of the likely health effects of climate change, almost all agree on the basics: they will be widespread and unpredictable, they are likely to be severe, and many, many people across the world will die as a result.

New Scientist magazine reported that ‘human disease is emerging as one of the most sensitive, and distressing indicators of climate change. “It is accepted by virtually all climate scientists that the likely increase in and spread of, potentially fatal diseases is likely to be the single most dangerous threat that climate change poses to human health.

Among the ten most dangerous diseases Malaria is the world’s most prevalent mosquito- borne disease. All experts seem to agree that one effect of climate change will be to increase the range of the malarial mosquito. Destruction of forests to create new human settlements can increase local temperatures by 3–4 °C and at the same time create breeding sites for malaria vectors. These phenomena can have serious consequences on malaria transmission in India, African highlands and other parts of the world.

And it is not just vector-borne diseases that are likely to take advantage of the changing climate. Other infectious killers are likely to enjoy a resurgence too, particularly diseases associated with water supply and sanitation. Climate change could have a major impact on water resources and sanitation by reducing water supply. This could in turn reduce the water available for drinking and washing, and lower the efficiency of local sewerage systems, leading to increased concentration of pathogenic organisms in raw water supplies.

More than 100 pathogens can cause illness if you drink or swim in water contaminated by sewage, including norovirus Norwalk and hepatitis A viruses and bacteria such as E. coli and campylobacter.

Several studies have shown that shifts brought about by climate change make ocean and freshwater environments more susceptible to toxic algae blooms and allow harmful microbes and bacteria to proliferate.

Global Warming will also increase rainfall intensity. Rainfalls will be heavier, triggering sewage overflows, contaminating drinking water and endangering beachgoers. Higher lake and ocean temperatures will cause bacteria, parasites and algal blooms to flourish. Warmer weather and heavier rains also will mean more mosquitoes, which can carry the West Nile virus, malaria and dengue fever. Fresh produce and shellfish are more likely to become contaminated.

Heavier rainfalls are one of the most agreed-upon effects of climate change. The frequency of intense rainfalls has increased notably in the Eastern India, China, Philippines, Korea and Japan.

Flooding may follow heavy rainfall. For developing nations there is evidence of outbreaks following floods. Outbreaks of leptospirosis in Rio de Janeiro (Barcellos and Sabroza 2001) and in the Philippines (Easton 1999) have followed floods. Hepatitis E, malaria and diarrhoeal disease have followed floods in Khartoom (Homeida et al. 1988; Novelli et al. 1988 ). Both acute diarrhoea and acute respiratory disease increased in Nicaragua following Hurricane Mitch and the associated flooding (Campanella 1999).

Temperature can affect both the distribution of the vector and the effectiveness of pathogen transmission through the vector. Gubler et al. (2001) list a range of possible mechanisms whereby changes in temperature impact on the risk of transmission of vector-borne disease:

  1. Increase or decrease in survival of vector
  2. Changes in rate of vector population growth
  3. Changes in feeding behaviour
  4. Changes in susceptibility of vector to pathogens
  5. Changes in incubation period of pathogen
  6. Changes in seasonality of pathogen transmission

By 2100 it is estimated that average global temperatures will have risen by 1.0–3.5 °C, increasing the likelihood of many vector-borne diseases in new areas. The greatest effect of climate change on transmission is likely to be observed at the extremes of the range of temperatures at which transmission occurs. For many diseases these lie in the range 14–18 °C at the lower end and about 35–40 °C at the upper end. Malaria and dengue fever are among the most important vector-borne diseases in the tropics and subtropics; Lyme disease is the most common vector-borne disease in the USA and Europe. Encephalitis is also becoming a public health concern. Health risks due to climatic changes will differ between countries that have developed health infrastructures and those that do not.

Human settlement patterns in the different regions will influence disease trends. While 70% of the population in South America is urbanized, the proportion in sub-Saharan Africa is less than 45%. Climatic anomalies associated with the El Niño–Southern Oscillation phenomenon and resulting in drought and floods are expected to increase in frequency and intensity. They have been linked to outbreaks of malaria in Africa, Asia and South America. Climate change has far-reaching consequences and touches on all life-support systems. It is therefore a factor that should be placed high among those that affect human health and survival.

Analyzing the role of climate in the emergence of human infectious diseases will require interdisciplinary cooperation among physicians, climatologists, biologists, and social scientists. Increased disease surveillance, integrated modeling, and use of geographically based data systems will afford more anticipatory measures by the medical community. Understanding the linkages between climatological and ecological change as determinants of disease emergence and redistribution will ultimately help optimize preventive strategies.


Barcellos, C. and Sabroza, P.C. (2001) The place behind the case: leptospirosis risks and associated environmental conditions in a flood-related outbreak in Rio de Janeiro. Cadernos de Saude Publica 17(suppl), 59–67.

Bouma MJ, Dye C, van der Kaay HJ. (1996) Falciparum malaria and climate change in the northwest frontier province of Pakistan. American Journal of Tropical Medicine and Hygiene,  55: 131–137

Bouma MJ et al. (1997) Predicting high-risk years for malaria in Colombia using parameters of El Niño–Southern Oscillation. Tropical Medicine and International Health, 2: 1122–1127.       

Campanella, N. (1999) Infectious diseases and natural disasters: the effects of Hurricane Mitch over Villanueva municipal area, Nicaragua. Public Health Reviews 27, 311–319.

Dengue in the WHO Western Pacific Region.(1998) Weekly epidemiological record, 73(36): 273–277.        

Easton, A. (1999) Leptospirosis in Philippine floods. British Medical Journal 319, 212.

Gubler, D.J., Reiter, P., Ebi, K.L., Yap, W., Nasci, R. and Patz, J.A. (2001) Climate variability and change in the United States: potential impacts on vector- and rodent-borne diseases. Environmental Health Perpectives 109(suppl 2), 223–233.

Homeida, M., Ismail, A.A., El Tom, I., Mahmoud, B. and Ali, H.M. (1988) Resistant malaria and the Sudan floods. Lancet 2, 912.

Novelli, V., El Tohami, T.A., Osundwa, V.M. and Ashong, F. (1988) Floods and resistant malaria. Lancet 2, 1367.

Poveda, G et al.(1999) Climate and ENSO variability associated with vector-borne diseases in Colombia. In: Diaz HF, Markgraf V, eds. El Niño and the Southern Oscillation, multiscale variability and regional impact. Cambridge, Cambridge University Press. 

Watts DM et al. (1987) Effect of temperature on the vector efficiency of Aedes aegypti for dengue 2 virus. American Journal of Tropical Medicine and Hygiene, 1987, 36: 143–152.