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The impact of climate changes on neglected tropical diseases is itself neglected

4 Mar 2008

Source: TropIKA

Xiao-Nong Zhou

National Institute of Parasitic Diseases, China CDC, Shanghai 200025, Email: ipdzhouxn@sh163.net

Former Vice President Al Gore, the Nobel Peace Prize winner for 2007, delivered a major speech on global warming at New York University Law School in September 2006, calling for an immediate freeze on carbon dioxide emissions to fight the effects of global warming. In his address he said, ‘This is an opportunity for bipartisanship and transcendence, an opportunity to find our better selves and, in rising to meet this challenge, create a better brighter future.’

The call of Vice President Gore was a response to the energy transition that has taken place on our planet, which has entered a warming phase that began in the early decades of the 18th century. Records, available for the past 150 years, suggest that the Earth has warmed by approximately 0.6°C over the past 100 years (1). Temperatures in the northern hemisphere have undergone many changes over the past 1000 years, for which natural variability and anthropogenic factors have both been responsible.

The main causes of climate change include solar variability, volcanism, and changes in greenhouse gases and tropospheric aerosols. The unusual warming seen in the 20th century has been particularly pronounced during the last three decades. The predominant greenhouse gas is carbon dioxide (CO2); its atmospheric concentration has increased from 290 parts per million (ppm) in 1890 to 373 ppm in 2002. In its 2001 report, the Intergovernmental Panel on Climate Change (1) predicted that the mean global temperature would increase by between 1.4°C and 5.8°C over the period 1990 to 2100. Recent model simulations conclude that a doubling of CO2 concentration will lead to global warming of 2.4–5.4°C within the same time frame (1).

Recent investigations attribute more than 150,000 deaths per year and a global disease burden of approximately 5 million DALYs annually to climate change. There is a growing body of literature documenting the impact of climate change on physical and biological systems, including human health ( 2, 3, 4, 5, 6, 7). An area that has received particular attention is the potential impact of global warming on shifts in the spatio-temporal distribution of disease vectors, and hence the frequency and transmission dynamics of vector-borne diseases ( 8, 9, 10). Vectors, pathogens (parasites), and hosts survive and reproduce within certain optimal climatic conditions. Changes in climate will alter the transmission of vector-borne diseases in different ways, such as changing the survival and reproduction rate of the vector and of the pathogen (parasite), as well as changing vector activities ( 9, 11).

The potential impact of global warming on the transmission of the neglected tropical diseases has received insufficient attention from researchers. Most studies on the impact of global warming on the transmission of tropical diseases have focused on malaria (10), and an early estimate suggested that its epidemic potential may increase by 12–27% as a direct consequence of higher temperatures (12). Very few studies have been carried out regarding the neglected tropical diseases, although some reviewers have discussed the situation as regards vector-borne diseases (13). Most of the vector-borne diseases considered by these reviewers are neglected tropical diseases, including filariasis, leishmaniases, trypanosomiasis, schistosomiasis, and arboviral diseases, (e.g. dengue, yellow fever, Japanese encephalitis).

One recently published paper on the potential impact of climate changes on schistosomiasis in China, based on a biological model, concluded that temperature increases will result in altered disease transmission, which will extend northwards into currently non-endemic areas (14). A detailed time series of instrumental records in combination with geographic information systems and spatial statistics estimated that 41,335 km2 and 21 million people have become at risk of S. japonicum transmission in central China over the past 30 years (15). Since, according to current estimates, 652 million people live in areas at risk of schistosomiasis worldwide ( 16, 17), an additional 21 million Chinese now at risk due to climate change translates to a 3% increase on a global scale. A map has been projected, coupled with predicted temperature increases, demonstrating that a surface area of 783,883 km2 might become at risk of schistosomiasis transmission in 2050. This translates to 8.1% of the total surface area of China ( 14, 18). Such a finding clearly establishes that global warming and climate change is leading to an increase in the prevalence of the neglected tropical diseases (18).

It should be noted that climate change not only results in higher temperatures, but also increases the frequency of extreme climate events, including floods (2). For example, a significant flooding event occurred along the lower reaches of the Yangtze River in 1998, which led to an excess risk of S. japonicum transmission ( 19, 20). Whether or not this particular event was due to climate change does, however, remain to be investigated.

There is an urgent need for researchers to investigate further the potential impact of climate changes on the transmission of neglected tropical diseases. The findings of such research are required so that populations might be able to adapt or, if necessary, migrate to overcome increased risks for transmission of neglected tropical diseases caused by climate changes (21).

References

1. (2001) Climate Change 2001: The Scientific Basis Cambridge, UK:Cambridge University Press .
2. Easterling DR, Meehl GA, Parmesan C, Changnon SA, Karl TR, Mearns LO, (2000) Climate extremes: observations, modeling, and impacts. Science 289: 2068-2074.
3. Beggs PJ, (2004) Impacts of climate change on aeroallergens: past and future. Clin Exp Allergy 34: 1507-1513.
4. Diaz JH, (2004) The public health impact of global climate change. Fam Community Health 27: 218-229.
5. Haines A, Patz JA, (2004) Health effects of climate change. JAMA 291: 99-103.
6. Knowlton K, Rosenthal JE, Hogrefe C, Lynn B, Gaffin S, Goldberg R, Rosenzweig C, Civerolo K, Ku JY, Kinney PL, (2004) Assessing ozone-related health impacts under a changing climate. Environ Health Perspect 112: 1557-1563.
7. Patz JA, Campbell-Lendrum D, Holloway T, Foley JA, (2005) Impact of regional climate change on human health. Nature 438: 310-317.
8. Reiter P, (2001) Climate change and mosquito-borne disease. Environ Health Perspect 109: 141-161.
9. Hunter PR, (2003) Climate change and waterborne and vector-borne disease. J Appl Microbiol 94: 37S-46S.
10. Sutherst RW, (2004) Global change and human vulnerability to vector-borne diseases. Clin Microbiol Rev 17: 136-173.
11. Patz JA, Graczyk TK, Geller N, Vittor AY, (2000) Effects of environmental change on emerging parasitic diseases. Int J Parasitol 30: 1395-1405.
12. Martens WJM, Jetten TH, Focks DA, (1997) Sensitivity of malaria, schistosomiasis and dengue to global warming. Clim Change 35: 145-156.
13. Campbell-Lendrun D, Pruss-Ustun A, Corvalan C, McMichael AJ, Campbell-Lendrum D, Corvalan C, Ebi KL, Githeko AK, Scheraga JS, (2003) How much disease could climate change cause?. Climate change and health: risks and responses Geneva:World Health Organization 133-155.
14. Zhou XN, Yang GJ, Yang K, Wang XH, Hong QB, Sun LP, Malone JB, Kristensen TK, Bergquist NR, Utzinger J, (2008) Potential impact of climate change on schistosomiasis transmission in China. Am J Trop Med Hyg 78: 188-194.
15. Yang GJ, Vounatsou P, Zhou XN, Tanner M, Utzinger J, (2005) A potential impact of climate change and water resource development on the transmission of Schistosoma japonicum in China. Parassitologia 47: 127-134.
16. Chitsulo L, Engels D, Montresor A, Savioli L, (2000) The global status of schistosomiasis and its control. Acta Trop 77: 41-51.
17. Utzinger J, Keiser J, (2004) Schistosomiasis and soil-transmitted helminthiasis: common drugs for treatment and control. Expert Opin Pharmacother 5: 263-285.
18. King CH, (2008) Where snails no longer fear to tread. Am J Trop Med Hyg 78: 185.
19. Zhou XN, Lin DD, Yang HM, Chen HG, Sun LP, Yang GJ, Hong QB, Brown L, Malone JB, (2002a) Use of landsat TM satellite surveillance data to measure the impact of the 1998 flood on snail intermediate host dispersal in the lower Yangtze River Basin. Acta Trop 82: 199-205.
20. Yang GJ, Utzinger J, Sun LP, Hong QB, Vounatsou P, Tanner M, Zhou XN, (2006) Effect of temperature on development of Schistosoma japonicum within Oncomelania hupensis, and hibernation of O. hupensis. Parasitol Res .
21. Zhou XN, Yang GJ, Sun LP, Hong QB, Yang K, Wang RB, Hua ZH, (2002b) Potential impact of global warming on the transmission of schistosomiasis. Chin J Epidemiol 23: 83-86.

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