China warming 'could widen scale of schistosomiasis'
[BEIJING] China's warming climate could put a large additional area of the country under the threat of schistosomiasis transmission by 2050, a new study reveals.
The study, carried out by an international team and published in the February issue of the American Journal of Tropical Medicine and Hygiene, looked at the distribution and temperature sensitivity of Schistosoma japonicum, the parasite that causes schistosomiasis in China and South-East Asian countries.
Increasing temperatures are believed to exacerbate climate-sensitive diseases in many parts of the world, including schistosomiasis (see Scientists: Warming 'could increase schistosomiasis').
But efforts to accurately predict the changes in the spatial distribution of schistosomiasis are insufficient and have produced conflicting results.
The researchers sampled schistosoma and snails — the intermediary for the parasite — and cultivated them in different temperatures for varying durations.
Combining this with historical data, the researchers determined that a mean temperature of zero degrees centigrade in winter months is the lowest the snails can survive at. They found that the parasites also thrive in warm conditions, with growth and reproduction occurring at a minimum temperature of 15.4 degrees Celsius.
They then used historical and projected temperatures in different Chinese regions to develop a model to evaluate where schistosoma could be transmitted, according to the lead author Zhou Xiaonong from the Shanghai-based National Institute of Parasitic Diseases, at the Chinese Centre for Disease Control and Prevention.
The researchers conclude that by 2050 schistosoma transmission will expand northward and westward to an additional 783,883 square kilometres of China — 8.1 per cent of the country's surface area.
The authors write that they have not fully considered rainfall or its potential interaction with temperature. But, "the rising floods in China linked to global warming will certainly facilitate the transmission of schistosoma," Zhou told SciDev.Net.
A separate study by researchers at the US-based Brown University, published in PLoS Neglected Tropical Diseases this week (5 March), claims that the impact of symptoms associated with Schistosoma japonicum is 7–46 times greater than current estimates by the WHO.
In an editorial published in the same journal, Charles H. King of the Center for Global Health and Diseases at US-based Case Western Reserve University School of Medicine, says the analysis should provide a focus for planning future surveillance and control of schistosoma epidemics.
American Journal of Tropical Medicine and Hygiene 78, 188 (2008)
PLoS Neglected Tropical Diseases doi 10.1371/journal.pntd.0000158 (2008)
PLoS Neglected Tropical Diseases doi 10.1371/journal.pntd.0000203 (2008)