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Land use in China threatens progress against schistosomiasis
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A half-century of progress fighting schistosomiasis in China is being jeopardized by land-use patterns and landscape connectivity, which are expanding the range of the snails that host Schistosoma japonicum parasites, researchers said.
For example, villages with more irrigation channels were more likely to both attract new and retain native Oncomelania snails, according to Justin V. Remais, PhD, MS, associate professor of environmental health services in the University of California, Berkeley, School of Public Health, and colleagues.
“We found that distances traveled by snails over a few generations in this study are comparable to the dispersal of other vectors, such as the malaria vector Anopheles gambiae, and the dengue vector Aedes aegypti,” Remais and colleagues wrote in PLoS Neglected Tropical Diseases.
“Given that both active and passive mosquito movement patterns are considered important to the spread of vector-borne diseases and can occur at scales similar to snail movement patterns, thorough investigation into the transmission impact of the snail dispersal … is warranted.”
Expanding the range of Oncomelania snails
According to Remais and colleagues, control measures have led to a decline in the number of cases of schistosomiasis in China from 11 million in the 1950s to approximately 115,000 today.
However, they noted that the range of Oncomelania snails has expanded and the disease has re-emerged in some areas of the country.
Oncomelania snails release larval forms of S. japonicum parasites into fresh water, where humans can be infected.
Implications for control programs
Remais and colleagues sampled 833 Oncomelania hupensis robertsoni snails from 29 villages in Sichuan Province and used gene sequencing to determine their migration patterns.
Between 14.4% and 32.8% of the snails sampled from the sites were new to their location within one or two generations — snails infected with S. japonicum have an average estimated lifespan of 171 days — and there was evidence of migration between sites up to 44 km apart, Remais and colleagues said. Snail populations in 20 of the 29 sites contained more than 20% migrants.
Remais and colleagues developed nine connectivity models — Euclidean, topography, incline, wetness, land use, distance from watershed, stream use, streams and channels, and stream velocity — to measure connectivity between the study sites. In all nine models, greater distance between sites corresponded to a decrease in the odds of snail migration between them.
They also investigated the propensity of sites to both attract migrant snails and retain native snails by using models based on village elevation, the number of incoming and outgoing streams and irrigation channels, population, cultivated crop and rice areas, the number of reservoirs, and land cover classification.
The findings showed a significant correlation between land use patterns and snail populations. The researchers found that irrigation channels made villages more likely to both attract migrant snails and retain native snails, and villages that used more land for agriculture were more conducive to migrant snails.
“The progress China has made toward elimination of schistosomiasis is threatened by an expansion of territory suitable for intermediate hosts under future climate and land use scenarios. This study has important implications for snail control,” Remais and colleagues wrote. “A combination of the analyses reported here with control programs based on landscape suitability could yield new control strategies that target where snails currently are, as well as where they may migrate, providing barriers to the establishment (or re-establishment) of host populations in new areas.” – by Gerard Gallagher
Disclosure: The researchers report no relevant financial disclosures.
Perspective
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Asian schistosomiasis, which is caused by Schistosoma japonicum and transmitted by Oncomelania snails, has plagued China for centuries and has even had an important impact on China’s modern history. In my book “Forgotten People, Forgotten Diseases: The Neglected Tropical Diseases and their Impact on Global Health and Development,” I pointed out how the acute form of the disease (known as Katayama fever) may have thwarted Mao’s amphibious assault on Taiwan in the early days following China’s 1949 liberation. Schistosomiasis was a major target of Mao’s disease control campaigns during the Great Leap Forward in the late 1950s. Through traditional snail and animal reservoir (water buffalo) control and mass drug administration approaches over the last 65 years, government and university scientists and public health workers have achieved remarkable success in knocking down the prevalence of schistosomiasis more than 2 logs from 100 million to less than 1 million, but refractory pockets of the disease remain. During the last decades of the 20th century, evolutionary malacologists showed how understanding the biology of Oncomelania snails in depth could lead to innovative approaches to further reducing transmission. The recent paper in PLoS Neglected Tropical Diseases by Head and colleagues provides important additional information that could inform an approach to wiping out the last vestiges of this ancient scourge.
Disclosure: Hotez is an editor-in-chief of PLoS Neglected Tropical Diseases. The Sabin Vaccine Institute is working toward developing a schistosomiasis vaccine.
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