The impact of land use and landscaping on disease spread.

Disease spread has been linked to land use,landscaping and agricultural practices as the aggressive use of land for various purposes is extending the reach of infectious agents,allowing colonization in new areas thus increasing spread of infection. Deforestation is not only contributing to climate change but creating an avenue for movement of more wild animals into contact with man and domestic animals and livestock,thus creating a cycle of infection which is shorter and also of economic importance. A recent research shows buttress this fact , In China the progress fighting schistosomiasisis recorded for over a century is being jeopardized by land-use patterns and landscape connectivity, which are expanding the range of the snails that host Schistosoma japonicum parasites as villages with more irrigation channels were more likely to both attract new and retain native Oncomelania snails. Justin V. Remais, PhD, MS, associate professor of environmental health services in the University of California, Berkeley, School of Public Health, and colleagues wrote in PLoS Neglected Tropical Diseases 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. 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. 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. Analyzing the results with different models 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.