Fig.1. Estuarine clam Potamocorbula amurensis with oval-shaped pits induced by trematode worms (Credit: John Warren Huntley, 2015)
Last week, former President Jimmy Carter announced that the end was in sight for eradicating the waterborne Guinea worm disease. This parasitic affliction could be the second disease in human history to be eradicated. Unfortunately, just as the Guinea worm is declining, other parasitic diseases are on the rise. One researcher examined fossils from thousands of years ago to understand how parasites might change in the 21st century.
One man and (many) wormsJohn Huntley, an assistant professor at the University of Missouri, found evidence of a greater risk of parasitic infections due to climate change. His study of prehistoric clam shells from the Pearl River Delta in China revealed that the rise of sea levels and temperatures more than 9,300 years ago caused a surge in parasite populations. Huntley examined these mollusks for trematodes, which are internal parasitic flatworms known as flukes. Although the fluke did not survive the millennia, pits in the fossilized shell showed parasite frequency and movement over time.
“Infected clam shells develop oval-shaped fits where the clam grew around the parasite in order to keep it out; the prevalence of these pits and makeup provide clues to how the clams adapted to fight trematodes,” Huntley said.
He found a clear pattern: when sea levels increased, so did trematode populations in mollusks. This connection has unfortunate implications for the next century. Modern day trematodes infect mollusks, which are eaten by shorebirds and then mammals (including humans). At least 56 million people around the world suffer from one or more foodborne trematode infections. Huntley believes that number will increase as global temperatures and sea levels increase, especially in coastal communities.
Evidence in collectionsAlthough other parasitic diseases are projected to increase with temperature, some believe that the link between climate change and disease prevalence is more complicated than previously thought. A study done by researchers at Princeton University argued that parasites will shift north with the temperature increase as opposed to generally expand in range. Such models are useful, but disease prevalence is also affected by factors such as sea level rise, deforestation, habitat loss, exposure to new populations, and more. Scientists are particularly worried about climate-dependent diseases, such as malaria, dengue, West Nile Virus, and Lyme disease. Climate change currently is and will be great concern for global health.
Huntley’s research, however, offers at least part of a solution for mitigating the spread of new diseases -- and he is not alone. His research involves active fieldwork in collecting shell fossils, but other researchers have drawn from natural history collections to map the course of a disease over time. As the environment changes, it is vital that we are ready for the entrance of disease into new and unrealized environments. Studies on ancient specimens or collections allow for us to connect infection events to historical climate events and ultimately prepare for the future.
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Huntley, J.W., Fürsich, F.T., Alberti, M., Hethke, M., & Liu, C. (2014 Dec 23). A complete Holocene record of trematode--bivalve infection and implications for the response of parasitism to climate change. PNAS, 111 (51): 18150-18155. doi: 10.1073/pnas.1416747111.
Irfan, U. & ClimateWire. (2011 Nov 21). Climate Change May Make Insect Borne Diseases Harder to Control. Scientific American. Retrieved from http://www.scientificamerican.com/article/climate-change-may-make-insect-born-diseases-harder-control/.
University of Missouri. (2015) Ancient Fossils Reveal Potential Risk of Rise in Parasitic Infections Due To Climate Change [Press release]. Retrieved from http://munews.missouri.edu/news-releases/2015/0112-ancient-fossils-reveal-potential-risk-of-rise-in-parasitic-infections-due-to-climate-change/.