As large animal populations decline, disease risks increase

Rodents have long been known as capable carriers of disease. In the Middle Ages, flea-carrying rats helped spread the Black Plague, and modern rodents continue to help piggybacking pathogens find human homes. It would follow then that large wildlife -- which have a significant effect on the size of rodent populations -- would play a role in hindering or helping the spread of disease. And that's exactly what ecologist Hillary Young, assistant professor at University of California Santa Barbara, found. When the numbers of large wild animals decreased, or disappeared, an increase in the risk of human disease via expanding rodent populations regularly followed. Young and her research colleagues used the East African savanna ecosystem as their laboratory, comparing the effect of large wildlife presence on rodent populations and Bartonellosis, a group of bacterial pathogens. "We were able to demonstrate that declines in large wildlife can cause an increase in the risk for diseases that are spread between animals and humans," Young explained. "This spike in disease risk results from explosions in the number of rodents that benefit from the removal of the larger animals." The researchers found that when elephants, giraffe and zebra were kept out of large plots of land in Kenya, rodent populations doubled -- which in turn doubles the number of disease-carrying fleas. "This same effect ... can occur almost anywhere there are large wildlife declines," Young said. "This phenomena that we call rodentation -- the proliferation of rodents triggered by large wildlife loss -- has been observed in sites around the world." Downturns in wildlife numbers can cause rodent increases in a variety of ways, including by providing more access to food and better shelter. "The result is that we expect that the loss of large animals may lead to a general increase in human risk of rodent borne disease in a wide range of landscapes," Young said. The research has considerable significance as modern coronaviruses like SARS and MERS continue to crop up, and as biodiversity continues to decline around the world. Young's research was published this week in in the Proceedings of the National Academy of Sciences Early Online Edition.