Around the world, large animal species are dying off rapidly in the face of hunting, deforestation, and other human activity. As the numbers of large mammals decline, the numbers of small mammals, such as rodents, increase—including populations that carry some nasty diseases.
“It has tremendous consequences for us,” says coauthor Rodolfo Dirzo, a professor of environmental science at Stanford University and senior fellow at the Stanford Woods Institute for the Environment. “Our data suggest that maintaining healthy populations of mega fauna helps us stay in good shape in terms of avoiding nasty bacteria.”
To study the potential health impacts of these large animal population declines, researchers fenced off multiple 2.5-acre (4-hectare) plots of savanna land in Kenya to prevent access by elephants, giraffes, zebras, and other large animals. Over the course of two years, the number of rodents in the study areas doubled, most likely because of increased availability of food and cover, among other reasons.
With more rodents came more of the pathogen-infected fleas they carry and increased disease risk for people.
Published in the Proceedings of the National Academy of Sciences, the study specifically examined the prevalence of Bartonella, a group of bacteria found throughout the world that can cause long-term damage to the human heart, brain, lungs, and spleen.
Typhus and the Plague
“This is an underappreciated and insidiously simple route by which human change can drive disease risk,” says lead author Hillary Young, who conducted the research as a biology graduate student at Stanford and is currently an assistant professor at the University of California, Santa Barbara.
More than 60 percent of all human disease originates with pathogens carried by animals. Flea-transmitted pathogens are found everywhere, from suburban enclaves to tropical forests. In East Africa, where rodent-borne disease is common, typhus and even the plague can spread via human contact with infected rodents. Most health clinics in the region are unprepared to detect, let along treat, some of these diseases.
Dirzo, Young and colleagues, including Woods Institute research associate Dan Salkeld and former biology graduate student Douglas McCauley, plan to expand the research to look more closely at the cascade effect of large animal decline on human health.
Among other analyses, they intend to study how different types of land use affect the prevalence of a wide variety of rodent-borne pathogens and how the risk of disease from these pathogens matches up with actual disease prevalence in people.