Startling Discovery About the Hidden Role of Wildlife in Human Health

Elephants in Safari

Story at-a-glance -

  • Smithsonian scientists have discovered a connection between decreases in large wildlife populations and increases in zoonoses (diseases that can be transmitted from animals to humans).
  • Their study is one of the first to offer clear experimental evidence that declining large animal populations correlate to significant increases in rodents that carry human disease-causing bacteria.
  • The study took place over a three-year period during which researchers evaluated rodents both inside and outside a 24-acre savanna in Kenya that had been free of large wildlife for over a decade.
  • The researchers found that rodent populations doubled inside the 24-acre fenced area. The increase was directly attributable to the lack of competition by large animals for food. The scientists also found that just as rodent numbers doubled, so did the number of rodents infected with Bartonella. Bartonellosis is an infectious zoonotic disease. The same team of scientists plans to conduct future studies to include a broader range of infectious diseases.
  • These study results point to the possibility that wildlife conservation efforts provide benefits not only to endangered species and their ecosystems, but also in less obvious ways to the humans living alongside them.

By Dr. Becker

Researchers from the Smithsonian Institute have discovered a link between declining populations of large wildlife and increases in zoonotic diseases across the globe. Zoonotic diseases are those transmitted from animals to humans (and humans to animals).

According to their study, published in the April issue of the Proceedings of the National Academy of Sciences,1 the researchers learned that in East Africa, a decrease in the number of large wildlife was associated with a substantial increase in rodent populations. And rodents, as we know, can harbor bacteria that causes disease in humans.

Smithsonian Scientists Show That Declining Large Wildlife Populations Result in Increasing Rodent and Rodent Flea Populations

Large wild animals like antelope, elephants, giraffes, and zebras exert a natural and profound influence on their ecosystems through their dietary and other habits. When declining numbers of large animals live in an environment, that environment changes in many ways. The Smithsonian scientists set out to discover if the loss of large wildlife increased the risk of rodent-related zoonotic diseases in humans.

According to Hillary Young, lead study author:

"Understanding the linkages between biodiversity loss and zoonotic disease is important for both public health and nature conservation programs. While this correlation has been the topic of much scientific debate, ours is one of the only studies to offer clear experimental evidence."

In Kenya, the researchers spent three years evaluating rodent populations inside and outside a 24-acre savanna that had been fenced off from large wildlife for several years.

The rodents included several species of mice, rats, and gerbils. Each time rodents were captured they were marked, and blood samples and fleas, if present, were collected. The animals were then released where they had been captured.

The scientists monitored Bartonella infections in the rodents and their fleas. Bartonellosis is an infectious zoonotic disease that in humans can cause memory loss, joint swelling, liver damage, and other symptoms.

The research team discovered that rodent and rodent flea populations doubled inside the 24-acre fenced area that had not been inhabited by large wildlife. The increase in rodents was directly attributable to the lack of competition by large animals for food. The scientists also found that just as the rodent and flea numbers doubled so did the numbers infected with Bartonella.

Increases in Rodent-Borne Zoonotic Diseases May Be Directly Linked to Loss of Large Wildlife Populations

According to a Smithsonian news release,2 based on these study results, declining populations of large wildlife could be directly linked to the increase in rodents and rodent-borne diseases that threaten humans. The news release goes on to suggest that a partial solution to the rise in zoonotic diseases could involve wildlife conservation efforts.

According to Helgen:

"Africa's large wildlife faces many threats—elephants, rhinos and other large mammals continue to decline in the face of growing human populations, expanding agriculture and the impacts of poaching and wildlife trade. While we know that conservation is good for wildlife and for economies reliant on tourism, our study shows a less-intuitive dimension of conservation that could greatly benefit the people living alongside wildlife."

The same research team plans to conduct several more studies to include a broader range of infectious diseases, as well as in "real world" locations where humans have already changed the landscape and eliminated much of the large animal population.

The scientists realize their research has application well beyond Africa, because rodent-borne diseases are a worldwide concern. "Our study shows us that ecosystem health, wildlife health and human health are all related," said Helgen.