Here’s Where Animal Diseases Are Jumping into Humans

The Black Death killed up to 200 million people during its fatal infestation of Europe. The culprit? A fugitive pathogen called Yersinia pestis that likely hitched a ride on flea-infested rats.

Zoonotic diseases like the Black Death—that is, infectious viruses, bacteria, parasites, and fungi transmitted from animals to people—are as old as human life itself. The earliest known documentation of zoonosis is a cautionary tale about rabies-infected dogs in the Mesopotamian Codex of Eshnunna around 1930 BCE. Today, the Ebola virus, Zika, severe acute respiratory syndrome (SARS), and Lyme disease are just a few of the nearly 1,000 zoonotic diseases that pose a risk to humans.

Yet, despite the fact that most human pathogens originate from animals, disease ecologists know strikingly little about how they spread and the paths they take. And now, according to new maps and research published this week in Trends in Parasitology, it appears the global hotspots for zoonotic outbreaks aren't where we thought they were.

A team of ecologists from the Cary Institute of Ecosystem Studies and the University of Georgia used real-time medical databases to map the ranges, distribution, and intensity of mammal-borne zoonotic diseases around the world.

If a tropical region is rich in potential disease hosts, shouldn't we see more spillover transmission to people? Not necessarily.

"I was rather surprised to see that hotspots of zoonotic diseases didn't match hotspots of biodiversity more closely," co-author Barbara Han, a disease ecologist at the Cary Institute of Ecosystem Studies in New York, said in a statement.

"For example, there is high species diversity in the tropics, so I expected to see a similar pattern of more zoonotic parasites and pathogens in the tropics as well. We do find more zoonotic hosts in the tropics, but we find more zoonotic diseases in temperate regions, possibly because these diseases can occur in multiple host species."

Even though more species live in tropical countries, fewer of those animals were found to carry zoonoses. On the contrary, species abundant in northern latitudes, such as the Arctic Circle, were more likely to be vectors for infectious diseases.

Han and her team also challenged our preconceptions of "contagious" animals. For example, while more than 10 percent of rodent species are zoonotic hosts, a greater proportion of primates—approximately 21 percent of them—harbor human infectious diseases. And bats, with their unfairly nasty reputation, carry far fewer zoonoses (only 25 of them) than all other orders.

Coincidentally, these new maps also illustrate the consequences of human-wildlife conflict and large scale agriculture. Unsurprisingly, diseases from wild animals were more common in areas with greater biodiversity. But by contrast, zoonoses transmitted by domesticated animals were inextricably linked to high human population density, and not at all to levels of species richness. Curiously, disease-carrying rodents were abundant in regions with low mammal overall biodiversity.

However, right now, health organizations are growing more concerned that climate change will cause zoonotic diseases to become more prolific and wide-spread.

The group admits it's possible that results may be skewed by the reporting bias of zoonotic diseases in medical records that favor areas with greater population density, such as Europe and Asia. Though if this was indeed a significant study error, their findings would have likely shown zoonotic hotspots in the United States and Canada as well.

"Understanding where animals are distributed and why may not seem applicable to our day-to-day lives," Han said. "But the big breakthroughs that we need as a society…rely on exactly this kind of basic scientific knowledge."