In a forest in Cameroon, a tiny, virulent virus leapt from a single infected chimpanzee carcass into a human's blood, giving him HIV. Near a tree in the Guinean village Meliandou, children at play may have captured and eaten a tiny Angolan free-tailed bat, ingesting the Ebola virus that would launch a deadly epidemic. Somewhere on the Arabian peninsula, a herdsman got too close to the runny nose of a camel and unwittingly infected himself with Middle East Respiratory Syndrome.
Of all the ailments that plague people, the majority began like these three: with a bacteria, virus, parasite or other pathogen that jumped from an innocuous animal host to the human population. Figuring out where the next spillover will take place is a question that keeps public health experts up at night. But there's no global database documenting where these kinds of infections — called zoonoses — might be lying in wait.
So Barbara Han, a disease ecologist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y., decided to map them.
"What we really want to do is shift the strategy from one of being defensive — always running around putting out fires — to one that's preemptive," she said. "One step toward that goal is to figure out where things are, what's carrying the known diseases and what's their distribution."
Drawing on hundreds of studies of emerging zoonotic diseases that come from mammals (other mammals, being our close relatives, are the largest source of zoonotic outbreaks in humans), Han tracked what classes of creature harbor the most known human pathogens, and where those reservoirs are most likely to be found. The results, published Tuesday in the journal Trends in Parasitology, are complicated and at times surprising. She uncovered no overarching trends in the prevalence of zoonoses, no single factor that health officials can hone in on when pinpointing spots with the potential for an outbreak. But she did identify several classes of animals and regional hot spots to keep an eye on.
"It's a hard game to play because there's hundreds and hundreds of combinations of different zoonoses and carriers," Han said. She compared the study to "opening a Pandora's Box."
"We'd hoped to find a unifying theme, and instead there's just 45 more questions that need to be answered."
Going into the review, Han had assumed that zoonotic diseases would be focused in the tropics, which have the greatest number of mammal species. "Biodiversity begets biodiversity," Han said. "That pattern is sort of biogeographical dogma."
Instead, the number of zoonoses was no greater near the equator than it was anywhere else in the world. Indeed, one of the geographic hot spots Han identified was a band across the sparsely populated global subarctic. Although there were relatively few reservoir species in Alaska, northern Canada and northern Russia, the number of zoonoses there was about the same as in the tropics. This meant that each host was likely to harbor a larger and more diverse set of zoonoses, increasing the likelihood that one could make the jump from animals to humans.
Other hot spots included Europe, which has a huge population of rodent and small insectivores, and Southeast Asia, which possesses a worrying combination of mammalian diversity and large human population packed into a relatively small land mass.
The research also gives a sense of what sorts of creatures public health experts should be keeping an eye on, and it's not necessarily the ones you'd expect. Although rodents are the most diverse order of mammals, with more than 2,200 species, they harbored about the same number of zoonotic diseases as the carnivores, which have just 10 percent as many members. The carnivores are also most likely to be reservoirs for disease: Of the 285 member species, almost exactly half carry pathogens that can infect humans.
Han noted that there are many gaps in her data: She was surveying previous research, so she could measure only the number of pathogens for species other researchers had documented. She's aware that some of the data may be skewed by sampling bias. For example, perhaps the apparent prevalence of zoonoses in Europe is due to the fact that governments in wealthy nations have more money to spend on public health research on potential zoonotic reservoirs.
But the maps are a good start, Han said. Combined with research on a range of other factors influencing disease transmission to humans, they can help officials identify places with the potential for an outbreak and devise ways to mitigate that risk. For example, she said, a country might choose not to develop in a forest that is home to a zoonotic hot spot in order to avoid increasing the rate of contact between humans and disease carriers.
"I'm hoping to work together with people who really understand public health to think about the wildlife human interface, and the cultural things that permit or prevent that from happening," Han said. "This is a multifaceted question and very complex ... and the devil's in the details."