When it comes to carrying viruses that can jump to other species — so-called "zoonotic" viruses — bats may be in a class of their own. The flying mammals are reservoirs for more than 60 viruses that can infect humans, and host more viruses per species than even rodents do, new research shows.
"There seems to be something different about bats in terms of being able to host zoonotic infections," study researcher David Hayman, a wildlife epidemiologist at Colorado State University (CSU), told LiveScience.
In recent years, bats have received a lot of attention for their virus-hosting abilities. They've been shown to carry a number of harmful infections, including rabies and viruses related to SARS (severe acute respiratory syndrome). Moreover, research suggests bats may be the original hosts of nasty viruses such as Ebola and Nipah, which causes deadly brain fevers in people.
This impressive track record left Hayman and his colleagues wondering: Are bats somehow special in their ability to host zoonotic viruses?
Bats vs. rodents
To find out, the researchers compared the virus-hosting abilities of bats with those of rodents —which are also known to transmit various zoonotic viral pathogens — by compiling and analyzing databases of every virus identified in the animals. They found that rodents host 179 viruses, 68 of which are zoonotic; bats, on the other hand, harbor 61 zoonotic viruses, with 137 viruses in total. Though rodents carry slightly more human-infecting viruses, bats host more zoonotic viruses per species — on average, each bat species hosts 1.79 zoonotic viruses, compared with rodents' 1.48 viruses per species. [10 Deadly Diseases That Hopped Across Species]
"It doesn't seem like a lot," said study co-author Angie Luis, a CSU disease ecologist. "But when you consider that there are twice as many rodent species as there are bat species, it's highly significant."
The researchers also looked at different factors that could help explain bats' surprising zoonotic viral richness. One factor stood out: sympatry, in which multiple species of an order of organisms inhabit the same geographical area. Bat and rodent species were more likely to host multiple viruses if their habitat ranges overlapped with the ranges of other bat and rodent species, respectively. And though rodents experience more range overlaps than bats (due to their greater number of species), the effect of sympatry was almost four times stronger for bats.
Compared with rodents, "adding one more bat species to another bat species' range will have a greater effect on the number of viruses it has," Luis told LiveScience. Essentially, bats share their viruses more than rodents do, a fact that's especially evident when one examines the number of host species each virus has. Each bat virus, on average, infects 4.51 bat species, while each rodent virus infects only 2.76 rodent species. This greater sharing of viruses likely results because bats typically live in very close quarters with millions of other bats, Luis said. Physical and genetic similarities between different bat species may also help the viruses spread more easily.
In addition to sympatry, certain life-history traits appear to affect the number of viruses a bat species has. Bat species that live longer, have greater body masses, smaller litter sizes and more litters per year tend to host more zoonotic viruses, the scientists found.
Public health concern?
"I think it's a very interesting study, the first of its kind," said Jamie Childs, azoonotic disease epidemiologist at Yale University, who was not involved with the research. "It confirms in many ways the importance of bats as reservoirs for viruses."
But while the study is a good first step, it "vastly underestimated the number of viruses bats host," Childs told LiveScience. Recent research looking at bat guano suggests there are a number of viral agents we don't have much information on, he said. [Tiny & Nasty: Images of Things That Make Us Sick]
Underestimation or not, the study highlights the public health concern of bat viruses, Childs said. Though humans don't often come into direct contact with bats, they can catch their viruses through contact with infected domesticated animals, including horses, cattle and cats. The majority of human rabies outbreaks have been linked back to bats, and transmission of the Nipah and Hendra viruses have resulted in large outbreaks with lots of fatalities, he said. Even outbreaks of the Ebola and Marburg viruses, which don't often kill a lot of people, catch the attention of the worldwide community.
But is there anything we can do to prevent future outbreaks of bat viruses?
Childs doesn't think so, given how the infections spread and the number of different species the viruses can jump to. "It's very hard to control the emergence of these infections with humans," he said.
Hayman, on the other hand, thinks that studying the interactions between bats, humans and domestic animals could be helpful. "We should to be looking at what we are doing to make bats come into contact with humans and domestic animals, and try to mitigate that," he said.
The study was published online recently in the journal Proceedings of the Royal Society B.
Correction: This article was updated on Sept. 10, 2013, to correct the definition of "zoonotic."