TORONTO - What is it about bats and nasty diseases?
With last week's announcement that a type of fruit bat may be a source of the deadly Marburg virus, scientists are again mulling over how it is that these nocturnal winged creatures can host, sometimes with no harm to themselves, some of the deadliest viruses known to humankind.
If researchers can puzzle out how a bat can survive infections that are more lethal to other mammals, the findings could provide clues for the design of drugs or treatments to help the human immune system mimic the effect, they say.
"If one can pattern and model a system or a mechanism whereby different animals have learned to deal with different infections, well potentially that can lead to better tools in the (medical) toolbox," says Dr. Charles Rupprecht, head of the rabies program at the U.S. Centres for Disease Control in Atlanta.
Bats are believed to be a reservoir for a number of virulent zoonotic (meaning transmitted from animals) infections that can afflict humans. The SARS coronavirus. Ebola and now Marburg. Nipah virus. Hendra virus. Rabies.
Last week a team of scientists from the CDC and research institutes in Gabon published a paper revealing they had found the strongest evidence to date that the virus that causes Marburg hemorrhagic fever resides in at least one type of fruit bat, Rousettus aegyptiacus.
The lead author, the CDC's Dr. Jonathan Towner, admits the international team of researchers currently trapping bats near a Ugandan mine where the two most recent cases of Marburg fever were exposed have been mulling the question "Why bats?" as they do their work.
"We've been talking about it. We don't know. The immune system of bats is not terribly well studied," Towner, who works in the CDC's special pathogen's branch, said from Ibanda, in western Uganda.
Rupprecht concurs: "We know relatively little about the immune systems of bats - I mean, beyond being mammals."
"But obviously, they're doing things that some of the rest of us in the mammalian group haven't found ways to do. Meaning, they seem to be able to live reasonably well with some of these agents."
The potential scientific gains may be tempting, but bats aren't an easy mammal to study. There are nearly 1,000 species and these viruses aren't carried by all types, or even all bats in a particular species. In fact, infection may be transient, meaning finding the source of a virus involves finding the right bats at the right time.
That, of course, is made more difficult by bats' ability to fly.
"They're the undiscovered country, if you will," Rupprecht says.
He and others suggest that something about bats' ability to hibernate contributes to their capacity to withstand infection with the array of viruses, bacteria, fungi and protozoa that they are found to carry.
As mammals, they are warm blooded. But bats allow their body temperatures to fall to ambient levels to over-winter. The slowing down of their metabolism may also, for a time, inactivate viruses they are in carrying. That mechanism may not only help ensure their survival, it may help the viruses they host to survive into the next active season.
"There are intrinsic biological and ecological and socio-biological variables of bats that make them an ideal reservoir for a whole suite of things," Rupprecht says, suggesting many scientists believe more research ought to be done on these mammals.
He says, though, that the field has years of work ahead to get to the point where the role bats play in virus survival and transmission is well understood.
Dr. Daniel Bausch, a hemorrhagic fevers expert at Tulane University in New Orleans, says one of the problems is a lack of laboratory tools such as tests and reagents specifically designed for bats.
"It sounds silly when you talk about it but it's actually been a huge impediment," he insists.
"Because some of the testing that you need to do in the laboratory requires you to have antibody against a bat, for example. We use mice and non-human primates and some other animals in research enough that people make all that stuff. But if you try to call somebody up and say: 'Send me some anti-bat,' ... (the response is) 'Well, we just don't make that.' "
