I read a News & Analysis article in the journal Science (30 August 2013, vol. 341, pp. 948-949) entitled Link to MERS virus underscores bats’ puzzling threat. It seems that bats carry more viral diseases than they are supposed to, and more than their share of the dangerous ones like Nipah, Hendra, SARS and its predecessor, Ebola, Marbourg, Australian bat lyssavirus, rabies, KasokeroDuvenhage, and Menangle.
Scientists have put the blame on a variety of causes like hibernation, bat immune systems, and even echolocation! Others think the reasons are simply their high numbers and lifestyle.
One reason often cited is the high number of species of bats – more than 1300 species, or a fifth of all mammal species. More bat species means more parasitic species means more virus species means more diseases – simple biology. Angela Luis published an enumeration of rodent and bat viral diseases, finding that bats harbor 61 and rodents 68, yet there are roughly twice as many species of rodents as bats, so perhaps there’s more to the matter.
Bats are on all continents except Antarctica, are abundant, can fly, some species migrate up to hundreds of miles, maternity colonies contain huge numbers of densely-packed bats and susceptible young, and species intermingle when roosting and hibernating. These factors force them to be exposed frequently to viruses, often at high levels of contamination.
I like another factor that was not mentioned in the Science article, and that is the microhabitat conditions within ceiling pockets, or bell-holes, occupied by bats during warm-month daytimes. Bell-holes are upside-down, bowl-shaped pockets in the ceilings of caves, and can vary widely in size up to a few meters with depths of up to a half-meter or so. A typical bell-hole is about a third-meter wide and half that deep. Bats often occupy bell-holes, and I suspect that bats are responsible for their creation via the same actions responsible for making bell-holes favorable for transmitting viruses.
A bat flying up into a bell-hole to roost grabs the ceiling with its claws and holds on, thus creating an opportunity for mechanical erosion of the rock. The bat breathes out warm, humid air that is slightly acid due to carbon dioxide from respiration, and contains water vapor, aerosolized water droplets, and viruses. This relatively warm mixture of gases and particulates rises up into the bell-hole and its acidity eats away at the rock, enlarging it beyond what claws and hydrology also erode. I don’t know how long viruses can survive outside the bodies of bats within bell-holes, but I suspect that the bell-hole micro-environment might sustain viruses at least a little longer than cold cave air would.
A much more complex process is proposed by Linfa Wang that invokes bat immune systems. Wang’s group sequenced the genomes of a small insectivorous bat and a large fruit-eating bat, and found that both were missing a hunk of DNA that senses microbial DNA and initiates an inflammation response. This might make bats more susceptible to disease. Wang thinks that the high energy demands of flight caused the release of metabolites that damaged DNA so much that bats evolved special DNA repair mechanisms, but bats suffer less damage from them because their immune systems use some of the same molecules that are also involved in DNA repair. In other words, a genetic coincidence results in bats being more vulnerable to infection but less vulnerable to serious disease and death. Wang’s hypothesis is rather complex compared to simpler environmental explanations, and Occam’s razor shaves me, but it will be an interesting story to follow.