Some animals have few human friends, like bats. We had a little brown bat tucked inside folded wings, like dead leaves, hanging over a rocker in our porch. It was a welcome guest. But ask the public about the most despised animals and bats come near the top of the list, whether in myth or fact—they give people creeps at Halloween, they hang out with witches, they are revenants haunting graveyards, they drink blood, and the latest accusation is they gave us the Ebola virus. So this week I’m defending bats but mostly writing about zoonoses (What?). I’ll try to explain…
There’s no smoking gun that would convict bats in a fair court of law. It’s true that maps for bat density and infectious diseases are closely matched, and fruit bats can be found roasting on charcoal fires in Guinea where the first cases of Ebola were reported. Circumstantial evidence is usually sufficient for prejudice when a creature is ugly, loathsome, and gets tangled in your hair (sic)! It matters not that “nice” animals like antelopes and chimpanzees harbor the same virus, and likely many other mammals too when the research is done. At one time, rabies was believed to be confined to canids, but we now know that every mammal can transmit the virus. It seems that every species that was crowded onto the Ark is a potential vector for a zoonosis that Noah’s family might have caught. Yes, there’s a microbial zoo in furry animals, the ones we think are cute and the others we loathe. All this said, I admit we can’t let bats off the hook, but we should agree the culprit for Ebola is still unknown.
Bats carry a menagerie of over thirty viruses, probably not a record load and certainly not all harmful to us, but alien to human hosts. Bats tolerate most of them without harm to themselves (they succumb to rabies) because microbes have piggy-backed their hosts for eons, time enough for host and pathogen to adapt through mutation and natural selection for virulence to taper off. This biology is a brilliant challenge for evolution deniers to deny. It’s not in the interest of a piggy-backer to break the back of the one carrying them, because when that one dies so does the other. And that is why new host-pathogen associations are so much more dangerous than old ones.
There are historical precedents where the introduction of a pathogen to an immunologically naïve population was devastating. Take the story of measles and smallpox brought by Europeans to the Americas, or Americans building the Panama Canal in the jungle who brought yellow fever home, or when raccoons deliberately moved across state lines in the 1970s transferred rabies to local wildlife. The history of HIV crossing to humans is still unsettled, although the retrovirus evidently has progenitors in healthy chimpanzees today. I published a speculation that it jumped species in the 1920s during human surgery involving grafted organs from chimpanzees, although that theory is dangling. The likelihood of hosting a foreign microbe is much greater between closely related species like mammals and to a lesser extent birds, than, say, reptiles or amphibians. Certainly, the reservoir of potential pathogens in animals will never run dry, but their abundance is not as alarming as the opportunities for exposure.
Some two decades ago, Laurie Garrett wrote The Coming Plague as a reaction to the complacent belief that infectious diseases had been largely overcome (smallpox, etc.), and the redirection of research endeavor to degenerative diseases. She was running with the tide because while drafting her book the HIV-AIDS epidemic kicked in, TB became resurgent, Legionnaire’s disease grabbed headlines, Lyme was discovered, and Ebola, etc. Tellingly, she chose as her subtitle, Newly Emerging Diseases in a World out of Balance.
There was a time when our distant ancestors were “in balance with nature” in the sense that their relationship with nature hardly changed over eons, except for the strains of climate. When microbes jumped on hunter-gatherers from another species (perhaps their prey food), the infection probably fizzled out without triggering an epidemic, either because their immune system quenched the invaders or the microbes slew them. The new diseases were unlikely to be transmitted between the small bands of thinly dispersed people, and large parts of the world were uninhabited until recent millennia. How different today! Our species has grown enormously in number and now extends its reach into every corner of the world, except the depths of the ocean which are much safer from pathogens. We are climbing a gradient of risk as rural populations move into crowded cities, as tropical rainforest destruction brings new exposures, as climate changes encourage migration of tropical diseases, as air circulating in airplanes efficiently spreads airborne infections, and in so many other ways. A perfect storm.
Well over 300 new infectious diseases have been recorded in my lifetime—SARS, HIV-AIDS, West Nile, MRSA, Ebola, Nipah, Hanta, and you name them—as well as countless others that are not yet attributed to a specific pathogen or go unnoticed in rural obscurity or poverty stricken regions.
Optimists look forward to the conquest of heart disease and cancer, but the tide of infectious diseases will continue to run over the feet of scientific Canutes for ages to come. They will wash up on the shores of human communities so long as we share habitats and have farm animals. To adapt a famous quotation, the price of biosecurity is eternal vigilance. New infections need to be nipped in the bud before they become runaway epidemics. As new diseases steal up on us, it is hard for public health bodies like the WHO and CDC to prepare for new threats, and to strike a tolerable balance between public risk and individual liberty. I have a personal example of how overreaction follows a feeble response to a threat.
In the 1990s, a few dozen unlucky people in the UK and one in France received the awful news that they had contracted “mad cow disease,” which is caused by a prion protein. This new variant of Creutzfeldt-Jakob disease was introduced to the human food chain when up to a million cows were fed rendered products of dead cows. I remember when the then Minister of Agriculture, John Gummer, declared on TV, “There is nothing wrong with British beef!” between nibbles of hamburger with his daughter. What a bummer! The burger didn’t kill him, but public ridicule destroyed his political career. The lesson learned, American authorities were far more cautious, as I discovered after moving to the States. I had lived in the UK in the years when egregious farm practices still existed. In consequence, I am officially deemed to be an unsafe blood donor, which is a pity for someone with the universal donor blood group. I am still waiting for my mad cow diagnosis.
I find cycles of public anxiety about Ebola rather ironic while our attitudes towards the otherwise beneficial and marvelous bat family are consistently grim. Rather, they should be reversed. We ought to worry whenever we hear news of a bat species in trouble, and be constantly alert for ill news of a new zoonosis of whatever origin. New pathogens are far more threatening to national security than terrorism which is currently gripping the public imagination, or at least until a sophisticated bioterrorist has the know-how to manipulate a pathogen for causing a pandemic. Perhaps then the word zoonosis will emerge from its obscurity at the back of the dictionary.