Since human beings have been on the Earth they have treated the planet like they own the place. Appropriating land and water resources, humans have prospered and multiplied while other living residents with equally valid claims have declined. Human self-centeredness has become deadly; we are causing a sixth mass extinction event (1).
The Hole Story
Extinctions and mass die-offs leave holes in ecosystems. Species may pass away quietly with little awareness on our part, but when prominent trees like Elm and Ash species are decimated by the arrival of invasive diseases (2) the physical voids are more obvious. We plant new tree species and other varieties eventually fill in the gaps in our forests. To the casual observer everything seems fine, but nothing is ever really the same because these human-caused disease invasions alter and sometimes erase completely the structure and function of our living world.
The Little Nothing Changes that Changed Everything
Self-anointed masters of the world, humans have only vague notions about how ecology works. Our careless ignorance has been costly; Minamata disease and the global spread of threatening viruses like yellow fever and Zika have been the unforeseen outcomes of the traditional ways of doing business. The harsh reality is we have virtually no capacity to anticipate some disastrous events until they unfold around us.
A minor change in livestock husbandry practices, collecting the waste materials from slaughtered cattle known as offal and feeding it back to dairy and beef herds, did not elicit much concern. That simple change could have ended up killing us all. The new recycling practice precipitated the bovine spongiform encephalopathy (BSE or mad cow disease) epidemic which ultimately invaded humans as new variant Creutzfeldt-Jakob disease (vCJD) (3). Named for the characteristic expansive holes they produce in brain tissue, these invariably fatal diseases are caused by a unique class of transmissible disease agent, the prions (4). Pathologic prions are mis-folded versions of normal cell proteins, something far different from more familiar infectious microorganisms or viruses. As we learned the hard way with diseases like Kuru, BSE and vCJD, prions can be transmitted through dietary practices (4). Turning cattle into cannibals by feeding them meat and bone meal recovered from slaughterhouse offal led to the unwitting amplification of pathologic prions. Only when an epidemic of the signs and symptoms of this slow-progressing neurologic disease appeared in dairy cattle was it realized something was terribly wrong. How many persons will ultimately die from consuming prion contaminated food? The experiment is in progress and it will take a long time to find out.
Prions Out of Control
Another TSE known as chronic wasting disease (CWD), is now a focus of growing concern (5). CWD is known to afflict deer, moose, caribou and elk and some recent experiments suggest it could pose a threat to humans consuming meat from infected animals. CWD has been particularly associated with game farms where animals are held in high densities because these prions are spread readily through direct contact with feces, saliva, blood and urine (6). Human desires are not always compatible with ecosystem logic; the spread of CWD has clearly been facilitated by game farming practices. All indications reveal the CWD epidemic is expanding with some groups estimating that between 7,000-15,000 infected game animals are consumed annually in North America (3).
Once generated, pathologic prions are extraordinarily resistant to inactivation. There are no drug treatments; to stop the BSE epidemic in Great Britain infected animals were culled and their carcasses destroyed. CWD is on the loose in nature and as stricken wild animals die, plants and soils – the environment itself – may become laden with pathologic prions that could remain infectious for years (7). Worse, some work suggests that CWD might infect other species including mice and voles (3). These animals are important components of food webs and while the implications of such CWD susceptibility are unknown, the possibility that the function of entire ecosystems could be disrupted is disquieting.
Perhaps we are seeing the first infectious prion-mediated self-consumption of ecosystems. Analogous to the fatal spongiform pathology seen in the brains of its victims, rampant prion contamination of the environment could leave gaping holes in the structure and function of vital ecosystems. If that is the case, it seems unlikely humans will emerge unscathed from the self-inflicted sixth mass extinction event.
(1) Damian Carrington. Earth’s Sixth Mass Extinction Event Under Way, Scientists Warn. The Guardian, 10 July 2017. https://www.theguardian.com/environment/2017/jul/10/earths-sixth-mass-extinction-event-already-underway-scientists-warn
(2) Damian Carrington. Red List: Ash Trees and Antelopes on the Brink of Extinction. The Guardian, 14 September 2017. https://www.theguardian.com/environment/2017/sep/14/red-list-ash-trees-and-antelopes-on-the-brink-of-extinction?CMP=twt_gu
(3) Mo Costandi. Mad Cows, Cannibalism and the Shaking Death. The Guardian, 26 September 2013. https://www.theguardian.com/science/neurophilosophy/2013/sep/26/mad-cows-cannibalism-kuru
(4) John Collinge. Mammalian Prions and Their Wider Relevance in Neurodegenerative Diseases. Nature 539:217-226. https://www.nature.com/nature/journal/v539/n7628/pdf/nature20415.pdf
(5) Dan Zukowski. Venison, Elk May No Longer Be Safe to Eat – Study: Deadly Chronic Wasting Disease Could Be Moving to Humans. Enviro News, 15 August 2017. https://www.environews.tv/081517-venison-elk-may-no-longer-safe-eat-study-deadly-chronic-wasting-disease-moving-humans/
(6) Centers for Disease Control. Chronic wasting disease. https://www.cdc.gov/prions/cwd/transmission.html
(7) Carl Zimmer. Fire May Be the Only Remedy for a Plague Killing Deer and Elk. The New York Times, 26 June 2017. https://nyti.ms/2tbtMrN