I went out last week to look for a tree, a rather specific tree, in the forest behind our home. I hoped to see this tree again. I first saw it last summer when the trees were still in leaf. Now in the winter, on a subzero morning, making my way on snowshoes, everything looked quite different. A Maine state forester had pointed out the tree while he was walking our property line with us. It is a healthy beech tree (Fagus grandifolia), a bit more than a foot in diameter with beautiful bark that reminds me of a young elephant’s skin (or at least what I imagine it would look like). We had been talking about how sick our beech trees were and the peculiar nature of the disease killing them. Beech trees where we live all suffer from beech bark disease caused by the beech bark scale. Their bark looks like a terrible case of acne as the trees slowly die, rotting away at their core.
Beech bark scale, Cryptococcus fagisuga, is an odd little insect. It only exists as females: no males have ever been found by persons able to identify them. The females reproduce parthenogenically, that is without needing fertilization. They produce one brood a year, the eggs hatching in early summer. During their first summer, they crawl about finding a nice cozy crack in a tree’s bark to inhabit. They molt, overwinter and in their second season emerge as soft legless non-mobile little creatures that adhere to their chosen spot on the tree. They use a kind of snout sucker to pierce cells deep in the tree and live off nutrients they suck up from cells deeper in the tree. They hide themselves under a covering of secreted whitish wax.
These sucking scales predispose the trees to infection by several species of fungi: mostly Neonectria coccinea, sometimes N. galligena, and in Maine, Xylococculus betulae. The combination of scale and fungal infections sickens the tree and leads to an early death. The disease arrived in Nova Scotia from Europe in a shipment of seedlings in the 1890s and has been slowly spreading since. reaching the U.S. in 1920 and now slowly spreading west; it’s now as far west as Wisconsin and covers about a third of the beech’s North American range. Infected trees are easy to spot. Their bark develops nasty looking eruptive lesions, an arboreal version of cystic acne. At the same time the trees rot in their core.
It’s predicted that 90% of trees will eventually succumb to this disease though for many it will be a chronic slow wasting illness. Management techniques such as insecticides may be successful for preventing disease spread between individual trees, but there is no workable option for treating the large number of beech trees in a forest.
Finding a healthy beech tree was a big thing. A tiny percentage of beech trees do have a natural resistance to the disease. Some lucky fluke of genetics has given them some still unknown physical characteristic that prevents the scale from setting up housekeeping on the tree. These trees will be the survivors and in time they and their genes will come to dominate the beeches that inhabit our forests. Until then our forest is a mess as sick beech trees slowly die, rotting on the inside until they snap unexpectedly in the wind and coming crashing down. We stay inside when the wind blows.
A story similar to what is happening to beech trees now occurred to humans some 700 years ago and caused a shift in our gene pool. It happened during the Black Plague and left us better able to defend against certain infections, that of Y. pestis in particular, but more prone to develop autoimmune disease.
In October 2022 the journal Nature published a fascinating study by Jennifer Klunk and colleagues.[i] These scientists knew that infectious disease is one of the strongest selective pressures to drive human evolution. They investigated how Yersinia pestis, whichcaused bubonic plague, shifted human DNA. The Black Death, as the pandemic is referred to, is the greatest mortality event in recorded history. An estimated 30-50% of the humans in Europe, Africa and Asia died. (There is debate about this as some authorities suggest the Flu Pandemic of 1918 killed more people.)
Klunk and her team compared human DNA during a hundred-year period before, during and after the Black Death struck London in the mid-1300s. They collected, extracted, and analyzed 516 DNA samples from the remains of people buried in London cemeteries and in the East Smithfield plague pits used for mass burials in 1348-1349. Bodies were also exhumed from five locations in Denmark. The scientists identified genetic adaptions that occurred during the plague. Four genes were identified that had shifted in frequency after the plague. These genes are all involved in the production of proteins that defend against invading pathogens and found that versions of those genes, called alleles, either protected against or rendered a person more susceptible to plague. People who had two identical copies of a gene, called ERAP2, were about 40 to 50% more likely to survive the plague than people without this version. These survivors passed on these good versions of the gene. These versions remain more common in humans today.
The plague returned to Europe repeatedly over the centuries and with each wave of infections, mortality rates decreased as greater numbers of people possessed these protective genes.
ERAP2 plays other important roles in immune regulation. You might say it increases immune responsiveness to threat. On the other hand, this same gene is a known risk factor for Crohn’s disease. This variant, rs11571319, is also associated with risk for rheumatoid arthritis and systemic lupus. While these genetic shifts bestowed protection against the plague, the same genetic changes left modern day humans susceptible to a range of chronic autoimmune diseases. You might say the gene increases immune response to any perceived threats as well, whether real or not.
Many of our patients suffer from autoimmune disease these days and they often ask us for an explanation, for us to tell them why. We’ve resorted to excuses about living in too clean an environment (the hygiene theory) or too dirty a world (toxic exposure) and other explanations. Yet Klunk et al. may have found the real explanation. Our immune balance shifted centuries ago so that our species might survive a catastrophic pandemic and we’ve inherited the genes that allowed that survival.
I failed to find that healthy beech tree on that snowy day, but I’ll look again come Spring; and come Fall I’ll hopefully collect beach nuts to plant the following year and start raising a generation of scale-resistant trees. I might be retired from naturopathic practice, but the old impulse to help nature along and leave the world a touch healthier has not abandoned me.
- Klunk J, VigalysTP, Demeure CE, et al. Evolution of immune genes is associated with the Black Death. Nature. 2022;volume 611, pages 312–319.
Published May 18, 2024
About the Author
Jacob Schor, ND, now retired, had a general practice with a focus on naturopathic oncology in Denver, Colorado. He served as Abstract & Commentary Editor for the Natural Medicine Journal for several years (https://www.naturalmedicinejournal.com/) and posts blog articles on natural therapies, nutrition, and cancer (https://drjacobschor.wordpress.com/). He was a board member of CoAND and past president of OncANP, and is someone who is happier outdoors than inside.