About two years after its debut on the global stage, COVID-19 continues to find new ways to morph, mutate, and spread among its hosts. There was “Delta,” now there is “Omicron,” and there is already talk of another variant, lying in wait.
The symptoms of COVID-19 are just as confounding. Is it a cold? Is it a flu? Why does the illness ravage certain patients’ organs, while leaving others asymptomatic?
The first wave of answers reminds us that the body’s systems are deeply interwoven. (Of course they are; how else do we walk down the street?)
Scientists are a long way from painting the full picture of COVID-19—but they have nonetheless started to sketch and fill in the lines as to why certain symptoms come to pass.
A diagram of the different types of cells that make scent possible! Source: https://hms.harvard.edu/news/how-covid-19-causes-loss-smell
Take anosmia, for example. People with the virus may unhappily find that their noses have appeared to stop working. Anosmia (the permanent loss of ability to smell) already occurs in 3 percent of the U.S. population. Yet eight in ten patients with COVID-19 report losing their sense of smell—a number so high that the symptom is often considered a better indicator of the virus than a fever or cough.
In the early days of the pandemic, scientists attributed this misfortune to the virus damaging the neurons responsible for carrying scent to the brain. Last July, researchers at Harvard Medical School found that “olfactory sensory neurons do not express the gene that encodes the ACE2 receptor protein, which SARS-CoV-2 uses to enter human cells.”
In other words, if the virus did indeed infect those neurons, it would leave behind some sort of genetic trace. The researchers did, however, find the expressed gene in nerve cells in the upper nasal cavity. Says one of the authors of the research: “Our findings indicate that the novel coronavirus changes the sense of smell in patients not by directly infecting neurons but by affecting the function of supporting cells.”
If we paid attention in science class, then we know all about red and white blood cells (right?!). But the human body houses around 200 types of cells, each with its different function—and certain types of cells are more vulnerable than others to coronavirus infection.
There are sustentacular cells, which wrap around sensory neurons and provide structural and metabolic support. There are basal cells, which help regenerate a specific tissue responsible for odor detection in the nasal cavity. And there are pericytes, or blood vessel cells, which are responsible for blood pressure regulation, blood-brain barrier maintenance and inflammatory responses. All three of these cell types exhibited the gene that indicates coronavirus infection.
The finding that the virus infects “support” cells that have wide-ranging functions throughout the human body explains why COVID-19 can have devastating effects on a variety of organs—lungs, heart, and brain. As another example, National Institutes of Health researchers recently studied the brains of 19 patients who had succumbed from COVID-19.
They found no evidence of the virus in brain tissue. Instead, they found “thinning and leaky blood vessels” in the tissue. The brain tissue in these patients was compromised not by a direct attack of the virus, but instead by the body’s “inflammatory response.”
The above examples offer us a reminder to think of the human body as a complex web: to pull on one thread is to pull on another. The second thread will pull on the third, and so on, and so forth.
The body, as Anatomy in Clay® Learning System founder Jon Zahourek has often said, is a weaving!
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