Since news of the new coronavirus first emerged
in January 2020, doctors around the world
have thought of it as a disease of the lungs.
But a flood of new reports—describing damage
to the heart, blood vessels, guts, kidneys,
brain, and even fingers and toes—suggests
that COVID-19 is a disease of the entire body.
This is how it begins its journey—and ultimately
does its damage.
The virus—SARS-COV-2—typically enters
through the mouth or the nose.
Using its protein spike, it latches onto a cell-surface
receptor called Angiotensin-Converting Enzyme 2,
or ACE2.
Once inside the cell, the virus
makes many copies of itself.
If the immune system can’t fight off the
virus in these early stages, it moves down
the windpipe and into the lungs.
There, the virus seeks out cells rich in ACE2
receptors, in air sacs known as alveoli,
which transfer oxygen to tiny blood vessels that
circulate it to the rest of the body.
As immune cells rush in to halt the invasion,
a mixture of fluids and dead cells are left
behind from the battle.
They clog the delicate air sacs, making breathing even harder.
Damage can be extensive.
In this 3D model, yellow regions indicate scarred lung
tissue in a 59-year-old man who later died of COVID-19.
Doctors can also see this destruction using x-rays—
ravaged lung tissue appears white,
in places where there should be black spaces of air.
From the lungs, the virus continues its journey.
In critical cases, it can enter the blood stream, where it
binds to ACE2 receptors on cells lining the vessels.
This can trigger body-wide inflammation and blood clots—
which can in turn cause heart attacks, pulmonary
embolisms, strokes and even damage to tiny
blood vessels in the fingers and toes.
In the rest of the body, the virus can wreak havoc, too.
In the heart, tissues may swell, the heart
rhythm may alter, and the weakened muscle may
fail to pump enough blood to the rest of the body.
In the liver, damage can result from an immune
system in overdrive, and from the very drugs
patients are taking to stem the disease.
In the kidneys, even more extensive harm
could come from a direct viral attack,
or from a whole-body event like plummeting blood
pressure.
But there are two other odd places where damage
is showing up: the intestines and the brain.
The lower digestive tract is rich in ACE2
receptors, and many patients—up to 50% in
some studies—have diarrhea.
Viral RNA has also been found in many stool samples.
A few labs have reported finding whole, live viruses–
not just snippets of viral genetic code–in feces.
But there’s not yet firm evidence
that the virus could spread this way.
Scientists do have a theory of how the virus
could travel to the brain: from the nose,
and then upward through the olfactory bulb,
which connects to the brain.
This route could explain the puzzling loss of smell
experienced by some patients.
In the meantime, doctors are seeing a multitude of
issues in the brains of some patients, including strokes,
seizures, confusion, and inflammation.
And, especially in the sickest patients, doctors can
see inflammation in the conjunctiva—
the membrane that lines the front of the eye
and inner eyelid.
The most troubling thing for researchers now
is that while they know what damage is taking place,
they don’t always know why.
Better understanding the pathology of COVID-19
is already changing treatment options—
like the usage of anti-inflammatory drugs and blood thinners.
As more details come into focus,
clinicians can tailor treatments that they
hope will save lives.