Much more than just the flu

The respiratory system

The coronavirus first takes hold in the upper respiratory tract — the nose, mouth, throat — and can provoke the same symptoms seen in other respiratory infections, notably fever and cough.

But the virus can also work its way deep into the lungs’ tiny air sacs. Those air sacs are loaded with ACE2, the protein that the coronavirus uses as a gateway into cells, where it replicates and emerges to infect adjacent cells.

Severe infection can lead to pneumonia as the lung tissue fills with fluid and pus, reducing the air sacs’ ability to transfer oxygen into the blood. This can progress to acute respiratory distress that requires treatment with mechanical ventilation.

COVID-19 pneumonia is distinct from other viral pneumonias. One study found that COVID-19 patients have characteristic “ground glass” opaque spots on their chest CAT scans, and are more likely to have pneumonia in both lungs rather than one. Often, their lab tests point to abnormalities in other organs, particularly the liver.

The lungs can also be damaged by two other uniquely awful features of COVID-19: the immune system overreaction, or cytokine storm. And excessive blood clotting.

The circulatory system

The cells that line blood vessels throughout the body help regulate blood pressure, inflammation, and clotting. These “endothelial” cells also produce ACE2, the surface protein that the coronavirus uses to break in.

This viral invasion sets off a cascade of destruction, as researchers explained in May in Nature Reviews Immunology. Vessels become leaky, the lungs fill with fluid, and the blood thickens and clots. All of this is intensified by inflammation — normally the immune system’s healing response to injury.

Some scientists now view COVID-19 as primarily a vascular disease, not a respiratory illness. That could explain some of the strange complications, particularly blood clots that have caused heart attacks, strokes, and limb tissue death that led to amputations, even in previously healthy young adults. Circulatory problems could also help explain why preexisting hypertension, diabetes, and heart disease raise the risk of severe COVID-19. It might even explain reddish-purplish toe rashes, dubbed “COVID toes.”

The heart

Blood clots generated by the coronavirus can block a heart vessel, triggering a heart attack.

But there is mounting evidence that COVID-19 can damage the heart, sometimes fatally, in a completely different way: by causing inflammation of the heart muscle, or myocardium.

The condition, called myocarditis, is a recognized complication of some other viral infections, and it usually goes away with rest. But it can also cause temporary or permanent heart problems, including abnormal rhythms, progressive heart failure, even sudden cardiac death.

In hospitalized COVID-19 patients, myocarditis is relatively common — affecting 7% to 23% of intensive-care patients, studies suggest — and dramatically increases the risk of death.

But in rare, unpredictable cases, myocarditis develops in elite athletes, even though they appear to have recovered from a coronavirus infection. That’s one reason sports leagues at all levels have debated whether to suspend competition during the pandemic. A recent cautionary case was Michael Ojo, 27, a former Florida State University basketball player who collapsed and died while training in Serbia. He had reportedly tested positive for COVID-19 and recuperated.

The brain and central nervous system

When a coronavirus-related blood clot blocks or bursts a vessel in the brain, the severing of the blood supply — better known as a stroke — begins to kill brain cells in minutes. This may impair speech, movement, and thinking, depending on the location and severity of the stroke.

But the virus can also interfere with brain signaling, in ways that are not yet understood. An early clue to this insidious effect: COVID-19 often causes temporary loss of smell and taste, usually without nasal congestion.

Recent studies and surveys have tied more serious, long-lasting psychiatric and neurological complications to COVID-19, including psychosis, PTSD, depression, dizziness, nerve damage, and a dementia-like syndrome.

However, the link is not clear-cut; many of these complications are also aftereffects of intensive-care and life-supporting devices.

The renal system

Initial reports from China, where the coronavirus first emerged, suggested that kidney damage was rare, and found mostly in people with underlying kidney problems due to diabetes or other chronic illnesses.

But researchers mining more recent data from New York City found that more than a third of patients admitted to the hospital developed acute kidney injury. Among those who needed intensive care, 78% had kidney damage, and 35% needed dialysis.

Why are these vital blood-filtering organs so vulnerable? Johns Hopkins University nephrologist C. John Sperati explains that research points to four possibilities: The virus can directly invade kidney cells. Low levels of oxygen in the blood, an inevitable part of severe COVID-19, can damage the kidneys. Clots generated by the virus can clog the kidneys. And fallout from the immune system overreaction (cytokine storm) can trigger kidney failure.

The liver

What the coronavirus does to this organ is a particular mystery. Some hospitalized COVID-19 patients have abnormally high levels of liver enzymes, indicating at least temporary damage.

However, it is not clear whether this is related directly to the infection, or to other factors, according to the U.S Centers for Disease Control and Prevention.

And while it’s reasonable to assume that COVID-19 patients who already have chronic liver diseases such as cirrhosis or hepatitis B would be at higher risk of serious liver damage, the CDC says “more research is needed” to prove it.

The gastrointestinal system

Doctors have come to realize that nausea, vomiting, and diarrhea — although not the usual symptoms of COVID-19 — may be part of the disease. What’s more, numerous studies have found at least circumstantial evidence that the coronavirus can infect the digestive system, including the pancreas, which regulates blood sugar.

The evidence includes finding viral RNA in human feces, imaging scans showing bowel abnormalities, a correlation between digestive symptoms and a positive stool test, a review of 52 COVID-19 cases that found 17% had pancreatic injury, and a case report of a woman diagnosed with pancreatitis (sudden inflammation of the pancreas) who turned out to have COVID-19.

“Our understanding of clinical manifestations of COVID-19 continues to progress, and this case illustrates that [the coronavirus] can precipitate acute pancreatitis,” wrote the authors, led by Mark M. Aloysius of Geisinger Commonwealth School of Medicine in Scranton.