The novel coronavirus seems able to infiltrate just about every inch of the human body, from the brain to the heart and lungs, into the gut, and right down to the toes, causing a dizzying array of symptoms ranging from annoying to fatal. It’s a list that doctors expect to grow even longer.
“It’s been unprecedented in many ways,” says Robert Salata, MD, a professor of medicine in epidemiology and international health at Ohio’s Case Western Reserve University. “In terms of the complications we’re seeing, it’s incredible.”
Covid-19 is among the most dangerous and intractable new viral diseases seen in years, perhaps decades. It “can attack almost anything in the body with devastating consequences,” says Harlan Krumholz, MD, a cardiologist at Yale New Haven Hospital. “Its ferocity is breathtaking and humbling.”
Severe cases often involve two phases, explains Mahalia Desruisseaux, MD, an associate professor of internal medicine focusing on infectious diseases at Yale University School of Medicine. As with the flu or other viral infections, Covid-19 typically subsides after several days, because the immune system mounts a response and neutralizes the virus, Desruisseaux says. But for some people, especially those over 65 and those with underlying health conditions and sometimes even in healthy adults and even children, a second phase kicks in.
“The inflammatory response to the virus gets disproportionately exaggerated, triggering massive inflammation in several organs,” Desruisseaux says. “The flu-like viral illness seems to start to get better, and then boom, the overactive immune response just hits.”
Cells in key organs and entire body systems can become inflamed and damaged irreparably, and the risk of deadly blood clots skyrockets. “It’s our own immune system going haywire,” Salata tells.
Here are most of the known symptoms of Covid-19, followed below by deeper explanations for some of the strangest aspects of the disease.
The strangest cases
The coronavirus infection begins mainly in the throat, and for many people, it may not go beyond that. But in some cases, the virus dives deep into the lungs, or the mucus-encased viruses can slide from the throat into the stomach. From there, it can spread elsewhere through the entire digestive system and into the bloodstream.
The lungs contain a large concentration of cells that have receptors for SARS-CoV-2, meaning the virus can easily gain entry to the cells, hijacking their genetic machinery in order to reproduce. This process damages or destroys lung cells and triggers the massive immune response, often leading to hospitalization.
But those same receptor cells exist in blood vessels, in the brain-blood barrier, in the intestines, and in nerve endings, possibly explaining why this coronavirus is able to wreak havoc in so many areas of the body.
Brain swelling and confusion
Fever and headaches can be signs of many illnesses, and they are symptoms of Covid-19, too. Some people have gone to health care facilities in states of confusion or disorientation, sometimes with headaches and fever, sometimes not, only to be diagnosed with Covid-19.
In one case study, brain scans revealed swelling on the brain of a Covid-19 patient, a condition known in other viral infections to cause altered mental status and seizures. One study of 214 Covid-19 patients found 36.4% had neurological symptoms, including dizziness, headaches, impaired consciousness, and seizures.
It’s not yet known if the coronavirus actually infects the brain or if the brain-related symptoms happen because the disease’s impact on the lungs also rob the brain of oxygen.“It is very difficult to separate the two,” says Chethan Rao, MD, a practicing physician and associate professor of neurology and neurosurgery at Baylor College of Medicine Medical Center.
When people infected with Covid-19 develop the classic symptom of breathing difficulty, it’s because the coronavirus has infected the lungs and the immune system is at work.
Most patients recover. But if the infection persists, the immune system goes into overdrive, releasing a flood of inflammatory proteins, called cytokines, that contribute to cell death and lead to acute respiratory distress syndrome.
When a cytokine storm strikes, lung tissue can become irreversibly scarred, the lungs fill with fluid, and patients often die. The flood of cytokines also fuels an overload of white blood cells, one of the body’s primary protectors which, in the confusion, begins to attack healthy cells, according to a May 13 study in the journal Frontiers in Public Health. This overaction by the immune system causes hyperinflammation in tissues that can also affect other parts of the body by causing leaky blood vessels, extremely low blood pressure, and a lack of oxygen in the blood.
Silent hypoxia and Covid pneumonia
Some people with Covid-19 develop dangerously low levels of oxygen in their blood but otherwise don’t feel terribly sick. They might complain of mild flu-like symptoms and perhaps a little shortness of breath or maybe just tiredness.
The condition has been called silent hypoxia and resembles the low oxygen levels a person suffers when breathing the thin air at high altitudes. These people are in critical condition, though they don’t realize it. (X-rays reveal their air sacs in the lungs are filled with fluid or pus.) The low oxygen levels, untreated, can quickly cascade into serious breathing problems and damage to other organs.
Scientists don’t know exactly what’s going on, but some doctors are calling the condition Covid pneumonia. Autopsies of a small number of people who died from Covid-19 find, because of all the water, lungs weighing more than four pounds, and in one case seven and a half pounds, compared to the normal average of under two pounds.
Only recently have doctors realized that Covid-19 seems to be infecting blood and damaging blood vessels. In one study, 31% of people in intensive care with Covid pneumonia had blood clots. These clots, which the body creates in an effort to stop bleeding, can break up and travel to the lungs or cut off blood flow to the brain, causing a stroke.
“Blood thinners don’t reliably prevent clotting in people with Covid-19, and young people are dying of strokes caused by the blockages in the brain,” science writer Cassandra Willyard notes in a review of the early research on the topic for the journal Nature.
The presence of blood clots can be indicated by levels of protein fragments from the clot, called D-dimer. Doctors are seeing unusually high D-dimer levels in many Covid-19 patients.
“We’ve never seen such high levels before,” says Salata, the Case Western physician. One woman recently had levels that were “just not heard of before,” he said. He and his colleagues are finding major clots as well as unusual numbers of smaller clots that can damage the heart, lungs, kidneys, and other organs.
Inflammatory syndrome in children
In late April and early May, a mysterious toxic shock syndrome began showing up in children in various countries, causing at least three deaths in New York. Some cases have been firmly tied to Covid-19 diagnoses; others have not.
The symptoms of pediatric multisystem inflammatory syndrome, as it’s being called, resemble an unrelated syndrome called Kawasaki disease, a leading cause of heart disease in children. Both are marked by high fever, rash, cracked lips, and bloodshot eyes.
It’s not clear yet if Covid-19 and Kawasaki disease are perhaps occurring simultaneously, but Salata thinks the new cases are probably yet another severe immune-system reaction to Covid-19 that only looks like Kawasaki disease. “We don’t know for sure,” Salata says, “but that’s what it really looks like.”
Loss of smell and taste
The loss of smell, called anosmia, can be caused by the common cold, when congestion mucks up nose and nasal passages. Other diseases can trigger anosmia by disrupting or killing the olfactory nerves high in the nasal cavity.
But doctors were surprised earlier this year when people with anosmia and no other symptoms, or only mild symptoms, tested positive for Covid-19. It’s not yet certain how Covid-19 triggers anosmia, but remember those receptor cells that readily receive the coronavirus, present in the lungs and elsewhere? Similar cells exist in the olfactory epithelium, a layer of skin containing the neurons that detect scents.
Yet another bizarre symptom of Covid-19 is ischemia of the fingers and toes, a reduction of blood flow that causes red or purplish lesions at the ends of the digits. Dubbed Covid toes, the condition can be painful and, if left untreated, can lead to tissue death. It’s showing up mostly but not exclusively in younger people, who sometimes have few other symptoms or none.
Like anosmia, Covid toe tends to heal without long-term complications, but both can be signs that someone has Covid-19 and is infectious, even if they feel no other symptoms.
Like many other complications of Covid-19, researchers have barely had time to study the reasons beyond this symptom, but one analysis, published April 15 in The Lancet, suggests it’s due to reduced function in small blood vessels.
Covid-19 is not alone among germs in generating a range of mild to serious outcomes.
“Viruses are so weird,” says Yonatan Grad, MD, an assistant professor of immunology and infectious diseases at Harvard T.H. Chan School of Public Health. Grad doesn’t mean to discount the devastating effects viruses can have, but he says they can bring “all sorts of unusual manifestations.”
The herpes virus causes unsightly cold sores, for example, but can also prompt a deadly swelling of the brain. The poliovirus generates no visible symptoms in most people while about 1 in 200 ends up with parts of their bodies paralyzed.
Meanwhile, Covid-19 likely has more surprises in store.
Robert Roy Britt.
Robert is an Explainer of things, former editor-in-chief of Live Science and Space .com, author of the science thriller “5 Days to Landfall.”