Doctors identified a new symptom of COVID-19, caused by the novel coronavirus, informally dubbed “COVID toes.”

The presence of purple or blue lesions on a patient’s feet and toes puzzles infectious disease experts.

“They’re typically painful to touch and could have a hot burning sensation,” said Dr. Ebbing Lautenbach, chief of infectious disease at the University of Pennsylvania's School of Medicine.

What doctors said is most interesting about "COVID toes" is that they appear in COVID-19 patients who don’t exhibit any other symptoms. Similarly, the loss of taste and smell was found to be associated with COVID-19 among otherwise asymptomatic patients by the American Academy of Otolaryngology – Head and Neck Surgery and ENT UK in late March.

“This is a manifestation that occurs early on in the disease, meaning you have this first, then you progress,” Lautenbach said. “Sometimes this might be your first clue that they have COVID when they don’t have any other symptoms.”

“COVID toes” in some people can disappear in the course of a week to 10 days, but others progress to respiratory symptoms, he said.

The new symptom appears in more children and young adults than any other age group. Ebbing suggested this may be because children and young adults have better immune systems.

According to Lautenbach, “COVID toes” were discovered in March by Italian doctors. Once experts were informed of the strange symptom, they began recognizing more and more cases in the USA.

There are two running hypotheses on what could cause “COVID toes." One possible explanation, Lautenbach said, is that there is an inflammatory response more localized to a patient’s foot and toes. Or it could be a clotting of blood vessels.

“The short answer is nobody knows,” he said.

Susan Wilcox, chief of critical care for the emergency department at Massachusetts General Hospital, saw the purple lesions on her most critically ill COVID patients and suspected it was purpura fulminans. This occurs when inflammation from a severe infection causes the body to make micro-clots in tiny blood vessels in the toes, fingers and even nose, she explained.

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Wilcox said this is most common in patients who develop acute respiratory distress syndrome, or ARDS, the majority of which die of intense, systematic inflammation.

“You get the infection, and then your body will release a cascade of inflammation,” she said. “In many ways, it’s beneficial, but then sometimes it can either be too much, so the inflammation can lead to its own damage.”

Wilcox said she’s seen this happen in cases of viral pneumonia or a bad flu, so it wasn’t surprising when she saw it in COVID-19 patients.

While experts learn more about the disease and how it relates to “COVID toes,” Lautenbach said an abundance of caution and a low bar of suspicion will stop the spread of the coronavirus.

Although patients with “COVID toes” are more likely to test negative for COVID-19, as the virus is in its earliest stages, he said patients should quarantine at home and monitor closely for developing symptoms.

Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT.