Powerful Immune System Response May Be Behind ‘Covid Toes’

In the early weeks of the Covid-19 pandemic in 2020, medical professionals were puzzled by an unusual symptom cropping up in some of their patients: red, frostbite-like sores on their toes. The condition informally called "Covid toes" was appearing in teenagers and young adults who were testing negative for the virus, but had other coronavirus-like symptoms, including loss of taste and smell. A new study suggests that the toe lesions may be a side effect of the immune system’s aggressive defensive response to the Covid-19 virus, which can damage cells and tissues in the process. 

The sores, which can cause an itching or burning sensation, were "extremely painful and many patients couldn't sleep or walk," says Charles Cassius, lead author on the study and a researcher at France’s Hôpital Saint-Louis, to Insider’s Marianne Guenot. "During the first wave of the pandemic...patients with Covid toe were afraid they were developing severe COVID-19 and that they could transmit it," he says.

When patients started arriving in hospitals with these mysterious toe lesions, doctors debated if it was linked to Covid-19. Some dermatologists suggested that the sores were caused by inflammation, while others thought they might result from small blood clots, reports Nicoletta Lanese for Live Science. The blistered, swollen, and itchy skin—called chilblains—can also appear on some peoples’ fingers. The lesions appear red or purple in people with light skin tones, and often purplish or brownish in people with dark skin tones, reports Roni Caryn Rabin for the New York Times.

In the study, published by French researchers in the British Journal of Dermatology last week, they analyzed blood samples and skin biopsies from 50 patients sent to the hospital in April 2020. When they examined the blood samples, the team found evidence that two parts of the immune system may be behind the lesions. Patients with chilblains had high levels of a type 1 interferon, a protein that kicks the immune response in gear but can also damage the body. The team also found elevated levels of an antibody that can turn on the body’s own cells. Lastly, they discovered that the frostbite-like sores may be due to changes in the lining of blood vessels, which could cause damage in parts of the body where vessels are most narrow, like toes and fingers. Like a biological wrecking ball, an aggressive immune response can successfully ward off viruses but also damage healthy cells in the process.

For each of the patients in the study, it was the first time they’d developed chilblains. Though more than half of the patients studied had symptoms consistent with Covid-19, like shortness of breath and loss of taste and smell, all of them tested negative for the virus. Although the relationship between being infected with Covid-19 and "Covid toes" is still being debated, the authors conclude that "peaks of chilblainlike lesions [accompanying] peaks of Covid-19 deaths in 2020 strongly suggest that this disorder is closely related to SARS-CoV-2 infection." Their work will help guide medical care for patients with "Covid toes," who can easily treat their lesions with anti-inflammatory medication.

The findings of the recent study echo a small study done earlier this year, according to the New York Times. The study, which only three male patients, also suggested that the bizarre toe and finger rashes were linked to a strong immune response. Cases of "Covid toes" seemed to peak during the first waves of the pandemic, and have become less common during the rise of the Delta variant says dermatologist Veronique Bataille to the BBC's Michelle Roberts. Increased immunity from vaccines and previous infections could be a reason behind the drop in cases of "Covid toes."

"Presentations after vaccination are much rarer," says Bataille.

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Corryn Wetzel is a freelance science journalist based in Brooklyn. Her work has also appeared in Audubon magazine, National Geographic and others.