Covid toes’ may be caused by an immune system overreaction, a new study finds.

Shortly after the pandemic erupted last year, doctors were baffled by a surge of patients, mostly teenagers and young adults, who came in complaining of chilblains — painful lesions on their toes, and sometimes also on their fingers.

The condition came to be called Covid toes. They were seen, like the loss of smell and taste, as yet another strange telltale sign of the disease, even though most of the patients tested negative for coronavirus. Physicians were hard-pressed to explain the association.

The lesions are red or purple in white people and often purplish or brownish in people of color. They cause painful burning or itching sensations and sometimes make it difficult for people to wear shoes or walk.

Now a new study from France, published in the British Journal of Dermatology, sheds some light on the causes of Covid toes. The research indicates that the lesions may be a side effect of the immune system’s shift into overdrive in response to exposure to the virus, which can damage cells and tissues in the process.

The French researchers analyzed blood samples and skin biopsies from 50 patients who had chilblainlike lesions for the first time in April 2020, and who were referred to St.-Louis Hospital in Paris. Slightly more than half of the patients had other symptoms suggestive of Covid-19, like coughing, shortness of breath and loss of smell, but all of them tested negative for the virus on PCR tests.

The samples showed high levels of Type 1 interferon, a protein that activates the body’s immune system to fight viruses, but which may also cause damage. The researchers also found high levels of an antibody that can inadvertently attack the body’s own cells.

Abnormal changes in the linings of the blood vessels may also play a role in the lesions, the study suggests.

Although the relationship between coronavirus infection and chilblainlike lesions “is still controversial,” the authors wrote, the “peaks of chilblainlike lesions concomitant with peaks of Covid-19 deaths in 2020 strongly suggest that this disorder is closely related to SARS-CoV-2 infection.”

The explanation for Covid toes is not entirely surprising; one of the hallmark features of the disease is an immune system overreaction called a cytokine storm, which may ultimately cause more illness than the virus itself.

German scientists published a paper last year saying they had found a strong localized interferon-driven response in three young men who came in with chilblains. That paper suggested that the men, who tested negative for the coronavirus, may have developed chilblains several weeks after an initial infection caused mild or asymptomatic disease, and that the interferon-driven immune response may have led to early control of the virus and prevented respiratory disease.

Dermatologists say that people with Covid toes generally do well and are unlikely to develop severe Covid, and that the symptoms reflect a healthy immune response to the virus.

The new study suggested treating Covid toes with local or systemic anti-inflammatory agents may be effective.