Covid tongue? Why new Covid-19 symptoms keep popping up.

It’s been more than a year since Covid-19 landed in the United States. And the once-perplexing array of symptoms like coughing, fever, shortness of breath, and loss of taste and smell are now very familiar to doctors around the country.

The range of possibilities when someone shows up to a doctor’s office with new respiratory symptoms “is very, very narrow these days,” said Anish Mehta, medical director for clinical quality and virtual health at Eden Health, and an assistant clinical professor of medicine at the Icahn School of Medicine at Mount Sinai. “It’s probably Covid if you have fever and you’re coughing, or if you have a fever and feel muscles aches.”

Medical researchers have also learned more about how a Covid-19 infection can ripple through the circulatory, nervous, and immune systems with symptoms like rashes, blood clots, strokes, and even foot lesions dubbed “Covid toes.” They’re also more familiar with the more than 10 percent of Covid-19 survivors who are reporting long-term symptoms, including difficulty thinking and focusing, heart palpitations, hair loss, and mood swings.

Yet even now researchers are finding new symptoms. Tim Spector, a professor of molecular epidemiology at King’s College London, has been studying Covid-19 throughout the pandemic through a Covid-19 Symptom Study smartphone app. He recently started receiving reports of mouth ulcers and something he calls Covid tongue — a fuzzy yellow-white coating on the tongue.

“It came about because people sent me images of their tongue,” said Spector. “I posted them and then people started … realizing that’s what they had originally when they had Covid. It’s a strange phenomenon that no doctor thought was related.”

Though cases and hospitalizations are falling, and multiple Covid-19 vaccines are rolling out, the SARS-CoV-2 virus that causes Covid-19 is still spreading, and new variants are threatening to undo some of the progress made during the pandemic. The more the virus spreads, the more likely it is to acquire new mutations. Changes in symptoms could be a warning sign of these changes to the virus. So doctors have to remain on their toes.

“I think the lesson is unusual symptoms can come out of the blue with no clear explanation,” said Andrew Chan, a professor of immunology and infectious disease at the Harvard T.H. Chan School of Public Health, who collaborated with Spector on the symptom study app. “It has to be on every health care provider’s mind.”