Health care workers are calling attention to a potential new symptom of a novel coronavirus infection: the loss of one’s sense of smell.

Clinicians in the U.S. and elsewhere around the world have reported, anecdotally, that some patients infected with the virus experience anosmia, or an inability to smell. The American Academy of Otolaryngology recently called for anosmia to be added to a list of screening tools for Covid-19, the disease caused by the virus. On Monday, World Health Organization officials announced they were further probing a possible link between anosmia and coronavirus — though they noted the evidence of a potential connection remains preliminary.

“A loss of smell or a loss of taste is something that we’re looking into,” Maria Van Kerkhove, the head of WHO’s emerging diseases and zoonoses unit, said on a briefing call with reporters on Monday. “We are reaching out to a number of countries and looking at the cases that have already been reported to see if this is a common feature. We don’t have the answer to that yet.”

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There are a number of unanswered questions about the new potential symptom, which at this point, is thought to be temporary. Experts aren’t sure how common it is, or at what point in the progression of the disease a patient loses the ability to smell. They also want to be sure the new symptom isn’t merely a sign of allergies, a cold, or the seasonal flu, all of which can lead a person to develop anosmia or ageusia, the reduced sense of taste that can sometimes accompany an inability to smell.

“It’s certainly popping up as a symptom, but how universal it is, we don’t know,” said Eric Holbrook, director of rhinology at Massachusetts Eye and Ear, a Boston hospital. Holbrook said he started hearing about the possible tie between anosmia and coronavirus in case reports from clinicians in Europe about a week ago.

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Those reports are among a growing amount of anecdotal evidence that suggests experts might want to consider anosmia as a potential sign of infection. In report published on March 16 in the South Korean newspaper Chosun, clinicians estimated that roughly 30% of patients who tested positive for coronavirus had temporarily lost their sense of smell. In Germany, clinicians at the University Hospital in Bonn surveyed 100 patients with coronavirus and found that up to two-thirds “described a loss of smell and taste lasting several days,” Hendrik Streeck, head of the hospital’s virology institute, told the German news site Frankfurter Allgemeine.

The known symptoms of Covid-19 have evolved as cases have climbed globally. The Centers for Disease Control and Prevention lists fever, cough, and shortness of breath as the core coronavirus symptoms. Difficulty breathing, persistent chest pain or pressure, confusion, and bluish lips or face are listed as emergency symptoms for which people should seek immediate care. The WHO says other symptoms can include tiredness, fatigue, and in some patients, “aches and pains, nasal congestion, runny nose, sore throat, or diarrhea.”

The WHO and CDC haven’t added anosmia to their coronavirus symptom lists, which health care workers use to screen patients who might have been infected with coronavirus. But some health care workers are encouraging their peers to consider the symptom when caring for patients they suspect might have be infected.

“I wouldn’t use it alone [to screen patients], but I think if it’s added to some of the other symptoms, it adds to the possibility,” said Holbrook. “If someone says yes, it would definitely make me more suspicious that they need further work-up.”

Using the new symptom as a screening tool is a delicate task, experts noted. Emphasizing it too early, or too heavily, could mean clinicians might miss patients who haven’t lost their sense of smell, but who are infected and have other symptoms. Ignoring a lost sense of smell, however, could mean doctors don’t detect infection in cases where patients have only mild symptoms or are asymptomatic. In those cases, using anosmia as a screening tool could help clinicians catch cases that would otherwise fly under the radar and give those infected individuals more guidance on self-isolation.

But to do that, clinicians need more data on the role anosmia might play in Covid-19.

“One of the things I think a lot of physicians and patients are struggling with is trying to determine whether these mild symptoms are related to, say, allergies or a mild cold,” Holbrook said. He added that it’s also difficult to distinguish between a loss of the sense of smell and a loss of the sense of taste, which patients can sometimes mix up.

“But anything that gives us a heads-up into who needs to be isolated or tested, that might be a good screening tool,” Holbrook said.

To get a better handle on the new symptom and its ties to coronavirus, Holbrook said he and his colleagues will be looking at people with anosmia and confirmed Covid-19, as well as those experiencing anosmia who have tested negative. That will allow them to compare both groups and understand whether anosmia is truly a core coronavirus symptom.

“That’s important data — data that we need. Many of us are trying to start doing those studies,” Holbrook said.

Helen Branswell contributed reporting.

An earlier version of this article mischaracterized South Korean research about anosmia. It was a non peer-reviewed report which suggested that roughly 30% of South Koreans with Covid-19 lose their sense of smell, not a Nature study.

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