NEW YORK — A mother who was infected with the coronavirus couldn’t smell her baby’s full diaper. Cooks who can usually name every spice in a restaurant dish can’t smell curry or garlic, and food tastes bland. Others say they can’t pick up the sweet scent of shampoo or the foul odour of kitty litter.

Anosmia, the loss of sense of smell, and ageusia, an accompanying diminished sense of taste, have emerged as peculiar telltale signs of Covid-19, the disease caused by the coronavirus, and possible markers of infection.

On Friday (March 20), British ear, nose and throat (ENT) doctors, citing reports from colleagues around the world, called on adults who lose their senses of smell to isolate themselves for seven days, even if they have no other symptoms, to slow the disease’s spread. The published data is limited, but doctors are concerned enough to raise warnings.

“We really want to raise awareness that this is a sign of infection and that anyone who develops loss of sense of smell should self-isolate,” Professor Claire Hopkins, president of the British Rhinological Society, wrote in an email. “It could contribute to slowing transmission and save lives.”

She and Professor Nirmal Kumar, president of ENT UK, a group representing ear, nose and throat doctors in Britain, issued a joint statement urging health care workers to use personal protective equipment when treating any patients who have lost their senses of smell, and advised against performing nonessential sinus endoscopy procedures on anyone, because the virus replicates in the nose and the throat and an exam can prompt coughs or sneezes that expose the doctor to a high level of virus.

Two ear, nose and throat specialists in Britain who have been infected with the coronavirus are in critical condition, Dr Hopkins said. Earlier reports from Wuhan, China, where the coronavirus first emerged, had warned that ear, nose and throat specialists as well as eye doctors were infected and dying in large numbers, Hopkins said.

The British physicians cited reports from other countries indicating that significant numbers of coronavirus patients experienced anosmia, saying that in South Korea, where testing has been widespread, 30 per cent of 2,000 patients who tested positive experienced anosmia as their major presenting symptom (these were mild cases).

The American Academy of Otolaryngology on Sunday posted information on its website saying that mounting anecdotal evidence indicates that lost or reduced sense of smell and loss of taste are significant symptoms associated with Covid-19, and that they have been seen in patients who ultimately tested positive with no other symptoms.

Dr Rachel Kaye, an assistant professor of otolaryngology at Rutgers, said colleagues in New Rochelle, New York, which has been the center of an outbreak, first alerted her to the smell loss associated with the coronavirus, sharing that patients who had first complained of anosmia later tested positive for the coronavirus. “This raised a lot of alarms for me personally,” Dr Kaye said, because those patients “won’t know to self quarantine.”

“Most ENTs have on their own accord tried to scale down,” she said, adding that her department at Rutgers had already started using personal protective equipment and stopped performing nonessential exams.

In the areas of Italy most heavily affected by the virus, doctors say they have concluded that loss of taste and smell is an indication that a person who otherwise seems healthy is in fact carrying the virus and may be spreading it to others.

“Almost everybody who is hospitalised has this same story,” said Dr Marco Metra, chief of the cardiology department at the main hospital in Brescia, where 700 of 1,200 inpatients have the coronavirus. “You ask about the patient’s wife or husband. And the patient says, ‘My wife has just lost her smell and taste but otherwise she is well.’ So she is likely infected, and she is spreading it with a very mild form.”

A study from South Korea, where widespread testing has been done, found that 30 per cent of some 2,000 patients who tested positive for the coronavirus reported experiencing anosmia.

Dr Hendrik Streeck, a German virologist from the University of Bonn who went from house to house in the country’s Heinsberg district to interview coronavirus patients, has said in interviews that at least two-thirds of the more than 100 he talked to with mild disease reported experiencing loss of smell and taste lasting several days.

Another physician who studied a cluster of coronavirus patients in Germany said in an email that roughly half of the patients had experienced a smell or taste disorder, and that the sensory loss usually presented after the first symptoms of respiratory illness, but could be used to distinguish people who should be tested.

Dr Clemens Wendtner, a professor of medicine at the Academic Teaching Hospital of Ludwig-Maximilians University of Munich, said that the patients regained their ability to smell after a few days or weeks, and that the loss occurred regardless of how sick they got or whether they were congested. Using nasal drops or sprays did not help, Wendtner said.

Several American patients who have had symptoms consistent with the coronavirus, but who have not been tested or are still awaiting test results, described losing their senses of smell and taste, even though their noses were clear and they were not congested.

Mr Andrew Berry, 30, developed a fever and body aches about 10 days ago, and then a sore throat and debilitating headaches. He tested negative for influenza and has not gotten the result of a coronavirus test taken four days ago, but his physician was convinced that he had the virus, he said.

Now, Mr Berry said, he literally cannot smell the coffee.

“Even with a clear nose, I just realised I couldn’t smell the food that I was cooking, and I couldn’t taste the food that I was making,” said Mr Berry, a tattoo artist based in Orlando, Florida. He was cooking a plantain dish with onions and vinegar, yet he could not smell it.

Ms Amy Plattmier, a woman from Brooklyn, was not tested for the coronavirus during a recent illness, but her husband then became sick and had a positive test. Ms Plattmier said she usually had a very sensitive nose, but now could barely smell anything — not the bleach she was using to clean the counters, which usually makes her feel nauseated, or the dog’s accident in the bathroom, which she cleaned up.

Mr Berry has also lost some weight, because he has not had much of an appetite. “Hopefully it’s not a prolonged effect,” he said. “I can imagine it changes the quality of life.” THE NEW YORK TIMES