BOSTON (NYTIMES) - For some Covid-19 patients, a diminished sense of smell, called anosmia, is the first symptom, and sometimes the only one.

Often accompanied by an inability to taste, anosmia occurs abruptly and dramatically in these patients, almost as if a switch had been flipped.

Most regain their senses of smell and taste after they recover, usually within weeks. But in a minority of patients, the loss persists, and doctors cannot say when or if the senses will return.

Scientists know little about how the virus causes persistent anosmia or how to cure it. But cases are piling up as the coronavirus sweeps across the world, and some experts fear that the pandemic may leave huge numbers of people with a permanent loss of smell and taste.

Smell is intimately tied to both taste and appetite, and anosmia often robs people of the pleasure of eating. But the sudden absence also may have a profound impact on mood and quality of life. Studies have linked anosmia to social isolation and anhedonia, an inability to feel pleasure, as well as a strange sense of detachment.

British scientists studied the experiences of 9,000 Covid-19 patients who joined a Facebook support group. Many members said they had not only lost pleasure in eating, but also in socialising. The loss had weakened their bonds with other people, leaving them feeling isolated, even detached from reality.

"From a public health perspective, this is really important," said Associate Professor Sandeep Robert Datta, a neurobiologist at Harvard Medical School.

"If you think worldwide about the number of people with Covid-19, even if only 10 per cent have a more prolonged smell loss, we're talking about potentially millions of people."

The most immediate effects may be nutritional. People with anosmia may continue to perceive basic tastes, but smell adds complexity to the perception of flavour via hundreds of odour receptors signalling the brain.

Many people who cannot smell will lose their appetites, putting them at risk of nutritional deficits and unintended weight loss.

Smells also serve as a primal alarm system alerting humans to dangers in our environment, such as gas leaks. A diminished sense of smell in old age is one reason older individuals are more prone to accidents, like fires caused by leaving burning food on the stove.