It is unknown why some COVID-19 patients develop an olfactory disorder that makes them temporarily unable to smell, but research is progressing.
Smell and taste disorders can occur with colds, which are often accompanied by symptoms such as runny and stuffy nose. However, COVID-19 seems to make it difficult for some patients to utilize their sense of smell and taste even without nasal congestion.
According to an Italian report on 59 hospitalized patients with COVID-19, about 34% had some smell or taste disorder. In a U.S.-British survey in which participants reported their symptoms via a smartphone app, about 65% of 7,200 or so people who tested positive for the virus said their sense of smell or taste had been affected.
The olfactory nerve, also known as the first cranial nerve, transmits sensory information related to smell. A joint research team of Harvard University and others, announced in July that cells around the olfactory nerve contain many ACE2 proteins, which bind to the coronavirus when it enters the cells.
There is no ACE2 in the olfactory nerve itself. It is understood that the virus attacks the surrounding cells instead of attacking the olfactory nerve, but this causes the nerve to lose its function. When nerve cells are damaged, regeneration takes time, but many patients with an olfactory disorder caused by the virus recover their sense of smell once they are completely cured of COVID-19. However, influenza and common cold viruses can directly attack the olfactory nerves, leaving some people with long-term problems with smell.
"It has been reported that 10% to 30% of people who have a smell disorder caused by the coronavirus will have symptoms for more than four weeks," said Prof. Takaki Miwa of Kanazawa Medical University, a specialist in otorhinolaryngology. Long-term evaluation of olfactory disorders is necessary after recovering from COVID-19.
The younger the patient, the more smell and taste disorders seem to appear. It is one of the conditions to take into consideration as signs that someone is infected with virus. "If you feel anything strange [with your sense of smell or taste], please get in touch with a consultation center for returnees and contacts," Miwa said.
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