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Paris (dpa) - A disturbed sense of smell or even its complete loss is one of the most common symptoms of an infection with Sars-CoV-2.
Patients with a mild course of the disease are apparently particularly affected, according to the conclusion of a European study published in the Journal of Internal Medicine.
While most of the sick can smell again after an average of three weeks, some report months later of disturbed sensory perception.
The study by the scientists led by the physician Jerome Lechien from the University of Paris-Saclay included data from 2581 Covid 19 patients from 18 European hospitals, which were collected from March to June 2020.
2194 of the patients had a mild Covid-19 course (85 percent), 110 a moderate (4.3 percent) and 277 (10.7 percent) became seriously ill.
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Based on a survey, olfactory disorders occurred in the group with mild courses in 85.9 percent, in moderate cases in 4.5 percent and in the group with serious to critical courses in 6.9 percent.
According to those affected, it took an average of three weeks before they could smell normal again.
Almost a quarter of those affected stated that they still had an impaired sense of smell even after 60 days.
The authors conclude that the survey results of the seriously ill could be influenced by the fact that many of them had to be fed through a nasogastric tube at times.
In special tests, impairment or loss of the sense of smell was still found in 54.7 percent of mild and 36.6 percent of moderate to critical Covid 19 cases.
According to a US meta-analysis, an average of 77 percent of all Covid 19 diseases are associated with odor disorders, regardless of the severity of the course.
In the epidemiological profile of Sars-CoV-2, the Robert Koch Institute indicates a disorder of the sense of smell and / or taste in only 21 percent of the recorded Covid-19 cases in Germany, but also emphasizes that the significantly higher prevalence in published studies «probably from the more intensive determination of such symptoms under study conditions compared to the information provided in the reporting system».
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Of the patients in whom a disorder of the sense of smell was confirmed by a special test, this was still impaired in the current study in 15.3 percent after 60 days and in 4.7 percent even after six months.
In fact, there are more and more stories from those affected who continue to have problems with their sense of smell weeks after an often mild Covid 19 disease, especially on the Internet.
In addition to its complete loss or partial failure, some report that they have noticed unpleasant smells since their illness.
This phenomenon, called parosmia, has already been described in isolated cases in earlier studies on Covid-19 symptoms and often means a high level of suffering for those affected - as well as the disturbance of the sense of smell in general.
Researchers at the Westfälische Wilhelms-Universität Münster and the Forschungszentrum Jülich have developed a “smell and taste check” that can be carried out online as a self-test.
All in all, the sense of smell recovers in most of the sick, according to the study.
"Olfactory dysfunction is more common in mild forms of Covid-19 than in moderate to severe forms, and 95 percent of patients regain their sense of smell six months after infection," summarizes lead author Lechien.
The scientists suspect that the disturbance of the sense of smell, especially in the mildly ill, could be due to a different response of the immune system than in the more severely ill.
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According to this hypothesis, although the virus would not spread as far in the body, it would trigger a local inflammatory reaction in the olfactory system.
According to the authors, another possibility is that the virus damages the nerve cells in the olfactory bulb (olfactory bulb) - the duration and severity of the olfactory disorders would then depend on how many olfactory cells are affected.
The nerve cells of the olfactory system are able to renew themselves, even if this process can take months.
For clarification, further analyzes would have to be carried out in which saliva and nasal secretions would be examined.
© dpa-infocom, dpa: 210108-99-939183 / 2