Loss of smell in COVID-19 explained by scientists

As the coronavirus pandemic progresses, the signs and symptoms that may indicate infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have evolved.

Initially, doctors in China described the common symptoms of coronavirus disease (COVID-19), including cough, fever, and difficulty of breathing. But as the disease spread across the globe, the list has expanded, adding the loss of smell, loss of taste, diarrhea, fatigue, swollen eyelids, and toes and among others.

However, one of the commonly reported symptoms by many patients infected with the novel coronavirus is the loss of smell. Most patients had reported being unable to distinguish scents during their battle with the infection. Now, a team of researchers at Harvard Medical school says they have found the reason why some people who get COVID-19 experience loss of smell.

Called anosmia, the symptom is known as the earliest and most reported indicator of SARS-CoV-2 infection. Health experts also said that anosmia could be a reliable and batter way to predict whether a person has the disease, other than other prevalent symptoms such as cough and fever.

Why anosmia?

The team set out to better understand how the sense of smell is affected in patients infected with SARS-CoV-2. They want to shed light and focus more on the cell types most susceptible to contracting SARS-CoV-2, the virus that causes COVID-19.

In the study, which was published in the journal Science Advances, the team noted that altered olfactory function is a common symptom of COVID-19, but it is unclear why it happens. To arrive at their findings, the team identified cell types in the olfactory epithelium and olfactory bulb that express SARS-CoV-2 cell entry molecules.

The team found that the virus attacks cells that support the olfactory sensory neurons, whose role focuses on detecting and transmitting signals from the nose to the brain. Also, olfactory sensory neurons do not express the gene that encodes the angiotensin-converting enzyme 2 (ACE2) receptor protein, which is used by the novel coronavirus to enter human cells. Instead, the ACE2 is expressed in cells that give metabolic and structural support to olfactory sensory neurons.

SARS-CoV-2 viruses binding to ACE-2 receptors on a human cell, the initial stage of COVID-19 infection. Imaghe Credit: Kateryna Kon / Shutterstock

SARS-CoV-2 viruses binding to ACE-2 receptors on a human cell, the initial stage of COVID-19 infection. Image Credit: Kateryna Kon / Shutterstock

This means that the infection of non-neuronal cell types may be responsible for the presence of anosmia in patients with COVID-19.

“Our findings indicate that the novel coronavirus changes the sense of smell in patients not by directly infecting neurons but by affecting the function of supporting cells,” Sandeep Robert Datta, associate professor of neurobiology in the Blavatnik Institute at HMS and senior author of the study, said.

She added that, in most cases, the infections are unlikely to cause permanent damage to the olfactory neurons and lead to persistent anosmia. When the infection clears, the patient can regain the sense of smell.

New clusters of symptoms

In a separate report, scientists have found that COVID-19 symptoms come in six different clusters, which can help predict the patients who are at a heightened risk of severe disease and may require respiratory support. The team utilized a machine-learning algorithm to study data from more than 1,600 patients in the United States and the United Kingdom.

The patients had COVID-19 and regularly logged their symptoms in an app called Zoe health from March to April. The team added the entries of an additional 1,047 patients in the U.S., U.K., and Sweden in May.

The team found that while cough, fever, and loss of smell were usually highlighted as the three main symptoms of COVID-19 recently, they had a long list of additional symptoms, such as muscle pain, headache, confusion, diarrhea, fatigue, loss of appetite, and shortness of breath, among others.

The six clusters include the “flue-like” with no fever, “flu-like” with fever, gastrointestinal, severe level one with fatigue, severe level two with confusion, and severe level three with abdominal and respiratory symptoms.

The first cluster included symptoms such as headache, loss of smell, muscle pains, cough, sore throat, and chest pain, without fever. The second cluster included headache, loss of smell, cough, sore throat, hoarseness, loss of appetite, and fever. The third cluster composed of headaches, loss of appetite, loss of smell, diarrhea, sore throat, chest pain, but without cough. The fourth cluster included headache, loss of smell, cough, fever, hoarseness, chest pain, and fatigue,

The fifth cluster included headache, loss of smell, loss of appetite, fever, cough, hoarseness, sore throat, fatigue, chest pain, confusion, and muscle pain, while the sixth cluster included headache, loss of smell, loss of appetite, fever, cough, hoarseness, chest pain, sore throat, fatigue, confusion, muscle pain, shortness of breath, abdominal pain, and diarrhea.

The team also studied whether the patients experiencing a particular symptom cluster were more likely to need breathing support, such as oxygen or ventilator. They found that about 1.5 percent of those with cluster 1, 4.4 percent of those in cluster 2, and 3.3 percent of those in cluster 3 needed breathing support. For clusters 4, 5, and 6, the figures were 8.6 percent, 9.9 percent, and 19.8 percent, respectively. Further, about half of those in cluster 6 needed admission to a hospital, compared to just 16 percent in the people in cluster 1.

“These findings have important implications for the care and monitoring of people who are most vulnerable to severe COVID-19. If you can predict who these people are at day five, you have time to give them support and early interventions such as monitoring blood oxygen and sugar levels, and ensuring they are properly hydrated – simple care that could be given at home, preventing hospitalizations and saving lives,” Dr. Claire Steves, consultant geriatrician, and study author, said.

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