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Why does COVID cause taste loss? New research: virus infects mouth’s cell

The mouth could play role "in transmitting SARS-CoV2 to the lungs or digestive system via saliva laden with virus from infected oral cells."
New international research conducted by the National Institutes of Health has provided a preliminary suggestion for why the novel coronavirus causes loss of taste.
The study – a joint effort by the National Institute of Dental and Craniofacial Research and the University of North Carolina at Chapel Hill that was published March 25 in Nature Medicine – showed that SARS-CoV-2 infected the cells in the mouth.
“The findings point to the possibility that the mouth plays a role in transmitting SARS-CoV2 to the lungs or digestive system via saliva laden with virus from infected oral cells,” a paper published on the subject by the NIH said. It could also help explain some of the other wide-ranging symptoms of COVID-19, “including oral symptoms such as taste loss, dry mouth and blistering.”
Researchers already knew that people with the virus have high levels of it in their saliva; saliva testing has come to be considered almost as accurate as the gold-standard PCR swab tests, which evaluate virus levels in nasal mucus. What scientists did not know is how the virus got into the saliva. 
The study included five aspects. First, researchers surveyed oral tissues of healthy volunteers to see if the mouth’s cells were susceptible to the novel coronavirus, meaning if they contained the necessary key entry-point proteins known as angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2). 
ACE2 and TMPRSS2 are the proteins that provide the entry point for COVID-19 to hook into and infect human cells.  
The researchers found that in certain cells of the salivary glands and tissues, these enzymes were present, sometimes in the same cells, which would increase their vulnerability to the virus.
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Second, they looked for evidence of infection in oral samples of people who were infected with coronavirus and found that SARS-CoV-2 RNA was present in more than half of the salivary glands they examined – including specific sequences of RNA that indicated the cells were actively replicating. 
The research team then evaluated whether the infected tissues could be a source of the virus in saliva and found that it could. They also tested in a dish if infected saliva extracted from COVID-19 patients would cause healthy cells to be infected and found that in some cases it did.
Finally, the team collected saliva from 35 NIH volunteers. Of 27 people who had symptoms, those that had coronavirus in their saliva were more likely to report loss of taste and smell. The researchers said that this suggests the direct connection between oral infection and oral symptoms.
“The study’s findings suggest that the mouth, via infected oral cells, plays a bigger role in SARS-CoV-2 infection that previously thought,” the NIH article said, noting that additional research is still needed on a larger cohort of volunteers to confirm these preliminary findings and to “determine the exact nature of the mouth’s involvement in SARS-CoV-2 infection and transmission outside the body.”
Dr. Blake M. Warner, study leader and assistant clinical investigator and chief of NIDCR’s Salivary Disorder Unit, said that, “By revealing a potentially underappreciated role for the oral cavity in SARS-CoV-2 infection, our study could open up new investigative avenues leading to a better understanding of the course of infection and disease. Such information could also inform interventions to combat the virus and alleviate oral symptoms of COVID-19.”
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