August 19, 2020
New loss of smell was one of the earliest COVID-19 symptoms identified in the pandemic, but it is also a possible symptom of the cold and flu.
So how can doctors tell the difference?
New research says with COVID-19, the loss of smell is sudden and severe. Another key tell is if there isn't an accompanying stuffy or runny nose.
COVID-19 patients in a study published in the journal Rhinology usually didn't experience stuffy sinuses or blocked nasal passages, unlike the patients with a bad cold. The COVID-19 patients also lost their sense of taste completely – to the point that they couldn't distinguish between bitter and sweet.
It is still not completely clear how the SARS-CoV-2 virus affects the sense of smell – whether directly or through a cytokine storm of inflammation. But the study's findings offer support to the theory that the brain and central nervous system are involved, researchers said.
With a cold or flu, any lose of smell typically is just the result of irritation to the lining in the nose or a blockage in the nasal passages.
The study included 10 COVID-19 patients, 10 people with bad colds and a control group of 10 healthy people. The researchers compared their smell and taste scores.
"We know that COVID-19 behaves differently to other respiratory viruses, for example by causing the body's immune system to overreact, known as a cytokine storm, and by affecting the nervous system," said researcher Carl Philpott, of the University of East Anglia. "So we suspected that patterns of smell loss would differ between the two groups."
"It is particularly interesting that COVID-19 seems to particularly affect sweet and bitter taste receptors, because these are known to play an important role in innate immunity," he added.
The "true" loss of taste that accompanied the loss of smell in COVID-19 patients was the biggest distinguishing factor in the study. Philpott and his team hope their findings can help develop smell and taste tests for COVID-19 screening.
"Although such tests could not replace formal diagnostic tools such as throat swabs, they could provide an alternative when conventional tests are not available or when rapid screening is needed – particularly at the level of primary care, in emergency departments or at airports," Philpott said.
Another recent study, published in the European Respiratory Journal, offered some illumination on how the coronavirus may be interacting with the nerve cells responsible for the senses of taste and smell.
To do this, Dr. Andrew Lane, a professor at Johns Hopkins University in Maryland, and his team studied tissue samples from the back of the nose and trachea of 23 patients. None of the patients had been diagnosed with COVID-19.
They found high levels of the ACE2 receptor in the olfactory epithelium, the area of the nose responsible for smelling. The levels were 200 to 700 times higher than those found in other areas of the nose and trachea. Most current research points to ACE2 receptors as the gateway the coronavirus most likely uses to infect cells in the body.
"We are now doing more experiments in the lab to see whether the virus is indeed using these cells to access and infect the body," Lane told the BBC. "If that's the case, we may be able to tackle the infection with antiviral therapies delivered directly though the nose."
Further research is needed to see if the virus is directly binding to the receptors in the olfactory epithelium. Most COVID-19 patients regain their sense of smell within weeks of recovering, both Lane and Philpott said.