Scientists think they’ve identified how COVID-19 causes patients to lose their sense of smell © Getty Images

Scientists think they’ve identified how COVID-19 causes patients to lose their sense of smell

New research suggests that the COVID-19 infection begins in the cells of the nose responsible for smell, which is why for some, anosmia is their only symptom.

The COVID-19 symptom of loss of smell, or anosmia, is unusual in the way that it differs from that of the common cold or flu.

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With these ailments, a blocked nose is usually the culprit, caused by inflammation swelling of the area. However, for some people that have COVID-19, anosmia is their only symptom – but it is ‘much more profound’ than with regular flu.

The reason for this could lie in the mechanism by which the virus infects our cells, new research suggests.

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The ‘entry point’ for SARS-CoV-2, the virus that causes COVID-19, is thought to be a protein found on the surface of some human cells. These are cells found in the heart, lungs, gut, throat and the nose.

The protein, a type of enzyme called the ‘angiotensin converting enzyme II’ (ACE-2), has a particular shape that is designed for its job – to take the hormone angiotensin, and convert it into angiotensin II, which is used around the body for things like regulating blood pressure. While ACE-2’s shape fits nicely with angiotensin, it also binds with the spiky outer proteins of the coronavirus.

By looking at tissue samples from patients’ noses, a team of scientists from John Hopkins University School of Medicine hoped to quantify how many ACE-2 proteins were found on each type of cell. They found that the levels of the protein on the olfactory epithelium, the tissue at the back of the nose which is used to detect smell, were ‘strikingly’ high, between 200 and 700 times higher than in other areas of the nose.

We are optimistic that this eventually goes away as the brain ‘re-learns’ to interpret signals
Dr Andrew P Lane, John Hopkins University School of Medicine

It’s thought that this finding could indicate potential avenues for treating the infection directly through the nose.

“These olfactory supporting cells are necessary to protect and maintain the delicate neurons in the nose that detect odours and signal that information to the brain,” said author of the study Dr Andrew P Lane.

“Generally speaking, when cells are infected with a virus, they undergo a process called pyroptosis – essentially hitting the self-destruct button to foil the virus. So, most likely, olfactory supporting cells destroy themselves, leading in turn to the death of sensory neurons and loss of the sense of smell.”

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Further studies looking at recovery of COVID-19 patients have found these neurons to return over time, though the long-lasting effects are still unknown.

“Some COVID-19 patients are reporting distorted smell – parosmia – persisting for months after their sense of smell has returned,” said Lane. “This long-lasting smell disturbance with COVID-19 is unusual and warrants further study.

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“It is possible that this altered smell function may be permanent, but it is too soon to know. We are optimistic that this eventually goes away as the brain ‘re-learns’ to interpret signals from the regenerated olfactory lining.”

What is viral load and why is it important to coronavirus?

Viral load is simply the amount of virus in the body. This varies in different parts of the body, and can change over time. Recent studies have shown, for example, that the viral load in the lungs of COVID-19 patients is greater than that in the nose.

A patient’s viral load increases as the virus replicates and disease symptoms get worse, and then decreases as the patient recovers. So monitoring the viral load can give us a useful indication of how a patient’s infection is progressing.

The amount of virus that you’re exposed to at the beginning of an infection is something different, and this is called the ‘infectious dose’. Studies on other viruses such as the flu and SARS have shown that the higher the infectious dose (the more virus you breathe in), the greater your chances of having more severe symptoms.

With one small exposure, your immune system may be able to fight off the virus before you get sick, but with repeated small exposures (such as touching your face throughout the day) or one large exposure (an infected person coughing in your face), the virus may grow faster than your body can control.

We don’t yet know if this link between infectious dose and disease severity holds for COVID-19, but it may do, and that’s why it’s so important to maintain physical distancing and keep the initial exposure as low as possible.

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