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Carl Philpott, University of East Anglia

Symptoms of novel coronavirus disease (COVID-19) typically include a dry cough, fever and shortness of breath. But evidence is beginning to emerge from other countries and now from the UK that sudden loss of smell - and in some cases, taste - is an early indication of the novel coronavirus infection.

So far, there is currently no hard evidence on this, although many have taken to social media to report smell loss alongside other COVID-19 symptoms. As yet, it has not been listed by Public Health England or on the NHS website as an official symptom to look out for. But in this rapidly evolving situation, this may change.

So why is it that people are reporting a loss of smell and taste from this virus? First, it's worth clarifying that when we eat, we smell and taste together. Try pinching your nose when you eat and see what your food "tastes" like. You'll find that the only things you will detect is if the food is salty, sweet, sour, bitter or savoury. This is because these elements of flavour come from the taste buds on the tongue. Losing the smell of food leads many people to think their taste has gone when in the vast majority of cases it will be intact.

Smell loss (also known as anosmia or hyposmia) can be caused by many things, including swelling in the nose and sinuses (such as chronic sinusitis), head injury, and nerve disorders (such as Parkinson's disease). In some cases, no cause is found.

Loss of smell because of a viral infection, such as the common cold, is the second most common cause of smell loss and accounts for about 12% of all cases of anosmia. These episodes typically happen when the virus infects the nose, giving rise to the usual cold symptoms, including a blocked or runny nose. Your sense of smell usually comes back once symptoms subside.

But sometimes even when other symptoms disappear, your sense of smell doesn't return - or in some cases it's reduced (hyposmia), or is distorted (parosmia). In these cases, the virus has damaged the smell receptors causing them to lose the fine, hair-like endings that allow them to pick up smell molecules from the nasal mucus. Previous studies have looked at which viruses cause this condition - and many have been implicated, including the coronavirus family of which COVID-19 is a member.

Permanent loss?

With COVID-19, however, there is a somewhat different infection pattern to other viral upper respiratory infections. First, smell loss may be the only symptom, and indicates someone who seems otherwise well - or has only very mild symptoms - could be a carrier of the disease.

Some people with this symptom appear to be younger - under 40 years of age. The fact that it has been reported in health care workers, too, suggests that COVID-19's ease of transmission from the nose is because the viral shedding (when the virus reproduces) is highest there - and even more so in severe cases. Those who have been affected also report that sensory loss comes back within seven to 14 days.

A virus usually enters the body by implanting itself and infecting host cells throughout the body, such as in the airways or the gut, then reproducing. The COVID-19 virus is believed to enter the nasal tissues through the angiotensin converting enzyme 2 (ACE2) receptor - though further research is needed to confirm whether this is the case. This protein is abundant in the nose, although the function of it is not clear. By entering the nose through this protein, it may cause temporary damage to the smell nerves. However, this damage appears to get better within one to two weeks after the onset of the disease.

Though most people who have reported this symptom get their sense of smell back, it's still too early to tell how many people may be left with more permanent smell loss after the virus has passed.

An international group of sense experts has been established to try and collect worldwide data on this issue, and determine the extent to which COVID-19 causes smell loss. If current trends continue as the weeks roll by and the pandemic peaks around the globe, we expect that people reporting smell loss will only increase in number. Smell loss as a first warning sign could be important in further preventing the spread.

Carl Philpott, Professor of Rhinology and Olfactology, University of East Anglia

This article is republished from The Conversation under a Creative Commons license. Read the original article.

TagsCOVID-19, Smell or Taste Loss, Symptoms of Coronavirus