What does the loss of sense of smell and taste mean in the context of COVID-19?
In the face of this pandemic, it has become clear just how much we don’t know about the COVID-19 disease, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Every day since its effects have been felt around the world, researchers have been focusing on developing a vaccine, on diagnostic methods, as well as modelling patterns and trends of the spread. The testing and screening efforts have been especially important, as the spread of the virus must be checked drastically by testing widely. There has been a massive movement of the scientific workforce to tackle the pandemic from many angles.
One of the causes of the alarming spread of the COVID-19 virus is the absence of symptoms in people who have been infected. This happens in two ways, firstly, with the incubation period varying between 1 day to 2 weeks, before people might experience COVID-19 symptoms, and secondly, in asymptomatic cases, where people are carriers of the infection, with little to no symptoms which could alert them to their own condition.
The absence of symptoms meant people travelled around the world, attended large gatherings, and all of them had no idea they were spreading the infection! Among many of these cases, there has been one common thread: that of changes to their sense of smell and taste. Of the asymptomatic patients, many later reported they experienced a loss of their sense of smell and taste.
This has led researchers to investigate the relationship between COVID-19 and dysfunction in the chemical senses. This area of research began based on the widespread anecdotal data, in various countries, including China, Belgium, France, Iran, Italy, Korea, Spain, USA, and the UK. People have reported the total loss of sense of smell (anosmia) and taste (ageusia), or a decreased sense of smell (hyposmia), even when they may not show other symptoms.
What we do know about chemosensory dysfunction in COVID-19?
Studies from different countries have shown quite clearly that there is olfactory and taste dysfunction among people who have suspected or confirmed cases of COVID-19, compared to those who test negative for the virus. The data diverges on the timeline, severity, and duration of this dysfunction. Because this data is limited to one hospital or country, there is not a clear picture of this symptom, and how it can be used to help fight the spread. The Centers for Disease Control and Prevention
(CDC) and others
have listed this as a symptom of COVID-19, but the information on this is still very incomplete. There is a need for a much wider assessment of these COVID-19-related changes in smell and taste.
There is evidence
that the SARS-CoV-2 gains entry to cells through two proteins: ACE2 and TMPRSS2. Both are expressed within the respiratory system, which could lead to the COVID-19 impacts on respiration. A few studies
have also demonstrated ACE2 expression within non-neuronal supporting cells
in the olfactory epithelium
. Together, these results suggest that COVID-19 may cause olfactory impairment through disruption of supporting cell function in the olfactory epithelium of the nose.
Though the COVID-19 disease might be the cause of rapid onset olfactory dysfunction, olfactory and taste dysfunction are present in various conditions. Factors which affect the chemical senses include medication, lifestyle, aging, smoking, genetics, environment, and the country or region the person is in. These have proved to be challenges in forming a cohesive understanding of what happens to the senses of smell and taste when people are infected with the COVID-19.
One of the reasons for the complexity of the data on the changes to the chemical senses is the research methodology differs. “There are also genetic variations in the COVID-19 viruses themselves in different regions, which could potentially change how they impact smell and taste,” says Dr Shannon Olsson, associate professor and chemical ecologist, at the National Centre for Biological Sciences. With a larger pool of responses, the potential understanding and usage of this symptom for screening could be invaluable.
A global study of the chemical senses and COVID-19
The data about anosmia and dysgeusia suggest it might be a good screening method for people to know they must self-isolate. To establish whether it is a reliable test, as well as a clear timeline for these symptoms, researchers from around the world have formed the Global Consortium of Chemosensory Research (GCCR). They are tackling this question with a survey, which has been translated to over 20 languages already, among them are 9 Indian languages: Bengali, Hindi, Kannada, Gujarati, Malayalam, Marathi, Punjabi, Tamil and Urdu.
Anyone who has experienced respiratory illnesses recently, including COVID-19 is requested to fill out this short and simple survey
, which will help scientists determine if this might be the first step to helping people identify their own COVID-19 infection and avoid spreading it unintentionally.
With the findings of this global survey, if there appears to be solid evidence for assessing smell and taste among suspected cases of COVID-19, the Indian collaborators will have more information for a test. This is being worked on by the Tata Institute of Fundamental Research (National Centre for Biological Sciences and TIFR-Hyderabad), Indraprastha Institute of Information Technology (IIIT), Delhi, Karunya Institute of Technology and Sciences, and the Council of Scientific & Industrial Research (CSIR)-Central Scientific Instruments Organisation (CSIO).
Pavithra Ashok Kumar works at the Communications Office, Bangalore Life Science Cluster, NCBS. She appreciates the feedback and information she received from Dr. Shannon Olsson on this article.