Anosmia (complete olfactory loss) can be experienced after viral infections of the upper respiratory tract such as the common cold, flu and sinus infections, or after traumatic brain injury. Recovery from olfactory loss can be slow and typically starts with the onset of parosmia (olfactory distortion) a few months after the initial smell loss. Parosmia is usually a temporary phase that can happen during the recovery period as the olfactory nerves regenerate. The smell of common food items, body care products, household cleaners and things in the environment become distorted and invoke a strong sense of disgust. Eating can become a challenge, and anxiety, depression and loss of quality of life is experienced by many. However, the onset of parosmia is usually a good sign, as it indicates recovery of the olfactory neurons and in many cases is the start of recovery to a ‘new normal’.
Anosmia is an officially recognised symptom of Covid-19 but the pattern of onset and recovery is slightly different to what we have seen in the past with other post-viral illnesses. The onset of Covid-19-related anosmia is sudden and occurs within hours1. If you experience this (with the emphasis on sudden), it is almost certain that you have Covid-19, as this is the best predictor of Covid-192. Recently it has been suggested that those who lose their sense of smell are less likely to get severe symptoms3. It is also quite common to lose your sense of taste at the same time, and your ability to detect heating and cooling sensations (chemesthesis)4 and this is quite different to other post-viral infections.
We estimate that about 80-90% of those with post-Covid anosmia will regain their sense of smell within a few weeks5. However, an unlucky few will find that recovery of their sense of smell takes much longer. As those with post-Covid-19 anosmia start to recover, we are observing unusual fluctuations in symptoms, sometimes a temporary return to (near) normal, with the onset of parosmia in many cases, a few months down the line.
This is a question which we are seeing in social media channels. It is important to distinguish between the early sudden onset of anosmia which indicates a live infection and the need to self- isolate, and the ‘residual’ anosmia which persists beyond the end of the infection. If in doubt, ask a health care professional, but in general after the 7 to 10-day quarantine period, the evidence suggests you are safe to go to work6.
There is some controversy over the efficacy of smell testing in the general population as poor olfactory function is not an indication of a live virus, and can also be related to chronic rhinosinusitis, neurodegenerative aging, Alzheimer’s disease, Parkinson’s disease and head injury. However, a sensory panel is one group for which regular smell testing could be recommended. In general, loss of the ability to smell is not insignificant, and hard to miss. Most people with smell loss will discover they cannot smell their soap or shower gel, cannot taste minty toothpaste, and do not smell their coffee or toast. Usually within 20 minutes, you are very aware of the problem, there is no need for further testing, and most people will notify their employers. However, for a sensory panellist, where their job relies on their olfactory acuity, there might be the temptation to under-report. For those with full anosmia, this will soon become apparent by checking panellist performance data, but some may return with a weaker, or fluctuating olfactory function. With this in mind we should consider regular checks of panellist olfactory acuity using standardised methods.
There are many ideas about the nature and extent of smell testing, but for a sensory panel, by far the most informative test would be to repeat the screening procedures carried out during recruitment. With sensory panels we are lucky in a sense, that pre-Covid-19 data is available for comparison with post-Covid data, and this will reveal any decline in olfactory function. However, if only simple odour identification tests were used for screening, it would be prudent to start using standardised methods, such as the Sniffin' Sticks test (Burghardt®, Wedel, Germany). This method considers odour thresholds, discrimination and identification of familiar odours.
Post-covid, a significant increase in olfactory dysfunction in the general population could be a problem. A recent study has shown that people tend to overestimate the recovery of their olfactory function. Objective tests showed that 55% of those reporting a return to a normal sense of smell still had a low olfactory function, and 5% were still functionally anosmic7 and this presents a potential problem for consumer trials. A typical consumer test using self-reported screening questions such as health information and familiarity with products, rather than sensory acuity, would not pick up those with low olfactory function, whether they were aware of it or not. If your consumer test focuses on product flavour, then it may be worthwhile to consider adding a smell test into your procedure. This could be as simple as identifying three familiar smells from a bottle, smelling strip or sniffing stick, or more complex sensory tests as required.
1. Gane, S. B.; Kelly, C.; Hopkins, C. Isolated sudden onset anosmia in COVID-19 infection. A novel syndrome? Rhinology 2020, 58 (3), 299-301. https://doi.org/10.4193/Rhin20.114
2. Menni, C.; Valdes, A. M.; Freidin, M. B.; Sudre, C. H.; Nguyen, L. H.; Drew, D. A. et al. Real-time tracking of self-reported symptoms to predict potential COVID-19. Nature Medicine 2020, 26 (7), 1037-1040. https://doi.org/10.1038/s41591-020-0916-2
3. Foster, K. J.; Jauregui, E.; Tajudeen, B.; Bishehsari, F.; Mahdavinia, M., Smell loss is a prognostic factor for lower severity of coronavirus disease 2019. Annals of Allergy Asthma & Immunology 2020, 125 (4), 481-483. https://doi.org/10.1016/j.anai.2020.07.023
4. Parma, V.; Ohla, K.; Veldhuizen, M. G.; Niv, M. Y.; Kelly, C. E.; Bakke, A. J. et al. More than smell - COVID-19 is associated with severe impairment of smell, taste, and chemesthesis. Chemical Senses 2020 45(7):609-622. https://doi.org/10.1093/chemse/bjaa041
5. Yan, C. H.; Faraji, F.; Prajapati, D. P.; Boone, C. E.; DeConde, A. S., Association of chemosensory dysfunction and COVID-19 in patients presenting with influenza-like symptoms. International Forum of Allergy & Rhinology 2020, 10 (7), 806-813. https://doi.org/10.1002/alr.22579
6. Centers for Disease Control and Prevention. Duration of isolation and precautions for adults with COVID-19. https://www.cdc.gov/coronavirus/2019-ncov/hcp/duration-isolation.html
7. Boscolo-Rizzo, P.; Menegaldo, A.; Fabbris, C.; Spinato, G.; Borsetto, D.; Vaira, L. A.; et al. High prevalence of long-term psychophysical olfactory dysfunction in patients with COVID-19. medRxiv 2021, 2021.01.07.21249406. https://doi.org/10.1101/2021.01.07.21249406
Written by Dr. Jane K. Parker FIFST in conjunction with IFST Sensory Science Group