- 02
- May
- 2024
The long-term effects of Covid-19 continue to pique researchers’ interests. A recent study delves into the enduring loss of smell and taste, one year after exposure to SARS-CoV-2.
The Covid-19 pandemic is far from over, and it seems this reality won't change anytime soon. Scientific studies are continuously yielding new insights into the virus' origin, its variants, and notably its long-term effects. In this regard, the sudden loss of smell (anosmia) and taste (ageusia) emerged early on as significant symptoms, even in the absence of other more typical clinical signs like fever, cough, or respiratory distress.
As initial reports merely mentioned this symptom among others — lacking quantitative data to substantiate these observations — a 2020 study led by Parma et al. corroborated this finding. The study, which involved approximately 4,000 Covid-19 patients from over 40 countries, provided evidence of a correlation between chemosensory deficits (mostly related to smell and taste) and the disease.
Unreliable Data
Anosmia and ageusia have often been reported as significant symptoms during the acute phase of SARS-CoV-2 infection. However, few studies have delved into the gustatory and olfactory functions of Covid-19 patients over an extended period of time, namely one year or more post-infection. Furthermore, many studies have relied on subjective self-reports rather than empirical tests, which provide an objective evaluation of chemosensory functions.
A meta-analysis published in 2023 in a journal edited by Oxford University Press uncovered that loss of taste was a distinct symptom of Covid-19, with an overall prevalence of 36.62%. However, only 3.4% of the studies included in this analysis incorporated empirical taste tests into their protocols.
New Insights
A recent cross-sectional study, published on April 23rd in JAMA Network Open, provides further insight into this matter. Its main objectives were to assess gustatory function using a validated test consisting of 53 items and to compare the results with those of a well-established olfactory test comprising 40 items. The study also aimed to determine the proportion of individuals still experiencing various degrees of gustatory or olfactory dysfunction one year after their initial infection and to explore any association between the predominant viral variant at the time of the initial diagnosis and test results. For this purpose, 774 individuals were selected to participate in the study: 340 had a history of Covid-19 and 434 did not.
Participants were enlisted through advertisements posted on a website or local notice boards in the United States of America. This process occurred between February 2020 and August 2023. On average, there was a 395-day gap between the tests administered and the diagnosis of Covid-19.
According to findings reported by Sharetts et al., gustatory function showed no difference between individuals who had contracted the infection a year earlier and those who had not. However, olfactory impairment was observed in 30.3% of individuals previously affected by SARS-CoV-2, compared to only 21% of those without a history of infection. While women scored higher than men, this difference was not statistically significant.
Strains and Variants
According to the same study, patients infected with the original untyped strain and the Alpha variant showed a higher incidence of olfactory loss compared to those affected by other variants. Among the 42 individuals affected by the Alpha variant, tests revealed that ten of them (23.8%) exhibited severe to complete anosmia, whereas among the 52 individuals who contracted the original untyped strain, seven (13.5%) showed a similar loss. However, among the 214 individuals affected by the Omicron variant, only ten, or 4.7%, suffered from severe to complete olfactory loss. In comparison, 2.8% of the 434 individuals without a history of Covid-19 exhibited this symptom. The authors concluded that newer variants of SARS-CoV-2, particularly Omicron, were associated with less frequent olfactory loss.
Olfactory-Gustatory Disorder
"The reports indicating that the loss of taste persists long after the initial infection are likely largely attributed to the confusion between the flavor of foods, which relies on both taste and smell," as explained by the authors of the publication.
In fact, taste differs from gustation: it involves a more intricate sensory process, primarily delivered by molecules reaching olfactory receptors from the oral cavity via the nasopharynx. Taste buds solely mediate oral perceptions of sweetness, sourness, bitterness, saltiness, and umami (the taste associated with glutamate).
The loss of smell persisted in nearly one-third of individuals exposed to the virus, potentially contributing to taste related complaints reported by many individuals affected by post-Covid-19 syndrome, the researchers concluded.
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