Even after recovering from COVID-19, many individuals continue to experience persistent alterations in taste. Recent research suggests that subtle molecular changes in taste receptor cells, rather than overt damage, may account for the ongoing disruption of sweet, umami, and bitter flavors long after the coronavirus infection has resolved.
A study published in the journal Chemical Senses delves into the mechanisms behind taste dysfunction associated with long COVID-19, shedding light on how SARS-CoV-2 impacts taste perception.
Infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which led to the global COVID-19 pandemic, has been linked to both olfactory and taste dysfunction. Whereas most people recover their taste abilities within weeks, a small subset continues to experience these disturbances for months. Symptoms that persist for over three months are categorized under long COVID-19, too referred to as post-acute COVID-19 syndrome (PACS).
Understanding Taste Dysfunction in Long COVID
The loss of taste, often accompanied by a notable reduction in taste perception, can severely affect individuals’ quality of life, including their appetite and nutritional intake. This necessitates specific interventions for those affected.
Interestingly, the prevalence of these taste-related symptoms was higher with earlier variants of SARS-CoV-2 compared to the Omicron variant. The current study primarily involved individuals infected with these earlier variants, suggesting that variations in taste symptoms may be influenced by different viral affinities for taste receptor cells and diverse immune responses.
Mechanisms Behind Taste Perception Disruption
Taste buds are comprised of epithelial sensory organs that contain up to 100 taste cells, half of which are taste receptor cells. These cells are specialized to correspond to specific taste subtypes, such as sweet, umami, and bitter. The functioning of these type II receptor cells involves PLCβ2-mediated signaling, which is triggered by various specific receptors like TAS1R2/R3 for sweet and TAS1R1/R3 for umami taste.
Type II cells also express ACE2 and TMPRSS2, the receptors that allow SARS-CoV-2 to attach and enter the host’s epithelial cells. This interaction might contribute to the damage observed in taste buds.
The study included 28 individuals diagnosed with COVID-19 who reported persistent taste dysfunction for over a year. To objectively assess taste dysfunction, researchers utilized the Waterless Empirical Taste Test (WETT), which quantitatively measures taste across the five basic categories: sweet, bitter, umami, sour, and salty. Biopsies of fungiform taste papillae were conducted on 20 participants to evaluate the structure of the taste buds.
Findings on Taste Loss and Molecular Changes
All participants reported normal taste function prior to contracting COVID-19, with a mean age of 50 years. The duration from initial infection to the taste assessment ranged from 13.5 to 27 months. While only three subjects had overall taste scores below the tenth percentile, 11 participants had completely lost one or more of the basic tastes, highlighting that a normal overall taste score does not rule out specific taste dysfunction.
The study revealed selective losses primarily involving sweet, umami, and bitter tastes. This was linked to lower expression levels of the mRNAs for PLCβ2 and TAS1R3, which are crucial for type II taste cell function. Though the individual scores for sweet and umami did not correlate directly with mRNA expression, a significant relationship was established when their scores were averaged.
Implications and Future Research Directions
While most taste papillae appeared structurally normal, some showed signs of disorganization, with PLCβ2-positive cells identified in biopsies from long COVID-19 subjects but absent in normal individuals. This suggests that long-term taste dysfunction is a measurable consequence of COVID-19, often associated with disruptions in the PLCβ2-dependent taste pathway.
Interestingly, the study did not detect viral RNA in the biopsied taste papillae despite the presence of ongoing taste dysfunction. This raises questions about potential mechanisms, such as reinfection, delayed viral clearance, or inflammation, which might impede the regrowth of taste buds.
The researchers also postulate that lasting molecular changes, including a phenomenon known as “inflammatory memory,” could be responsible for prolonged taste alterations. This suggests that further investigation into the epigenetic effects of COVID-19 on taste perception is warranted.
The study has its limitations, being a retrospective analysis without a control group and relying on self-reported pre-pandemic taste function. Nonetheless, its combination of subjective and objective assessments provides valuable insights into the long-term impacts of COVID-19 on taste.
Next Steps in Understanding Long COVID Taste Loss
the research underscores the prevalence of long-term taste loss in individuals with long COVID-19, emphasizing the need for larger, controlled studies to better understand the underlying mechanisms and prevalence of this condition. As scientists continue to explore these pathways, it remains crucial to address the quality-of-life impacts faced by those still grappling with taste dysfunction.
For those experiencing similar symptoms, sharing experiences and seeking professional guidance can be beneficial as the medical community works towards effective interventions. Comments and shares on this topic are encouraged to foster awareness and support among affected individuals.
Disclaimer: This article is intended for informational purposes only and is not a substitute for professional medical advice.
