Loss of smell, medically known as anosmia, emerged as a distinctive symptom of COVID-19, the disease caused by the SARS-CoV-2 virus. Unlike the temporary smell loss experienced with common colds, which often results from nasal congestion, COVID-19 related anosmia frequently occurs without a blocked nose or significant congestion. This unique presentation made it a notable indicator of infection, differentiating it from other viral respiratory illnesses. It quickly became recognized as a prominent neurological symptom, affecting a significant portion of individuals infected with the virus.
Typical Onset
The onset of smell loss in COVID-19 patients often occurs early in the infection, frequently as one of the initial symptoms. Some studies indicate that anosmia can even precede other symptoms of COVID-19. The median time from general symptom onset to smell loss is typically 2 to 3 days, though some research suggests it may appear slightly later, around 3.4 to 5 days.
For some individuals, this loss of smell can be the only symptom of COVID-19, particularly in mild or asymptomatic cases. The timing and severity of anosmia can vary widely among individuals, reflecting the diverse ways the virus affects different people. It has been noted that the severity of smell loss might correlate with the severity of other COVID-19 symptoms, such as shortness of breath, fever, and cough.
How COVID Affects Smell
COVID-19 causes smell loss through a mechanism distinct from other viral infections. The SARS-CoV-2 virus does not typically infect the olfactory sensory neurons directly. Instead, it targets the supporting cells within the nasal cavity’s olfactory epithelium. These sustentacular cells, which provide metabolic and structural support to the olfactory neurons, express the ACE2 receptor that the virus uses for entry.
When these supporting cells are infected, it triggers an inflammatory response. This inflammation involves immune cells like microglia and T cells, which release proteins that can alter the genetic activity of the olfactory nerve cells. The damage to these supporting cells disrupts their ability to maintain the necessary environment for the olfactory neurons, indirectly impairing the sense of smell. This indirect effect on the neurons, rather than direct infection, helps explain why smell loss can be temporary for many individuals, as supporting cells can regenerate.
Duration and Recovery
The duration of smell loss due to COVID-19 varies among individuals. For many, it is a temporary condition, with significant improvement often seen within four weeks. Approximately 90% of affected individuals can expect their sense of smell to improve within this timeframe. Overall, about 95% of people with COVID-19 related smell loss experience improvement within six months to a year.
During recovery, some individuals may experience distorted smells (parosmia), where familiar odors become unpleasant, or phantosmia, smelling things not present. Parosmia typically appears two to three months after infection, as smell begins to return. While distressing, these altered perceptions are generally part of the recovery process, though they can persist for weeks or months. Long-term smell loss, lasting months to years, can be linked to ongoing inflammation and reduced olfactory nerve cells.
When to Seek Medical Advice
If smell loss persists beyond a few weeks or significantly impacts daily life, consulting a healthcare professional is advisable. This is important to rule out other potential causes of anosmia, as various conditions beyond COVID-19 can affect the sense of smell. Healthcare providers can assess the situation and recommend appropriate steps.
For prolonged cases, interventions such as smell training may be suggested. This involves regularly smelling a set of specific scents, like rose, lemon, eucalyptus, and clove, to help retrain the olfactory system. Smell training can be performed twice daily for several weeks or months. The earlier smell training is started, the more beneficial it may be.