Influenza, commonly known as the flu, is a respiratory illness caused by a virus that affects the nose, throat, and sometimes the lungs. Typical symptoms include fever, body aches, and cough. A change in the ability to perceive scents and flavors can also occur during an influenza infection. This sensory disturbance is generally temporary and linked to the body’s reaction to the virus.
The Link Between Influenza and Sensory Loss
It is possible to experience a reduction in smell and taste perception with the flu, though it is usually a minor symptom compared to the more severe, systemic issues. Studies suggest that influenza viruses can cause some degree of olfactory dysfunction in approximately 25% of infected individuals. The loss of smell is termed anosmia, and reduced ability is called hyposmia.
The diminished sense of flavor is mostly due to the compromised sense of smell, as the two senses work together to create the full perception of flavor. Taste receptors on the tongue only detect five basic qualities: sweet, sour, salty, bitter, and umami. The complex array of flavors we experience comes from scent molecules reaching the olfactory system.
When a person has the flu, the sensory loss is typically partial, meaning the senses are dulled rather than completely absent. The severity and duration of this sensory change are generally mild and brief compared to other viral causes of anosmia.
The Biological Mechanism of Temporary Loss
The primary reason influenza causes a change in smell is a physical blockage known as conductive olfactory loss. The immune response triggers inflammation and swelling within the nasal and sinus passages, a condition called rhinitis or sinusitis. This swelling, along with increased mucus production, physically obstructs the pathway for odor molecules.
Scent molecules, called odorants, must travel through the nostrils to the olfactory epithelium, a patch of tissue high inside the nasal cavity. Olfactory sensory neurons, which contain the receptors for smell, are located within this tissue. When the nasal passages are congested, odorants cannot reach these receptors, blocking the signal to the brain.
This physical obstruction immediately causes food to taste bland because flavor is derived primarily from smell. The taste buds themselves are usually still functioning, but the brain receives insufficient information to register complex flavors. This mechanism explains why the loss of smell and taste often coincides with the peak of nasal congestion during the flu.
While less common, some influenza strains may cause a sensorineural loss, involving direct damage to the olfactory nerves or supporting cells. The influenza virus can potentially invade and damage the cells that maintain the health and function of the olfactory neurons. However, in flu cases, this direct nerve damage is a rare occurrence, with most sensory loss being purely conductive and temporary.
Differentiating Sensory Loss Caused by Other Respiratory Illnesses
Understanding the cause of sensory loss is important because the presentation differs among respiratory illnesses, particularly the common cold and COVID-19. The common cold, like the flu, causes conductive loss due to nasal congestion. Cold symptoms are often milder, meaning the associated smell loss is typically brief and less pronounced.
The sensory loss associated with COVID-19, caused by the SARS-CoV-2 virus, frequently presents differently from flu-related loss. COVID-19 often causes a sudden and complete loss of smell (anosmia), which can occur even without significant nasal congestion. This presentation suggests a more direct attack on the supporting cells of the olfactory neurons, classifying it as a sensorineural injury.
The prevalence also differs, with a higher proportion of COVID-19 patients reporting anosmia compared to those with influenza. Some studies found that the incidence of sensory disorder was over 11 times higher in COVID-19 cases compared to flu cases. The nature of the sensory loss—sudden and complete versus gradual and partial—can offer a clue to the underlying cause, although testing is needed for a definitive diagnosis.
Recovery Timelines and When to Consult a Doctor
For sensory loss linked to influenza, the prognosis for recovery is generally positive. Because the cause is typically nasal blockage, the sense of smell usually returns quickly once the inflammation and congestion subside. Most people notice their sense of smell and taste return to normal shortly after the other flu symptoms clear up, often within one to two weeks of symptom onset.
The majority of patients who experience this temporary loss report a complete recovery within 14 days. If the sensory loss persists beyond the resolution of other flu symptoms, or if it lasts for longer than three to four weeks, consult a healthcare professional. Persistent anosmia, regardless of the cause, warrants a medical evaluation to rule out other underlying issues.
A doctor, often an ear, nose, and throat specialist (otolaryngologist), can determine if the persistent loss is due to prolonged inflammation or a rarer, sustained nerve injury. They can offer guidance on potential treatments, such as nasal steroid sprays, or discuss options like olfactory training to help stimulate recovery.