Why COVID-19 takes away smell
The mechanism of anosmia and parosmia after COVID
The virus does not hit the neurons
For a long time it seemed obvious: if smell disappears after COVID, the virus must be killing olfactory neurons. It turned out not to.
In 2020 a group led by David Brann worked out which cells of the olfactory epithelium actually carry the proteins SARS-CoV-2 uses to enter a cell — above all the ACE2 receptor. The olfactory neurons themselves had practically none. They were abundant on the supporting cells — the sustentacular cells — and on vascular and gland cells.
So the virus damages the neuron's surroundings, not the neuron. Supporting cells maintain the environment: they keep the mucus layer at the right composition, supply the neuron, and support the cilia that carry its receptors. When that environment collapses, the neuron is physically intact but cannot work.
The good news in this explanation: since the neurons survive, there is something to recover from. The bad news: timing depends on several processes at once, not one.
Why smell comes back
The olfactory epithelium is one of the few nervous tissues that renew themselves throughout life. It contains basal cells that divide and mature into new olfactory neurons to replace those lost.
That is why most people recover their sense of smell on their own within a few weeks after an infection. The supporting layer is rebuilt, the cilia recover, the signal gets through again.
Difficulty starts when the damage reached the neurons as well, and recovery has to begin from scratch — growing the cell and routing its process into the olfactory bulb again.
Where parosmia comes from
Growing back is not enough for a new neuron. Its axon has to reach the bulb and connect to the correct glomerulus — the exact place where the brain expects that component of the odour.
When a whole population of cells recovers at once, some connect in the wrong place. The brain receives a familiar physical stimulus, but it arrives on the wrong wire. Coffee starts smelling of burnt rubber, meat of rot, a favourite perfume becomes unbearable. That is parosmia: not imagination and not a psychological problem, but a wiring error.
Paradoxically, parosmia usually signals that the process is under way: cells are recovering, they are simply connecting crookedly for now. Over time the connections sort themselves out and the distortions fade. Speeding up that sorting is exactly what the differentiation mode is for.
What to expect on timing
The careful answer: for most people, weeks; for some, months; for a few, recovery stays incomplete. Smell training does not guarantee a result, but it improves outcomes statistically, and the earlier it starts the larger the effect.
A reasonable rule of thumb: if smell has not returned on its own within two or three weeks after an infection, it is worth starting training and seeing an ENT specialist. Sudden loss without a blocked nose, one-sided loss, or loss after a head injury are reasons to see a doctor straight away, not in a month.
Sources
- Brann D. H. et al. Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID-19-associated anosmia. Science Advances, 2020. PubMed — the work showing the virus targets supporting cells, not neurons.
- Whitcroft K. L. et al. Position paper on olfactory dysfunction: 2023. Rhinology, 2023. PubMed — what to do about smell loss and when to see a doctor.
