Why does covid cause long term loss of smell? Scientists have a theory.

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Persistent loss of smell has some COVID-19 survivors looking forward to smell of your freshly bathed child or a bit of your favorite meal. Got other people used to the stink of garbage and accidentally drink spoiled milk. “Anosmia,” as experts call it, is one of the strangest symptoms of the long-running covid — and researchers may be one step closer to figuring out what causes it and how to fix it.

A small study published online Wednesday in Science Translational Medicine and led by researchers at Duke University, Harvard and the University of California San Diego offers theory and new insight into persistent loss of smell.

The scientists analyzed samples of olfactory epithelial tissue – where olfactory cells live – from 24 biopsies, nine of which were from post-Covid patients struggling with persistent loss of smell. Although the sample was small, the results suggest that the sensory deficit is linked to an ongoing immune attack on the cells responsible for smell – which persists even after the virus is gone – and a decline in the number of olfactory nerve cells.

Bradley Goldstein, an associate professor in Duke’s Department of Head and Neck Surgery and Communication Sciences and Department of Neurobiology, author of the paper, called the results “impressive” and said in a statement: “It’s almost similar to a kind of autoimmune disease process in the nose.”

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While there has been research that looks at short-term smell loss and uses animal models, the new study is notable because it focuses on persistent smell loss and uses high-tech molecular analysis on human tissue.

The study reflects the enduring interest in the mysterious symptom. In July, researchers estimated that at least 5.6% of COVID-19 patients develop chronic smell problems. That study, published in the peer-reviewed medical journal BMJ, also suggested that women, as well as those with more severe initial dysfunction, were less likely to regain their sense of smell. Seniors are also especially vulnerable, The Post reported.

Earlier this month, another small study of COVID-19 patients suggested that long-term olfactory dysfunction could lead to changes in brain regions corresponding to smell. A study published in February, on which the Duke study is based, found that individuals who died from Covid-19 had fewer membrane-embedded receptors for detecting odors than expected.

Loss of smell can have significant implications. It’s a mechanism for detecting threats – from the gas stove you accidentally left on to the stomach-churning smell of rotten eggs. And it is a sense closely associated with memories.

Carol Yan, an otolaryngologist and head and neck surgeon at the University of California San Diego like one of the authors of the new study, treated patients with persistent loss of smell. “It’s pretty devastating for them. And often, at this point, it’s been more than two years without a sense of smell,” she says. “They’re asking themselves, ‘Why me? Why do I still have a loss of smell compared to so many friends, colleagues and family who have recovered?’”

Doctors have struggled to explain what causes it. “Clinically, when you look at these patients and look into their noses, everything looks untouched,” she says. “So this is happening on a molecular level.”

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The study offers some hope, Yan says, because while some have suggested that the smell deficit is linked to the central nervous system, this research offers evidence that at least part of the problem is due to inflammation in the nose, where the virus originally attacked. This could mean there is potential for easier topical treatments.

For Yan, the research on the localized immune response supports other research she has done on platelet-rich plasma as a treatment for loss of smell. “What we found in the clinical trial is that PRP is actually more likely to improve outcomes for Covid-19-related smell loss compared to placebo,” she says, cautioning that PRP, which has anti-inflammatory qualities , is not “a silver bullet” and needs further research – but it looks promising.

And the stakes are high. With smell, Yan says, comes your ability to appreciate food and your surroundings. It even affects how you connect with others. “I’ve actually had some patients come to see me and say ‘I’m a little embarrassed to come see you.’ I didn’t think it was a big deal. I just lost my sense of smell, but it actually significantly affected my quality of life.’”