Why do some people not smell months after catching Covid? Their immune response could be the answer

For some, Covid knocked out the sense of smell for months without a precise scientific explanation of why and without therapeutic drugs specifically designed to restore it.

A research team led by Duke Health may be close to some answers.

Researchers have taken biopsies deep into the nasal passages of patients struggling to regain their sense of smell months after being infected with Covid. They found evidence of continued inflammation and an immune response that persisted after the virus was long gone, according to a small study published Wednesday in the journal Science Translational Medicine.

Millions of people around the world have lost their sense of smell for at least six months after contracting Covid. This new research could help develop specific drugs to address the problem in more difficult cases and could also help scientists better understand how long Covid affects other organ systems.

“I think this answers the question, ‘What is different about these people? What’s damaged and how might we fix it,’” said Dr. Bradley Goldstein, an author of the paper and an associate professor in Duke University’s department of head and neck surgery and communication sciences. “We clearly see a persistent unresolved local immune response.”

The study evaluated biopsy tissue from the olfactory linings of 24 people, including nine who lost their sense of smell for at least four months. The researchers collected data on tens of thousands of cells for individual patients, giving them specific insight into what was happening at the cellular level.

The system that replaces olfactory neurons appears to be vulnerable to ongoing damage from inflammation, Goldstein said. Those struggling to regain their smell had far fewer olfactory nerve cells than typical, and far more local immune cells in the olfactory lining where scientists wouldn’t normally expect to find them.

The analysis is small and more research is needed, said Dr. Aria Jafari, a rhinologist at UW Medicine Sinus Center in Seattle who was not involved in the study. But she added that the study does a good job of describing and providing evidence for a possible mechanism that would explain what she sees regularly in her clinic.

“That’s really what’s been missing in our observations of patients with smell loss — why,” she said. “I think this is the first step in really developing therapies in this patient population.”

Loss of taste and smell can have a profound impact on people’s lives. Loss of smell has been linked to higher mortality rates in the elderly and can have a major impact on people’s emotional and psychological well-being.

A study published this summer in The BMJ found that around 5% of patients with confirmed cases of Covid – around 27 million people worldwide at the time – suffered from a “persistent dysfunction” in taste or smell which lasted more than six months.

Therapies such as a steroid nasal rinse or smell training — in which patients sniff essential oils such as lemon, clove, eucalyptus and rose twice a day to stimulate different types of neurons — have had some success in restoring the sense.

Jafari said that over time it has become clear that a small percentage of patients suffer from a persistent loss of smell that lasts more than 18 months post-Covid.

“Those patients are more difficult to treat,” he said. “Smell training works best in patients within the first 18 months of the loss, and we don’t have many good options after those 18 months.”

As a next step, Goldstein said, researchers or pharmaceutical companies could try to develop drugs that block specific immune responses or that promote neuron repair.

Goldstein and Jafari also said the study findings could provide clues about how long Covid cases are affecting other body systems as well.

“There aren’t many studies that have obtained biopsies of organs affected by Covid for a long time,” Goldstein said. “Are there immune cell mechanisms … involved in other long Covid problems?”

This article was originally published on NBCNews.com

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