Loss of taste and smell are among the symptoms of COVID that stick around after the infection itself has subsided, and now scientists think they may be on to why the latter happens. It may all come down to an unusual immune response in which T-cells that cause inflammation invade the nasal passages, perhaps helping to decimate crucial nerve cells that enable us to smell things. The Guardian notes that ever since this particular symptom emerged in COVID patients, researchers have been scratching their heads over whether it's caused by damage done to sensory cells in the nasal passages, to centers in the brain that process olfactory information, or both. In research published Wednesday in the Science Translational Medicine journal, scientists looked more closely at activity happening in the nose.
The researchers biopsied tissue from the nasal passages of 24 patients, including nine who lost their olfactory senses for at least four months after contracting the illness; the Wall Street Journal notes some of the patients had smell loss for up to 16 months. They found that, compared with COVID patients who didn't lose their sense of smell, those who did had 75% fewer olfactory sensor neurons. This group also had more T-cells present making the substance interferon-gamma, which is tied to inflammation. This was found to be the case even in patients who no longer had any traces of the virus still in their system. The Journal notes the T-cells might not be attacking the olfactory neurons directly, but rather support cells that the neurons can't survive without. "We think the reduction of sensory neurons is almost definitely related to the inflammation," study co-author Bradley Goldstein says.
Their findings don't mean the brain doesn't play a part, too: Dr. Gwenaelle Douaud, a neuroscientist at Oxford, tells the Guardian that "persistent olfactory problems have been shown to be associated with the shrinking of brain regions related to our sense of smell." She adds that Goldstein's study "provides further evidence that such a specific brain loss could be related to persevering inflammation and loss of olfactory neurons in the nasal cavity itself." Goldstein, meanwhile, notes that to help the 15 million or so who've suffered a loss of smell after COVID infection—as well as other long COVID symptoms that may be tied to inflammation—it's important to figure out the underlying causes. One possible treatment: a T-cell-blocking spray or cream that can easily be applied in the nasal passages. "We are encouraged by these findings and are hopeful that new treatments may emerge," he notes. (Read more COVID-19 stories.)