Long covid: what we know about the loss of smell and taste



CNN

Imagine waking up one morning after recovering from Covid-19 to find your coffee smells like unwashed socks, your eggs smell like feces and your orange juice tastes metallic. Oddly enough, that’s a good thing: It’s a sign that you still have a working sense of smell, even if it’s wired incorrectly in your brain.

Your ability to smell may also disappear completely, a condition called anosmia. Without warning, you won’t be able to breathe in the sweet scent of baby’s skin, the roses your partner has given, or the pungent stench of your workout clothes.

Taste and smell are intertwined, so food can be bland or tasteless. Appetite and zest for life can decline, which previous studies show can lead to nutritional deficiencies, cognitive decline and depression.

Danger also lurks. Without smell, you may not recognize the telltale signs of fires, natural gas leaks, toxic chemicals, or spoiled food and drink.

That’s the reality of about 5% of global Covid-19 survivors who have now developed long-term taste and odor problems, according to a 2022 study. More than two years after the pandemic, researchers found that an estimated 15 million people still have problems have difficulty perceiving smells, while 12 million have difficulty with taste.

Support and advocacy groups such as AbScent and Fifth Sense have mobilized to help, with affirmation and hope, tips on scent training, and even recipes to stimulate appetite.

Fragrance or olfactory training encourages people to sniff essential oils twice a day, said rhinologist Dr. Zara Patel, a professor of otolaryngology, head and neck surgery at Stanford University School of Medicine.

“The way I explain it to patients is that if you had a stroke and your arm didn’t work, you would go to physical therapy, you would rehabilitate,” Patel said. “That’s exactly what smell training is for your sense of smell.”

As science learns more about how Covid-19 attacks and disrupts odor, “I think you’re going to see interventions that are more targeted,” said rhinologist Dr. Justin Turner, associate professor of otolaryngology, head and neck surgery at Vanderbilt University Medical Center in Nashville.

Anyone who is still struggling with a loss of smell and taste “must think positively and assume their sense of smell will return,” Turner said. “Yes, there are people who will not recover, so for those people we don’t want them to ignore it. We want them to take it seriously.”

For centuries people have lost their sense of smell and taste. Cold and flu viruses, nasal polyps, thyroid disease, severe allergies, sinusitis, and neurological conditions such as: Alzheimer’s disease, Parkinson’s disease and multiple sclerosis can all impair the sense of smell and taste — sometimes permanently.

So are head trauma, exposure to harmful chemicals, cancer treatments, smoking, gum disease, antibiotics and various blood pressure, cholesterol, reflux and allergy medications, according to the Cleveland Clinic.

Some people who have had Covid-19 have long-term problems with their sense of smell.

Aging is a major cause of smell loss as the ability of the olfactory neurons to regenerate decreases. A 1984 study found that more than 50% of people between the ages of 65 and 80 suffered from “severe olfactory disorders.” The number rose to more than 75% for people over 80.

As the virus causing Covid-19 invaded our lives, a condition that was relatively rare among people under 50 spread exponentially, affecting all ages.

“Covid-19 affected younger people much more than other forms of post-viral smell loss,” he said surgeon dr. Eric Holbrook, associate professor of otolaryngology and head and neck surgery at Harvard Medical School. “You wouldn’t see much scent loss in the pediatric population, for example, and now it’s very common.”

In fact, loss of smell was so widespread at the start of the pandemic that it was considered the canary in the coal mine – an early sign of Covid-19 infection, even if there were no other symptoms.

That is not true today. A study published in May found that 17% of people lost their sense of smell when infected with the Omicron variant, which predominant variant of the virus causing Covid-19 at the end of 2021. (this may change again) if the virus mutates.)

By comparison, people who got sick from the two original varieties, Alpha and Beta, were 50% more likely to lose their sense of smell or taste. Delta was almost as bad – 44% of people were affected, according to the study.

Statistics show that most people recover their taste and smell. An August analysis of 267 people who had a loss of smell and taste at least two years ago found that the majority fully (38.2%) or partially (54.3%) recovered their sense of smell and taste. That was especially true for people under 40, according to the study.

only 7.5% two years after their Covid-19 infection cleared, their sense of smell and taste had not recovered. Those least likely to recover were those with existing nasal congestion, more women than men, and those with greater initial severity of smell loss, the study found.

How does Covid-19 damage the olfactory system? Initially, scientists believed it infected neurons in the nose that are responsible for transmitting odors from the environment to the brain. Those neurons sit in the olfactory bulbs at the very top of each nostril and send axons or cables to unique sensory sites in the brain.

Soon, studies found that the virus doesn’t invade those neurons at all. Instead, it attacks sustentacular cells, also known as support cells, which provide nourishment and protection to nerve cells from birth. Unlike many other cells, neurons in the nose undergo a rebirth every two to three months.

“(Covid-19) infection of those supporting cells probably has some sort of long-term effect on those neurons’ ability to regenerate themselves over time,” Turner said.

“That’s one of the reasons we sometimes see a delayed effect. People can have some smell loss that recovers, and later they have a second wave of smell loss, parosmia, or other symptoms because that regenerative ability isn’t functioning properly,” he said.

Parosmia is the medical term for distorted smells, which can often be quite disgusting, Patel said.

“Unfortunately, there are these classic categories of really awful smells and tastes,” she said. “Sometimes it’s feces, garbage or old dirty socks. There may be some kind of sickly sweet chemical smell and taste. Oh, and rotting flesh is another common category.”

For many people, parosmia tends to occur or recur after three months, about the time olfactory neurons would naturally regenerate, experts told CNN.

“If the reconnection misses its target and hits another spot in the brain reserved for another smell, your perception of smell will be totally ruined,” Holbrook said.

“You have to rely on those axons’ ability to retract and then find their way to the right place,” he added. “Or if they’re not correct, wait for those neurons to die and let new ones come back and find the right spot.”

Science continues to discover ways in which the virus attacks. A February study found that it can also damage the olfactory receptors that sit on the surface of nerve cells in the nose. Those receptors bind odors and activate the nerve impulses that transmit the information to the brain.

There may also be a genetic component. A January study discovered a mutation in two overlapping genes, UGT2A1 and UGT2A2, which play a role in metabolizing odors. People with that mutation may be more prone to losing their sense of smell, but further research is needed to diagnose the viral disease association to the genes – if there are any.

People who are older and have chronic diseases that affect the nervous system, such as diabetes, are often more susceptible to olfactory damage, Patel said.

“It’s the very small blood vessels in the body, including the nose, that are affected by diabetes, disrupting the blood, nutrition and oxygen supply to these olfactory nerves,” she said. “People with chronic sinus or allergic inflammation in the nose — anything that makes it harder for our system to bounce back is also likely to be at greater risk.”

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