Barie Carmichael lost her sense of taste and smell while traveling in Europe. She remembers keeping a dinner date at a Michelin-starred restaurant but tasting nothing. "I didn't have the heart to tell my host," she said.
It may sound like a case of covid-19. But Carmichael, 72, a fellow at the University of Virginia's business school, lost her ability to taste and smell for three years in the 1990s. The only respiratory infection she'd had was bronchitis.
Medical scientists say that although the complications of covid have riveted peoples' attention, many symptoms -- like a loss of smell -- are not unique to covid. Heart inflammation, lung and nerve damage, and small blood clots in the lining of lungs occur in a small but noticeable percentage of patients who have had other respiratory and viral infections. And these patients, too, can have their own version of "long covid."
No one is saying covid is the equivalent of, say, the flu that circulates each year. The usual seasonal flu has not killed millions of people worldwide in a single year, including more than half a million Americans, while upending society and ravaging economies. But covid-19 is providing a new opportunity to understand the complications of many common viral infections.
Before the pandemic, research grants to study a loss of smell were hard to come by, said Danielle Reed, associate director of the Monell Chemical Senses Center, a nonprofit research group in Philadelphia.
"It seemed like nobody cared," she said. But now, "there is an explosive growth of interest among funders." (She added that most who say they have lost a sense of taste have really lost a sense of smell.)
Monell researchers want to compare how often people lose their sense of smell after a bout with the flu versus a bout with covid-19 -- and how long the loss lasts. Is there a genetic predisposition to this complication?
Researchers at other institutions want to know who is susceptible to heart infections, blood clots or lung damage after having a respiratory virus like the flu. For the most part, little is known. Part of the problem was that only a minority of patients with respiratory viruses were affected with these conditions, and until the coronavirus, that tended not to be a big number. Many of these effects were noticed but then forgotten.
Heart problems following a viral infection are among the best studied. Every year, myocarditis -- an inflammation of the heart muscle -- affects as many as 1.5 million people worldwide, most of whom had a previous respiratory virus infection. Most recover fully.
But symptoms like fatigue are often not recognized as being related to myocarditis. And Dr. Bruce McManus, an emeritus pathology professor at the University of British Columbia, suspects that the fatigue that sometimes follows a bout with covid-19 might be caused by this heart problem.
"We think of covid-19 and influenza as respiratory diseases, and in fact they are," McManus said. "But the reason many patients reach their demise in many instances is myocardial."
Some severely ill covid patients have lung damage. That, too, can also occur with other viruses, said Dr. Clemente Britto-Leon, a lung researcher at Yale School of Medicine. He lists some possibilities.
"You can have lung injury and scarring with influenza, with herpes viruses and with cytomegalovirus infections, for example," he said, referring to a common virus that usually causes no symptoms. All these viruses can wreak damage on rare occasions, he said. "You can have a very severe injury and a lot of tissue destruction."
Influenza can cause blood clots in the lining of the lungs that look just like the small clots seen in the lungs of some covid patients, said Marco Goeijenbier of Erasmus University in the Netherlands. It happens when flu viruses infect the lower respiratory tract, an unusual event because most people have some preexisting protective immunity.
Goeijenbier wants to study the blood clots that occur in these cases. Previously, with so few patients, he and others resorted to reproducing and studying the effect in laboratory studies and in ferrets -- the preferred animals to study flu.
"It was hard to get funding," he said. "Big journals or funders didn't think it was interesting enough."
Covid is changing that.
There is now "a huge cohort of people to study," said Pamela Dalton, a smell researcher at Monell. But "the big question is, even if you learn everything about SARS-CoV-2" -- the formal name of the coronavirus -- "how generalizable is it?"