Human metapneumovirus: Doctors say it’s the biggest virus you’ve never heard of


Last winter was heavy for respiratory viruses, dominated by outbreaks of RSV, influenza and Covid-19. But just as it was ending, a little-known virus that causes many of the same symptoms — a lower lung infection, dry cough, runny nose, sore throat and fever — was just picking up steam. magnitude.

Cases of human metapneumovirus, or HMPV, rose this spring, according to the U.S. Centers for Disease Control and Prevention’s respiratory virus surveillance systems. It has filled hospital intensive care units with young children and the elderly who are most vulnerable to these infections. At its peak in mid-March, nearly 11% of samples tested were positive for HMPV, a number about 36% higher than the pre-pandemic average seasonal peak of 7% test positivity.

However, most people who caught it probably didn’t even know they had it. Sick people are generally not tested outside of a hospital or emergency room. Unlike Covid-19 and the flu, there is no HMPV vaccine or antiviral drugs to treat it. Instead, doctors treat seriously ill people by attending to their symptoms.

An underestimated threat

Studies show that HMPV causes as much misery in the United States each year as the flu and a closely related virus, RSV. A study of patient samples collected over 25 years found it to be the second leading cause of respiratory infections in children behind RSV. A study in New York over four winters found it to be as common among hospitalized seniors as RSV and the flu. Like these infections, HMPV can lead to intensive care and fatal cases of pneumonia in the elderly.

Diane Leigh Davison caught human metapneumovirus at a family party in early April. Two weeks later, she was coughing so violently that she could no longer talk on the phone.

“I couldn’t get more than a few words out,” said Davison, 59, a Baltimore entertainment attorney. “I would go into a violent, violent cough to the point where I was literally almost vomiting.”

Her cough was so constant and deep that she was convinced she had finally caught the coronavirus after managing to avoid it throughout the pandemic. But she took six rapid tests for Covid-19, and all came back negative.

Davison is immunocompromised, so she has been cautious throughout the pandemic. Concerned about pneumonia, she had an x-ray at an x-ray clinic near her home and was told it was clear.

However, her doctor was not satisfied and sent her to the emergency room for further tests. Blood tests determined she had HMPV.

“I was like, ‘what?’ Because it sounds really disastrous,” Davison said. “I’ve never heard of it.”

Human metapneumovirus was discovered by Dutch virus hunters in 2001. They had 28 samples from children in the Netherlands with unexplained respiratory infections. Some of the children had been very ill and needed mechanical ventilation, but they did not test positive for any known pathogens.

The researchers cultured the samples in various cell types from monkeys, chickens and dogs, then examined the cultures under an electron microscope. They saw something that seemed structurally related to the Paramyxoviridae family, a group of viruses known to give people respiratory illnesses like measles, mumps, and respiratory syncytial virus, or RSV.

Closer examination of the virus’ genes revealed a close relative: avian metapneumovirus, which infects birds. The new virus has been dubbed the human metapneumovirus. Scientists believe it likely jumped from birds to humans at some point and evolved from there.

When researchers tested blood samples from 72 patients who had been in storage since 1958, all showed evidence of exposure to the mysterious virus, indicating that it had been circulating in humans, undetected, for at least half a year. -previous century.

Doctors and patients in the dark

Respiratory infections are the leading killer of children worldwide and the number one reason children are hospitalized in the United States, but scientists don’t know what causes much of it, says Dr John Williams, pediatrician at the University of Pittsburgh who has spent his career researching vaccines and treatments for HMPV.

Williams says there were extensive epidemiological studies conducted in the 1950s and 1960s looking at the causes of respiratory infections.

“Basically, they could only identify a virus in people about half the time. And so the question was, ‘OK, what about that other half?’ ” he said. Human metapneumovirus does not represent all unknown viruses, but it is a significant proportion – about as many cases as RSV or influenza.

But no one knows. Williams calls it “the most important virus you’ve never heard of.”

“Those are the three main viruses,” he said. “These are the big three of children and adults, most likely to send people to hospital and cause serious illness, most likely to sweep up nursing homes and make old people really sick and even killed.

Since testing for HMPV is rarely done outside of hospitals, it is difficult to know the true burden of the disease.

Blood tests show that most children have had it before age 5.

A 2020 study in the Lancet Global Health estimated that among children under 5 there were over 14 million HMPV infections in 2018, over 600,000 hospitalizations and over 16,000 deaths.

The infection, however, generates weak or incomplete immune protection and humans are reinfected throughout their lives.

Companies are working on vaccines against it. Covid-19 vaccine maker Moderna has just completed an initial study of an mRNA vaccine against HMPV and parainfluenza, according to the website.

The CDC recommends doctors consider testing for HMPV in winter and spring, when it tends to peak.

Doctors aren’t testing it primarily because of a lack of awareness of the virus, Williams said, but also because a test likely wouldn’t change the care they would give a patient. This would help them rule out other causes that have dedicated treatments, like Covid or the flu.

Davison said HMPV caused him severe bronchitis. She was briefly admitted to the hospital for observation. She eventually got better, but she was sick for a month.

She’s had respiratory infections before, of course, but she’s especially happy to be on the other side of the human metapneumovirus, she said – “It really was the worst I’ve ever had.”

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