CNN
—
Last winter was a tough one for respiratory viruses, led by outbreaks of RSV, influenza and Covid-19. But as it wanes, a little-known virus that causes many of the same symptoms — a lower lung infection, hacking cough, runny nose, sore throat and fever — is just picking up steam.
Cases of human metapneumovirus, or HMPV, spiked this spring, according to the US Centers for Disease Control and Prevention’s respiratory virus surveillance systems. It was filled hospital intensive care units in young children and the elderly who are most vulnerable to these infections. At its peak in mid-March, nearly 11% of tested specimens were positive for HMPV, a number about 36% higher than the average, before the pandemic. seasonal peak of 7% test positivity.
Most people who catch it probably don’t know they have it, however. People with the disease are not usually tested for it outside of the hospital or ER. Unlike Covid-19 and influenza, there is no vaccine for HMPV or antiviral drugs to treat it. Instead, doctors care for seriously ill people by treating their symptoms.
An underestimated threat
Studies show that HMPV causes as much suffering in the US each year as the flu and a closely related virus, RSV. A study of patient samples collected over 25 years found that it is the second most common cause of respiratory infections in children behind RSV. A New York study conducted over four winters found it to be as common among hospitalized adults as RSV and the flu. Like those infections, HMPV can lead to intensive care and fatal cases of pneumonia in adults.
Diane Leigh Davison contracted human metapneumovirus at a family celebration in early April. After two weeks, he was coughing so hard, he couldn’t talk on the phone.
“I couldn’t get more than a couple of words out,” said Davison, 59, an entertainment attorney in Baltimore. “I would go into violent, violent coughing fits to the point where I almost threw up.”
His coughing was so constant and deep, he was convinced that he had finally caught the coronavirus after managing to avoid it throughout the pandemic. But he took six rapid tests for Covid-19, and all came back negative.
Davison is immunocompromised, so he has been cautious throughout the pandemic. Worried about pneumonia, she got an X-ray from a radiology clinic near her home and was told it was clear.
His doctor was not satisfied, however, and sent him to an emergency room for further evaluation. Blood tests revealed that he had HMPV.
“I was like, ‘what?’ Because it’s really scary,” said Davison. “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 were very sick and required mechanical ventilation, but they did not test positive for any known pathogen.
The researchers cultured samples of different types of cells from monkeys, chickens and dogs, and then they looked at the cultures under an electron microscope. They found something that appeared to be structurally related to the paramyxoviridae family, a group of viruses known to give humans respiratory illnesses such as measles, mumps and respiratory syncytial virus, or RSV.
A close look at the virus’s genes revealed a close relative: avian metapneumovirus, which infects birds. The new virus is called 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 stored since 1958, all showed evidence of exposure to the mystery virus, indicating that it has been circulating in humans, undetected, for at least in the past half century.
Doctors and patients in the dark
Respiratory infections are the leading cause of death in children worldwide and the No. 1 reason children are hospitalized in the United States, but scientists don’t know what causes a large portion of them, said Dr. John Williams, a pediatrician at the University of Pittsburgh who has spent his career researching vaccines and treatments for HMPV.
Williams said there were extensive epidemiological studies conducted in the 1950s and ’60s, looking at the causes of respiratory infections.
“In general, they can only identify a virus in humans about half the time. And so the question is, ‘OK, what about that half?’ ” he said. Human metapneumovirus does not account for all unknown viruses, but it is a significant proportion – almost as many cases of RSV or influenza.
But no one knows about it. Williams called it “the most important virus you’ve never heard of.”
“Those are the three main viruses,” he said. “That’s the big three in children and the elderly, are the most likely to put people in the hospital and cause severe illness, are likely to sweep into nursing homes and make the elderly really sick and even kill them.”
Because testing for HMPV is rarely done outside of hospitals, it is difficult to know the true burden of the disease.
Blood tests indicate that most children have it by age 5.
A 2020 study in Lancet Global Health estimated that among children under 5 years of age, there were more than 14 million HMPV infections in 2018, more than 600,000 hospitalizations and more than 16,000 deaths.
Infection generates weak or incomplete immune protection, however, and people are reinfected throughout their lives.
Companies are developing vaccines against it. Covid-19 vaccine maker Moderna has just completed early studies of an mRNA vaccine against HMPV and parainfluenza, according to the website clinicaltrials.gov.
The CDC recommends that doctors consider testing for HMPV in the winter and spring, when it tends to increase.
Doctors don’t test for it because of a lack of awareness of the virus, Williams said, but also because a test probably won’t change the care they give a patient. This will help them rule out other causes with dedicated treatments, such as Covid or the flu.
Davison said HMPV gave him severe bronchitis. He was admitted to the hospital briefly for observation. Eventually he recovered, but he was ill for a month.
He’s had respiratory infections before, of course, but he’s particularly glad to be on the other side of the human metapneumovirus, he says – “It’s really kind of the worst I’ve ever had.”
