What is human metapneumovirus, the infectious respiratory disease on the rise this year?

John V Williams is Professor of Paediatrics, Microbiology and Molecular Genetics at the University of Pittsburgh.

In 2000, Dutch scientists embarked on a fact-finding mission – not to discover lands or riches, but to identify unknown causes of acute respiratory infections.

These diseases, from the common cold to pneumonia, have plagued mankind throughout history. Most are caused by viruses. So if a clinician has ever told you, “You probably have a virus,” they’re probably right. However, respiratory illnesses can be much more serious than the common cold.

Respiratory infections are the leading cause of death in children under the age of 5 worldwide and a major reason for children being hospitalized in developed countries. They are also a leading cause of illness and death in people at high risk of serious illness, such as premature babies, older adults and those with underlying conditions.

However, meticulous research studies by many groups over decades had not been able to identify a virus or bacterium in every person with an acute respiratory illness. Is this failure to detect a microbe due to tests that weren’t good enough, or to viruses that doctors and scientists didn’t know about? The answer was partly the first; Modern molecular tests are much better, allowing doctors to find more known viruses.

But the Dutch group discovered a new virus, human metapneumovirus, abbreviated HMPV or MPV, which is emerging as a major cause of respiratory infections. HMPV often presents like others common respiratory viruseswith constipation, cough and fever.

As a pediatric infectious disease specialist and virologist, I have led my team in HMPV research for over 20 years and have personally cared for many children with this infection. I have received emails from colleagues, doctors and parents from across the country and around the world with questions about serious and tragically fatal cases.

The US saw a spike in HMPV detections in the early months of 2023. This trend is similar to the higher-than-average fall rates of Respiratory syncytial virus or RSVand influenza in autumn 2022 and winter 2023, likely associated with reduced population immunity after two years of face mask wearing and social distancing.

Still, I find that many people are unfamiliar with this virus, even in healthcare.

Origins of the human metapneumovirus

Human metapneumovirus was isolated from humans with acute respiratory infection and sequenced in 2001 using a combination of specialized culture and molecular techniques.

It is related to RSV, which is the leading cause of serious respiratory infections in children and a major problem in adults. Both viruses belong to the same large group as measles, mumps and parainfluenza viruses, all of which are major causes of childhood illnesses.

However, a wealth of data shows that HMPV differs from its cousin RSV in many ways. First, the order of genes in its genome is very different. In addition, HMPV lacks two genes that RSV uses to overcome the immune response that normally aims to; However, HMPV has its own ways of blocking immunity.

Third, genetic analysis of several different groups shows that the closest ancestor of HMPV is an avian virus, avian metapneumovirus. This is an agricultural pathogen of chickens and turkeys. Evolutionary and genetic analyzes indicate that the human virus separated from the avian virus several hundred years ago. This is an example of a zoonosis: an animal virus that spreads to humans. In this case, HMPV has become established as a persistent human pathogen.

Understanding how HMPV successfully made the leap could help predict which other animal viruses might be able to transform into primary human pathogens. The recent H5N1 avian flu outbreak – which can only be transmitted to humans to a limited extent – ​​illustrates this risk.

HMPV in children

Although it was only recognized two decades ago, many studies have confirmed that HMPV is a major cause of respiratory infections in humans. Initial research groups focused on children and quickly discovered that HMPV caused respiratory infections in children worldwide, including Canada, Australia, Japan, Hong Kong, South Africa and Argentina.

In fact, HMPV is a common cause of acute respiratory disease in children in all countries surveyed, and most children first get the infection by age 5. A study of samples collected over 25 years in the US found that HMPV was the second most common cause of lung infections in children, after RSV. Other studies from several children’s hospitals in US cities found that HMPV was the second leading cause of respiratory infections leading to hospitalizations and pneumonia.

Children with underlying risk factors, such as those born prematurely and those with conditions such as asthma, or those with compromised immune systems, such as organ transplant recipients or children being treated for cancer, are at higher risk of severe HMPV. Most children hospitalized with HMPV are otherwise healthy before they get it, yet many require intensive care for the disease.

Not just for kids

HMPV is also a common cause of serious lung infections in adults. This is especially true for adults over the age of 65 or those with underlying medical conditions. A New York study spanning four winters found that HMPV is as common as RSV or influenza in hospitalized older adults, with similar rates of critical care and death.

Studies over three winters in Nashville of adults over the age of 50 found that HMPV hospitalizations and emergency room visits were similar to those for RSV and influenza. HMPV and RSV were more common than the flu in people over 65, presumably because many were vaccinated against the flu.

Another national study of adults hospitalized for pneumonia showed that HMPV was as common as RSV and almost as common as influenza. As in children, HMPV is a particular concern for adults with chronic conditions such as asthma, cancer, or chronic obstructive pulmonary disease, also known as COPD.

Similar to the devastating effects of influenza and COVID-19 in nursing homes, HMPV has also caused numerous outbreaks among vulnerable older adults in long-term care facilities.

Why HMPV is still so underestimated

Although HMPV is a common cause of severe respiratory disease, it is still underdiagnosed by physicians and little recognized by the general population. Most people with acute respiratory disease do not receive testing, and when they do, only complex molecular testing can detect HMPV. However, these tests are usually only performed on hospitalized patients in certain circumstances.

People tend to believe what they see, and as a result even medical professionals are keenly aware of the diseases they commonly test for. But HMPV circulates predictably each year, and in North America the peak is typically February through May. So if you had a recent cold this winter or spring, HMPV was a likely culprit. Children’s hospitals across the country are seeing an increased number of cases, including many in intensive care units. Based on previous research, this almost certainly happens in adults too — it’s just that only patients with severe disease are usually tested for HMPV.

A lack of treatments

There are currently no specific antiviral drugs to treat HMPV like the flu and COVID-19. As with many other respiratory viruses that cause colds, most infected people do well with rest and fluids.

However, some may develop breathing difficulties and need to see a doctor. Children or adults with serious underlying medical conditions should take extra care, and just as with COVID-19, using hand sanitizer and washing hands can reduce transmission.

Preventive vaccines and antibodies against HMPV are in development but still a long way off. So for now, wear a mask if you are sick and avoid others who are sick. You can avoid repeated exposure to this virus that you have not heard of.

The conversation

This article is republished by The conversation under a Creative Commons license.

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