John V. Williams is Professor of Pediatrics, Microbiology and Molecular Genetics at University of Pittsburgh.
In the year 2000, Dutch scientists set out on a mission of exploration – not to discover land or riches, but to identify unknown causes of acute respiratory infections.
These illnesses, from the common cold to pneumonia, have plagued mankind throughout history. Most are caused by viruses, so if a clinician ever told you “you probably have a virus,” they were probably right. However, respiratory diseases can be much more serious than simple colds.
Respiratory infections are the leading cause of death in children under 5 worldwide and a leading cause of hospitalization of children in developed countries. They are also a major cause of illness and death in people at high risk of serious illness, such as premature infants, the elderly, and people with underlying conditions.
However, meticulous research studies by many groups over decades had failed to identify a virus or bacteria in every person with acute respiratory illness. Was this inability to detect a microbe the result of tests that weren’t good enough or of viruses that doctors and scientists didn’t know about? The answer was partly the first; modern molecular tests are much better, so doctors find better-known viruses.
But the Dutch group discovered a new virus, the human metapneumovirus, abbreviated HMPV or MPV, which turns out to be a major cause of respiratory infections. HMPV often presents like the others common respiratory viruseswith congestion, cough and fever.
As a pediatric infectious disease specialist and virologist, I led my research team on HMPV for over 20 years and personally treated many children with this infection. I have received emails from colleagues, clinicians and parents across the country and around the world with questions about serious and tragically fatal cases.
The United States experienced a spike in HMPV detections in the first few months of 2023. This trend is similar to higher than normal case rates from respiratory syncytial virus or RSVand influenza in fall 2022 and winter 2023, likely linked to declining population immunity after two years of wearing face masks and social distancing.
Yet I find that many people, even in the health field, are unaware of this virus.
Origins of human metapneumovirus
Human metapneumovirus was isolated from people with acute respiratory infection and sequenced in 2001 using a combination of specialized cultures and molecular techniques.
It is linked to RSV, which is the leading cause of serious respiratory infection in children and a major problem in adults. Both viruses are part of the same large group as the measles, mumps, and parainfluenza viruses, all of which are major causes of childhood illnesses.
However, there is plenty of data to show that HMPV differs from its cousin RSV in many ways. First, the order of the genes in its genome is quite different. Additionally, HMPV lacks two genes that RSV uses to overcome the immune response that would normally target it; however, HMPV has its own ways of blocking immunity.
Third, genetic analysis by several different groups shows that the closest recent ancestor of HMPV is an avian virus, avian metapneumovirus. It is an agricultural pathogen of chickens and turkeys. Evolutionary and genetic analysis suggests that the human virus diverged from the avian virus several hundred years ago. This is an example of a zoonosis: an animal virus that is transmitted to humans. In this case, HMPV has established itself as a permanent pathogen for humans.
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 has only been transmitted to humans to a limited extent – illustrates this risk.
HMPV in children
Although it was recognized only two decades ago, numerous studies have confirmed that HMPV is a major cause of respiratory infection in humans. Early research groups focused on children and soon discovered that HMPV caused respiratory infections in children around the world, including Canada, Australia, Japan, Hong Kong, South Africa and Argentina. .
Indeed, HMPV is a common cause of acute respiratory illness in children in all countries reviewed, and most children first become infected by age 5. A study using samples taken over 25 years in the United States found that HMPV was the second most common cause of lung infection in children after RSV. Other studies of several children’s hospitals in US cities found HMPV to be the second most common 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 weakened immune systems, such as organ recipients or children being treated for cancer , are at higher risk of severe HMPV. Most children who are hospitalized with HMPV are otherwise healthy before acquiring it, but many require intensive care because of the disease.
Not just for kids
HMPV is also a common cause of serious lung infections in adults. This is especially true in adults over 65 or those with underlying conditions. A New York study over four winters found that HMPV was as common among hospitalized older adults as RSV or the flu, with similar rates of intensive care and death.
Studies over three Nashville winters of adults over 50 detected rates of HMPV hospitalizations and emergency room visits similar to RSV and influenza. HMPV and RSV were more common than influenza in people 65 and older, likely because many were vaccinated against influenza.
Another national study of adults hospitalized with pneumonia found HMPV to be as common as RSV and nearly as common as the flu. As in children, HMPV is a particular problem for adults with chronic conditions such as asthma, cancer, or chronic obstructive pulmonary disease, also known as COPD.
Similar to the disastrous effects of influenza and COVID-19 in nursing homes, HMPV has also caused numerous outbreaks among vulnerable seniors in long-term care facilities.
Why HMPV is still so little recognized
Although it is a common cause of serious respiratory disease, HMPV remains underdiagnosed by clinicians and poorly recognized by the general population. Most people with acute respiratory illness do not get tested, and if they do, only complex molecular tests can detect HMPV. But this test is usually only done for hospitalized patients under certain circumstances.
People tend to believe what they see and therefore even medical professionals are the most aware of the diseases they frequently test for. But HMPV circulates predictably every year, and in North America the peak is usually February through May. So if you had a recent cold this winter or spring, HMPV was probably the culprit. Children’s hospitals across the country are seeing a surge in cases, many of them in intensive care. Based on previous research, this almost certainly happens in adults too – it’s just that usually only patients with severe disease are tested for HMPV.
A shortage of treatments
Currently, there are no specific antiviral drugs to treat HMPV like there are for influenza and COVID-19. As with many other respiratory viruses that cause colds, most people infected will do just fine with rest and fluids.
But some may develop breathing difficulties and need to see a doctor. Children or adults with serious underlying illnesses should be especially cautious and, as with COVID-19, using hand sanitizer and washing hands can reduce transmission.
Preventive vaccines and antibodies against HMPV are under development but are still far from being achieved. So, for now, wear a mask if you are sick and avoid other sick people. You can dodge repeated engagement with that virus you had but hadn’t heard of.
This article is republished from The conversation under Creative Commons license.