In the year 2000, Dutch scientists went on a reconnaissance mission – 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 humanity throughout history. Most are caused by viruses, so if a doctor has ever told you “you probably have a virus”, they probably were right. However, respiratory illnesses can be much more serious than the common cold.
Respiratory infections are the the leading cause of death in children under 5 years of age worldwide and a major reason for hospitalization of children in developed countries. They are also a leading cause of illness and death in people at high risk of serious disease, such as premature infants, older adults and those with underlying conditions.
However, painstaking studies by many groups over the past few decades have failed to identify a virus or bacteria in every person with acute respiratory disease. Was the failure to detect a microbe the result of tests that were not good enough, or of viruses that doctors and scientists knew nothing about? The answer was partly the former; modern molecular tests are much better, so doctors find more known viruses.
But the Dutch group discovered a new virus, human metapneumovirus, abbreviated HMPV or MPV, which appears to be a major cause of respiratory infections. HMPV often presents like other common respiratory viruses, with congestion, cough and fever.
Like a specialist in childhood infectious diseases and virologistI led my team in HMPV research for over 20 years, and I have personally cared for many children with this infection. I have received emails from colleagues, clinicians and parents from around the country and the world asking about serious and tragically fatal cases.
The US saw one peak in HMPV detections during the first few months of 2023. This trend is similar to the higher-than-normal cases of pneumonia respiratory syncytial virus or RSVand flu in fall 2022 and winter 2023, likely related to reduced population immunity after two years of wearing face masks and social distancing.
Yet I notice that many people, even in healthcare, are not familiar with this virus.
Origin of human metapneumovirus
The human metapneumovirus was isolated from people with an acute respiratory infection and sequenced in 2001 using a combination of specialized culture and molecular techniques.
It’s related to RSV, which is the leading cause of serious respiratory infections in children and a major problem in adults. Both viruses are in the same large group with measles, mumps and parainfluenza viruses, all main causes of childhood illnesses.
However, abundant data shows that HMPV differs from its cousin RSV in many ways. First, the order of genes in it genome is very different. In addition, HMPV lacks two genes that RSV uses to overcome the immune response that would normally target it; yet HMPV has its own ways of block immunity.
Third, genetic analysis by different groups shows that the closest recent ancestor of HMPV is a bird virus, bird metapneumovirus. This 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 jumps to humans. In this case, HMPV was established as a permanent 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 outbreak of bird flu H5N1 – which has only limited transmission to humans – illustrates this risk.
HMPV in children
Despite being recognized only two decades ago, many studies have confirmed that HMPV is a major cause of respiratory infections in humans. The first research groups focused on children and soon discovered that HMPV caused respiratory infections in children worldwide Canada, Australia, Japan, Hong-Kong, South Africa And Argentina.
Indeed, HMPV is a common cause of acute respiratory disease in children any country investigated, and most children get the infection for the first time at age 5. A study using samples collected in the US over 25 years found that HMPV had the second most common cause of lung infection in children after RSV. Other studies from multiple children’s hospitals in U.S. cities found that HMPV was the second most common cause of respiratory infections, leading to hospitalization And pneumonia.
Children with underlying risk factors, such as who were born prematurely and people with conditions such as asthmaor those who have compromised immune systems, such as organ transplant recipients or children being treated for cancer, are on higher risk of severe HMPV. Most children who are hospitalized with HMPV are otherwise healthy before they get it many require intensive care of the disease.
Not just for kids
HMPV is also a common cause of pneumonia severe lung infections in adults. This is especially true for adults over the age of 65 or those with underlying conditions. A study in New York over four winters found that HMPV was as common in hospitalized older adults as RSV or the flu, with comparable rates of ICU care and death.
Studies over three winters in Nashville of adults over 50 years old detected percentages of HMPV hospital admissions And visits to the emergency department which were comparable to RSV and flu. HMPV and RSV were more common than flu in people aged 65 and older, presumably because many had been vaccinated against flu.
Another national study of adults hospitalized for pneumonia showed just that HMPV was as common as RSV, and almost as common as the flu. As in children, HMPV is a particular problem for adults with chronic conditions such as high blood pressure asthma, cancer or chronic obstructive pulmonary disease, also called COPD.
Comparable to the nasty consequences of the flu and COVID-19 in nursing homesHMPV has also caused numerous outbreaks among the frail elderly in long-term care facilities.
Why HMPV is still so recognized
Despite being a common cause of serious respiratory disease, HMPV remains underdiagnosed by clinicians and little recognized by the general population. Most people with acute respiratory disease do not get tested, and if they do, only complex molecular tests can detect HMPV. But this testing is usually only done for hospitalized patients under certain circumstances.
People tend to believe what they see, which is why even healthcare providers are most aware of diseases they often test for. But HMPV predictably circulates every year, and in North America the peak is usually February through May. So if you’ve recently had a cold this winter or spring, HMPV was a likely culprit. Children’s hospitals across the country are seeing a increased number of cases, including many in ICU. Based on previous research, this is almost certainly the case in adults, but only patients with severe disease are usually tested for HMPV.
A shortage of treatments
At this time, there are no specific antivirals to treat HMPV, such as there are for flu and COVID-19. As with the many other respiratory viruses that cause the common cold, most infected people will do just fine with rest and fluids.
But some may have trouble breathing and need medical attention. Children or adults with serious underlying conditions should be especially careful, and as with COVID-19, using hand sanitizer and hand washing can reduce transmission.
Preventive vaccines and antibodies for HMPV are in development but are still far away. So wear a mask for now if you are sick and avoid others who are sick. You can dodge a repeated involvement with this virus that you have had but never heard of.