Researchers are beginning to piece together the scope of this ubiquitous but overlooked bug, which now appears to afflict just about everybody, probably over and over.
Even though the virus seems to be rarely serious, its vast presence intrigues microbiologists, and it is one of the most talked-about topics at this week's meeting in Chicago of the American Society for Microbiology.
Experts say the metapneumovirus is almost certainly not a new bug but something that has been around for eons.
Like many other respiratory bugs, this one is most likely to cause severe illness in infants encountering it for the first time. Although repeat infections are thought to be common, they result in much less intense illness, often just an ordinary cold or perhaps no symptoms at all. However, the bug may cause more serious problems in the elderly and people with other medical conditions.
Although new microbes, such as the ones that cause AIDS and SARS, are occasionally recognized, it is unusual to discover a virus as omnipresent as this.
"Thousands of hospitalizations occur every year due to this virus in infants," said Dr. James Crowe Jr. of Vanderbilt University.
His research suggests that the metapneumovirus is second only to respiratory syncytial virus as a cause of severe lower respiratory infections in the young, occurring about two-thirds as often. Both viruses are members of the paramyxovirus family.
Crowe's team looked at nasal specimens taken from 2,000 children after they were treated for lower respiratory infections since 1976.
The newly discovered virus turned out to cause about 12 percent of these severe illnesses. They also caused 15 percent of common colds in children, including one-third of the colds complicated by middle-ear infections.
"When we put this in perspective, it appears to be the second most common cause of respiratory illnesses in children," he said.
The virus went undiscovered because it does not grow well in cell cultures, a standard tool for sorting out the viruses that cause human disease. It was identified in 2001 by researchers from Erasmus Medical Center in Rotterdam. They calculated that every child catches the virus by age 5.
As with many respiratory viruses, such as the rhinoviruses and coronaviruses that cause many colds, the body does not remember the metapneumovirus clearly and so contracts it over and over, even though repeat bouts seem to be milder.
Now that scientists know what to look for, they can seek out the virus' genetic footprints in nasal secretions. This way, they are trying to assess how often — and how severely — it makes people sick through life.
"At least in adults, we are figuring out whether it is a big deal. It is a little too early to say," said Dr. Ann Falsey of the University of Rochester.
Nevertheless, she said, it is probably a source of recurring colds, although they may be less common than ones caused by the rhinoviruses, long considered the classic cold bug.
Without testing, victims cannot tell what sort of virus is causing their sniffles. However, Falsey said it appears that hoarseness is more often a symptom of metapneumovirus colds.
In other studies at the conference:
Researchers from McGill University looked at elderly patients hospitalized with pneumonia or chronic obstructive pulmonary disease. The metapneumovirus was found in 2 percent, which was the same as influenza A.
A team from Hadassah University in Jerusalem looked for signs of metapneumovirus infection in children. They found that one-third had been infected by their first birthday, and half had it by age 2.
Researchers from Oxford University tested children up to age 12 being seen for coughs and fever and found that 8 percent had metapneumovirus. In these youngsters, flu was the most common cause of illness.
Whether the metapneumovirus plays some role in SARS is also a matter of debate. The primary cause of SARS is a newly discovered coronavirus. However, a recent study from Hong Kong found that half of the SARS patients tested during last spring's outbreak were also infected with the metapneumovirus.