Researchers at the Yale University School of Medicine found that 19 out of 296 New Haven, Connecticut, children who had respiratory infections of unknown cause were infected with hMPV. Symptoms included wheezing, cough and fever.

"I think this virus probably accounts for a small but significant portion of respiratory tract illness in children," study author Dr. Jeffrey S. Kahn told Reuters Health.

The findings appear in the latest issue of the journal Pediatrics.

In children, 15 to 34 percent of cases of pneumonia and a lung infection called bronchiolitis have no known cause. The most common causes are flu viruses, parainfluenza viruses and respiratory syncytial virus (RSV). Samples screened in the study were negative for these viruses.

"If you step back and look at pneumonia in general, about 50 percent of the time we can identify a cause," Kahn said. "Obviously, in the other 50 percent we don't know what causes it. So that suggests that there are unknown pathogens out there."

Human metapneumovirus came to light in 2001, when researchers in the Netherlands identified the virus in children.

"When it was first reported, everybody said, 'wow, that's very interesting,' and everybody started going back to their freezers where they kept samples, and started to probe samples. Now it's been found in many countries," Kahn said.

In March of this year, researchers reported finding hMPV in adults in Rochester, New York. That was the first indication that it is circulating in the United States. It has also been found in the UK, Canada, Australia, Japan, Finland, and France.

The implications of these discoveries are not yet fully understood. "The disease caused by this virus is really just beginning to be explored," Kahn said.

It's certain that hMPV can make people sick enough to land them in the hospital. But Kahn said that the severity of the illness it causes may change from year to year. More research will be needed to find out how the virus behaves and the extent of its impact.

The existence of hMPV may answer some nagging questions for pediatricians. "We see otherwise healthy kids getting very bad lung infections," Kahn said. It's possible that hMPV interacts with other viruses to cause more serious infections.

Before scientists determined that a new kind of coronavirus causes SARS, or severe acute respiratory syndrome, they thought that hMPV might play a role. Six out of eight SARS patients in Canada had both hMPV and the coronavirus, leading researchers to wonder if one of the viruses worsened the effects of the other. Later, they found that the coronavirus alone causes SARS.

SOURCE: Pediatrics 2003;111;1407-1410.