CHICAGO—Human metapneumovirus was discovered in 2001 by researchers from Erasmus Medical Center in Rotterdam, the Netherlands. They found that in children, the symptoms of metapneumovirus were similar to those of respiratory syncytial virus (RSV). The researchers also found that by age 5, nearly all children have been exposed to human metapneumovirus.[1]

At the 43rd Interscience Conference on Antimicrobial Agents and Chemotherapy, James E. Crowe, Jr, MD, and colleagues from Vanderbilt University in Nashville reported their findings from a study of more than 2,000 children over a 25-year period. Nearly 12% of all serious respiratory illnesses and 15% of all common colds in their population were caused by human metapneumovirus—making it the second most common respiratory infection after RSV.[2]

“We have run a very unique clinic since the 1970s in which we—as both pediatricians and virologists—have taken care of healthy infants and children as their primary care providers,” said Dr. Crowe, an Associate Professor of Pediatrics at Vanderbilt and one of the study’s lead investigators. “Whenever they were ill with respiratory infections we examined them and obtained nasopharyngeal washes, which were tested for respiratory viruses.”

In addition to testing nasopharyngeal samples, the researchers also collected clinical data on the children’s illnesses. “We have prospectively collected clinical data with matching specimens and looked, over a 25-year period, at illnesses for which there was no previous virus isolated,” explained Dr. Crowe in an interview.

What they found was that metapneumovirus was a significant cause of both upper and lower respiratory illness in children. “This is very important and has major implications for vaccine [and therapeutic] development for children,” he said.

GETTING TO KNOW METAPNEUMOVIRUS

While human metapneumovirus is newly discovered, it is by no means new. Dr. Crowe noted that the Dutch researchers who discovered the virus looked as far back as the 1950s, and serum samples from that period were testing positive for metapneumovirus. “It has been causing illness at similar rates as far back as we can look. It’s not an emerging illness or one that’s likely to be increasing in importance,” he said, adding that rather than being a cause for alarm, “it is actually rather exciting that we are able to understand an illness that has been occurring for a very long time that we previously didn’t understand.”

Metapneumovirus is common worldwide, showing no geographic or climactic preference. Studies have documented cases of metapneumovirus in Japan, the United Kingdom, and Australia, in addition to the Netherlands and the United States. In fact, said Dr. Crowe, “In collections and archives it’s one of the most common respiratory viruses in the world.”

Metapneumovirus seems to be most active in late winter and early spring—a period when many other respiratory viruses are also circulating.

Human metapneumovirus affects people of all ages. A study from the University of Rochester followed cohorts of adults during two winter seasons and evaluated them for human metapneumovirus infection. Overall, 4.5% of 984 illnesses were associated with metapneumovirus infection.[3]

INFORMATION STILL NEEDED

One big technical barrier—and one of the reasons the organism was not discovered earlier—is that metapneumovirus does not grow well in cell cultures. “All of the current tests on metapneumovirus are being done with nucleic acid detection, such as polymerase chain reaction,” said Dr. Crowe. “We need good cell-culture substrates, and we don’t have them right now.”

Additionally, the basic biology of the virus is not well understood. “Pneumoviruses are missing some of the nonstructural genes that related viruses have,” Dr. Crowe observed. “These nonstructural genes participate in acting against host responses, which suggests that metapneumovirus will have a different profile of pathogenicity than some of the other viruses. We need to understand the biology of the virus better. For example, what cells does it infect? How does it replicate?”

Currently, there is no diagnostic test available to clinicians for metapneumovirus, so most physicians are not able to identify or treat it. “I am optimistic that, in the next few years, diagnostic tests will become available for use in clinical labs,” predicted Dr. Crowe.

“Although everyone may become infected, it’s probably the very young and the very old who are at the highest risk of severe disease,” Dr. Crowe said. Defining high-risk populations is important, he continued. These could include premature infants, children with congenital lung disease, adults or elderly people with lung disease, and smokers. High-risk populations need to be defined because they will be the targets for therapies such as antibody prophylaxis or vaccination.

—Gale Jurasek

References
1. van den Hoogen BG, de Jong JC, Groen J, et al. A newly discovered human pneumovirus isolated from young children with respiratory tract disease. Nat Med. 2001;7:719-724.
2. Newly discovered respiratory virus is a major cause of illness in children [press release]. 43rd Annual Interscience Conference on Antimicrobial Agents and Chemotherapy; September 15, 2003; Chicago, Ill.
3. Falsey AR, Erdman D, Anderson LJ, Walsh EE. Human metapneumovirus infections in young and elderly adults. J Infect Dis. 2003;187:785-790.

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