A number of viruses are known to be common causes of acute respiratory tract infections, including influenza viruses, respiratory syncytial virus (RSV), parainfluenza viruses, adenoviruses, rhinoviruses, and coronaviruses. About one half of adult community-acquired pneumonia cases and up to one third of bronchiolitis and pneumonia cases in children are of unknown etiology, suggesting that other, as yet unknown, infectious agents exist. Dutch researchers first identified human metapneumovirus (MPV) in 2001, but its presence has since been demonstrated in stored samples from subjects with respiratory infections as far back as the 1950s. Hamelin and colleagues give a clinical review of this emerging human pathogen.

MPV belongs to the paramyxovirus family, which also includes RSV. MPV is difficult to grow in the laboratory, and the only animals shown to develop symptoms after inoculation are primates. The genome of this single-stranded RNA virus has now been mapped and shows similarities to avian metapneumovirus and human RSV.

Worldwide studies have confirmed MPV as a common cause of respiratory infections in healthy and immunocompromised persons. Seroprevalence research indicates that MPV infection is acquired by childhood almost universally, but cases of severe infection in adults indicate that prior infection is not protective and that reinfection readily occurs. There is seasonal variation to MPV infection, with most cases occurring in winter and early spring, a pattern that is similar to that of RSV. About 5 to 10 percent of respiratory infections in children leading to hospitalization appear to be caused by MPV, while the corresponding adult rates are not clear.

Similar to other respiratory pathogens, the most commonly reported symptoms with MPV infection are fever, cough, and dyspnea. Bronchiolitis is the most common clinical presentation of MPV infection in children. Infection occurs at an older age than that noted for RSV, and the severity of illness has been reported as both more and less severe than with RSV. In healthy adults, MPV infection tends to cause minor flu-like illness and colds, but case series also have described severe or fatal infections, especially in immunocompromised patients or others with serious underlying medical conditions. Possible synergistic effects, leading to increased severity of illness, have been described in co-infections with RSV and the new coronavirus that causes severe acute respiratory syndrome.

Because rapid antigen detection tests are unavailable and viral culture is difficult, reverse transcriptase polymerase chain reaction testing is the method of choice for detecting infection.

Ribavirin and intravenous immunoglobulin, which may be used for RSV infection, also have demonstrated in vitro activity against MPV, but palivizumab (a monoclonal antibody directed against RSV) has not been shown to be effective.