Human metapneumovirus.

Human metapneumovirus (hMPV) is a common pathogen that can cause both upper and lower respiratory tract infections, particularly in children, elderly adults, and immunocompromised hosts. Since its initial identification in 2001, hMPV has been isolated from individuals with acute respiratory tract infections (ARTIs) in virtually every continent. Serological studies indicate that it has caused human infection since 1958 or earlier. The epidemiology and clinical manifestations of hMPV are similar to those of the human respiratory syncytial virus (hRSV). HMPV has a seasonal variation: it circulates in late winter to early spring in temperate climates; late spring and summer in tropical regions. In young children, symptoms range from mild upper respiratory tract infections to severe lower respiratory tract infections (eg, bronchiolitis, croup, and pneumonia). In adults, hMPV reinfection typically presents with colds and flulike clinical manifestations. The disease is more severe (sometimes lethal) in immunocompromised hosts. Reverse transcription polymerase chain reaction (RT-PCR) is the most sensitive test with which to detect hMPV infection. Direct detection of hMPV antigens with an immunofluorescent antibody test is available but is less sensitive than PCR. Antibody testing is used mainly for retrospective diagnosis (≥ fourfold increase in titer or seroconversion) and for epidemiological studies. The mainstay of treatment of hMPV infections is supportive. Ribavirin has similar activity in vitro and in animal models against hRSV and hMPV, but its efficacy in vivo is unproven. Monoclonal antibodies have activity in murine models but are not available in humans. Several vaccines are promising in animal models, but their safety and efficacy have not yet been evaluated in humans.