Human metapneumovirus: A novel virus that is a ubiquitous and important agent of respiratory disease. The human metapneumovirus (hMPV) was discovered in 2001 in young children in The Netherlands with acute respiratory illnesses ranging from mild upper respiratory infections to severe bronchiolitis and pneumonia. It has since been found to cause severe acute respiratory infections all over the world in young children, the elderly, and people with weak defense systems (the immunocompromised). It accounts for 1 to 3% of all flu-like illnesses. (The influenza viruses, which belong to the orthomyxovirus family, cause most cases of flu and flu-like illnesses.)
Although hMPV is a distinct and different virus from respiratory syncytial virus (RSV), it shares many features with RSV. For example, it tends to strike in the winter months in temperate climates. And, as already indicated, the young, the elderly, and the immunocompromised are most vulnerable. The signs and symptoms of infection with hMPV also appear indistinguishable from those caused by RSV. Cough, sore throat, runny nose, and high fever are most common. Wheezing, difficulty breathing (dyspnea), pneumonia, bronchitis, bronchiolitis, conjunctivitis, and inflammation of the middle ear (otitis media) each occur in 10% or more of patients.
Studies of blood samples reveal that by the age of 5 years, virtually all children are infected with hMPV. Moreover, when someone contracts the virus, they usually fall ill. In other words, asymptomatic and subclinical infections with hMPV are rare.
Like the influenza viruses and RSV, the hMPV is a single-stranded RNA virus. Like the RSV, the hMPV belongs to the paramyxovirus family. Within that family of viruses, hMPV is genetically most similar to the avian pneumovirus and so was named "metapneumovirus."
The initial diagnostic method used to identify hMPV infections was the RT-PCR assay. Virus isolation, however, was relatively difficult. Hence, the belated discovery of the virus. Furthermore, genetic variation exists among hMPV isolates. That is, at least two genetic lines of descent (lineages) are known, each of which is subdivided into at least two sublineages. An important consequence of this variation is that viruses belonging to different genetic lineages may cause multiple hMPV infections in the same person.
Metapneumovirus is not a new virus. Serological studies of antibodies against hMPV indicate that the virus circulated in humans for at least 50 years before its discovery in 2001. Yet, it is now the agent of an "emerging infectious disease."