human metapneumovirus (HMPV)

Introduction

a negative-sense single-stranded RNA virus of the family Pneumoviridae (which also includes human respiratory syncytial virus / human orthopneumovirus (HRSV))
1st isolated in 2001 although appears to have been circulating at least 50yrs prior
- appears to have diverged from avian metapneumovirus around 1800AD
there are at least four lineages: A1, A2, B1 and B2
infects airway epithelial cells in the nose and lung
2nd most common cause of lower resp tract infection in young children (RSV is the most common cause) and tends to cause hospitalisation in a slightly older group than RSV: 6-12 months of age compared to 2-3 months of age for RSV
in most people it only causes a common cold like illness but it may be severe in the young (esp. premature infants), the elderly, asthmatics, those with COPD, or the immunocompromised
- nearly as common and as severe as influenza in the elderly
re-infections are common in older children and adults

global seasonal outbreaks similar to RSV and influenza, tending to occur in late winter and early spring in temperate climates
by age 5yrs virtually all children have been infected
likely that transmission occurs by contact with contaminated secretions, via droplet, aerosol, or fomite vectors