Human metapneumovirus (HMPV or hMPV) is a negative-sense single-stranded RNA virus of the family Pneumoviridae and is closely related to the avian metapneumovirus (AMPV) subgroup C. It was isolated for the first time in 2001 in the Netherlands by using the RAP-PCR (RNA arbitrarily primed PCR) technique for the identification of unknown viruses growing in cultured cells. As of 2016, it was the second most common cause—after respiratory syncytial virus (RSV)—of acute respiratory tract illness in otherwise-healthy children under the age of 5 in a large US outpatient clinic. The peak age of hospitalization for infants with HMPV is between 6 and 12 months, slightly older than the peak of RSV, which is around 2 to 3 months. The clinical features and severity of HMPV are similar to those of RSV. HMPV is also an important cause of disease in older adults and infants.
Taxonomy
Discovery and naming Human metapneumovirus (HMPV) was first discovered in 2001 in the Netherlands by Bernadette G. van den Hoogen and her colleagues. HMPV was first detected in the respiratory secretions of 28 young children in the Netherlands and had initially stood out from other common respiratory viruses because the testing methods van den Hoogen and her colleagues had tried using—immunological assays using virus-specific antibodies and PCR-based methods using virus genome-specific primers—were able to test only for known respiratory viruses and, therefore, were unable to identify the novel virus. When researchers began applying molecular biology techniques, the genetic characteristics and portions of the genomic sequences of the virus could be identified. These techniques included the randomly primed PCR technique, which obtained the limited sequence data needed to reveal a clear relationship between this new virus and the avian pneumovirus. The new virus, human metapneumovirus, is named for its close relationship to AMPV, indicating its identity as a metapneumovirus and its human host.
Epidemiology HMPV was responsible for 12% of cases of acute respiratory tract illness in otherwise-healthy children in a US outpatient clinic and 15% and 8% of cases (respectively) of community-acquired pneumonia requiring hospitalization in children under and over the age of 5 in the United States in 2010–2012. The virus is distributed worldwide and, in temperate regions, has a seasonal distribution generally following that of RSV and influenza viruses during late winter and spring. Serologic studies have shown that by the age of five, virtually all children worldwide have been exposed to the virus. Despite near universal infection during early life, reinfections are common in older children and adults. HMPV may cause mild upper respiratory tract infection (e.g., the common cold). However, premature infants, immunocompromised persons, and older adults >65 years are at risk for severe disease and hospitalization. In some studies of hospitalizations and emergency room visits, HMPV is nearly as common and severe as influenza in older adults. HMPV is associated with more severe disease in people with asthma and adults with chronic obstructive pulmonary disease (COPD). Numerous outbreaks of HMPV have been reported in long-term care facilities for children and adults, causing fatalities.Symptoms include rhinorrhea, nasal congestion, cough, wheezing and sore throat.
Genome
The genomic organisation of HMPV is similar to RSV; however, HMPV lacks the non-structural genes, NS1 and NS2, and the HMPV antisense RNA genome contains eight open reading frames in slightly different gene order than RSV (viz. 3’-N-P-M-F-M2-SH-G-L-5’). HMPV is genetically similar to the avian metapneumoviruses A, B and in particular type C. Phylogenetic analysis of HMPV has demonstrated the existence of two main genetic lineages termed subtype A and B containing within them the subgroups A1/A2 and B1/B2 respectively. Genotyping based on sequences of the F and G genes showed that subtype B was associated with increased cough duration and increased general respiratory systems compared to HMPV-A.
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