Human metapneumovirus is a respiratory pathogen first described in 2001. It was detected using a molecular approach on clinical samples from children with respiratory illnesses but with negative test results for known respiratory viruses. Human metapneumovirus is able to cause a wide range of respiratory illnesses from mild upper respiratory symptoms to severe lower respiratory tract disease in all age groups. In general, symptoms are similar to those caused by RSV.

Pathogenesis and Pathology

 Human metapneumovirus infects only humans and is related to avian metapneumovirus that causes rhinotracheitis in chickens. It consists of two subgroups and at least four genetic lineages. These viral lineages are distributed worldwide; multiple lineages can be circulating at the same time in the same location. It appears that the predominant strain in circulation may vary by geographic location and from year to year.

The incubation period for metapneumovirus is estimated to be from 4 to 9 days. The duration of shedding is about 5 days in children and several weeks in immunocompromised hosts.

Replication is limited to respiratory epithelial cells in infected hosts. The cell surface receptor for human metapneumovirus appears to be αvβ1-integrin. Cytopathic effects induced by human metapneumovirus in cultured cells, such as LLC-MK2 monkey kidney cells, are similar to those of RSV.

Clinical Findings

 Human metapneumoviruses are associated with a variety of symptoms of the respiratory tract. These symptoms cannot be distinguished from those induced by RSV. Children usually display rhinorrhea, cough, and fever and may develop acute otitis media. Lower respiratory tract illnesses may occur, including bronchiolitis, pneumonia, croup, and exacerbation of asthma. Bronchiolitis in children seems to be less frequently associated with metapneumovirus than with RSV.

Populations at risk besides children include elderly adults and immunocompromised individuals. Many children hospitalized because of metapneumovirus have underlying chronic conditions. Severe infections may occur in immunocompromised individuals, such as children or adults with cancer or bone marrow transplants, and in institutionalized elderly persons.

Healthy adults tend to develop cold and flulike symptoms in response to metapneumovirus infection. Asymptomatic infections are more common than for influenza virus or RSV in this population.

Immunity

 The prevalence of antibodies to human metapneumovirus increases in children from the age of 6 months and reaches nearly 100% by 5–10 years of age. Despite high rates of antibody in adults, reinfections are common. It has been suggested there may be limited cross-protective immunity among different strains of metapneumovirus and that antibody-mediated protection may not be adequate to prevent disease.

Laboratory Diagnosis

The best specimens for detection of human metapneumovirus are nasopharyngeal aspirates or swabs. RT-PCR assays are the methods of choice. Detection of viral antigens in respiratory specimens by direct immunofluorescence staining is sensitive for children due to higher viral loads but poorer for adults. Detection of antibodies in patient sera is useful primarily for research studies.

Epidemiology

Human metapneumoviruses are ubiquitous and distributed worldwide. Infections occur in all age groups, but especially in pediatric patients. It appears that infections with human metapneumovirus in young children are less common than with RSV but more common than with the parainfluenza viruses. The majority of infections occur in late winter to early spring in the United States. The median age of metapneumovirus-positive hospitalized children is 6–12 months, older than seen with RSV (2–3 months).

Different strains of human metapneumovirus are in circulation simultaneously, with predominant strains varying by location and over time.

Treatment and Prevention

 There is no specific therapy for human metapneumovirus infections, and no vaccine is available.

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