Human Metapneumovirus: Innocent Bystander or Important Pathogen?

Human metapneumovirus (HMPV), first discovered in 2001, is a major cause of acute respiratory disease in young children. The virus has been isolated — alone or as a copathogen — from many children with bronchiolitis. The clinical features of HMPV infection are similar to those of respiratory syncytial virus (RSV) infection.

Now, researchers in Israel have compared HMPV with other viral pathogens as a cause of community-acquired alveolar pneumonia (CAAP). They defined CAAP as “a dense opacity appearing as a fluffy consolidation of a portion, total lobe, or the entire lung, often containing air on bronchography and sometimes associated with pleural effusion.” Such consolidations are classically associated with bacterial pneumonia, but viruses are often isolated from children with CAAP.

Between November 2001 and October 2005, nasopharyngeal wash specimens were obtained from 1296 children aged <5 years who had been radiologically diagnosed with CAAP. The specimens were tested for RSV, influenza A and B viruses, parainfluenza viruses, and adenovirus by direct immunofluorescence assay, and for HMPV by reverse-transcriptase PCR.

Virus was detected in 608 (47%) of the patients. HMPV was isolated in 108 (8%), second only to RSV (23%). Overall, viral CAAP infections peaked during the winter months, with a minor peak in the spring; HMPV infections peaked during the winter, extending into spring and occasionally early summer. Among children with viral CAAP, RSV was isolated significantly more often than HMPV in those aged ≤2 years, whereas HMPV was isolated as often as (or slightly more often than) RSV in those aged 3 to 5 years. Compared with RSV-associated CAAP, HMPV-associated CAAP was seen more often in older children and in children with a history of wheezing or (even after adjustment for age) otitis media; it also had higher rates of wheezing and gastrointestinal symptoms.