It may be time to rethink our ideas about respiratory viruses. Polymerase chain reaction (PCR) testing and other molecular methods are showing that some novel viruses may be contributing to many upper respiratory tract infections.
Data presented at the Interscience Conference on Antimicrobial Agents and Chemotherapy suggest that 5 new viruses may be key players: human metapneumovirus (MPV), coronavirus, Mimivirus, a novel polymavirus (known as WU virus), and bocavirus.
Respiratory syncytial virus (RSV) is still considered the main player in children, and influenza and adenoviruses remain important. Rhinoviruses are likely key contributors to all acute respiratory disease, including bronchiolitis, pneumonia, and asthma exacerbations; however, there may be co-conspirators.
MPV was first described in the Netherlands in 2001. Today it may be causing serious respiratory illness in young children and in the elderly. Studies suggest that it is common worldwide and is likely an important cause of respiratory infections in all age-groups. The main symptoms are fever, cough, congestion, sore throat, and runny nose.
The importance of MPV in the elderly was illustrated in a recent study in a VA hospital, which showed that 42% of MPV-infected patients had pneumonia, 17% of whom died.
MPV is probably an important contributor to respiratory infections in children as well. Very consistent data now suggest that it may be the third most common respiratory virus (after rhinovirus and RSV) in children hospitalized with lower respiratory tract infections.
Clinically, MPV is very similar to RSV. It occurs during winter epidemics, bronchiolitis is the major manifestation in infants, and it is known to contribute to asthma exacerbations.
Differences between MPV and RSV are: Winter epidemics have not been consistent from year to year, and its pathogenicity may be less than that of RSV.
MPV has also been found in association with severe acute respiratory syndrome (SARS), and some recent data suggested an association with RSV, perhaps resulting in more severe illness.
They don’t know a great deal about these viruses in terms of their pathogenicity. So how dangerous is the human metapneumonvirus and other newly discovered ones? They think sometimes it may be that they are playing little or no role, sometimes a little bit, and sometimes quite a lot.
Bocavirus, another winter respiratory virus, is now found in about 5% of infants and children hospitalized for lower respiratory tract infections. Little is known about its pathogenicity; in animals, bocaviruses are important causes of diarrhea, which is also common in children with bocavirus-associated respiratory disease.
The power of PCR and other modern nucleic acid-based testing methods have also turned up 2 new coronaviruses, NL63 and HKU1, which have now joined SARS in this virus family. The 2 previously identified strains are 229E and OC43. It is believed that these viruses contribute to asthma exacerbations in children and bronchitis in adults, as well as to more serious respiratory illness in the elderly.
The mimivirus is a giant DNA virus. Believed to be the largest virus to infect humans, it is larger than some bacteria-approximately the size of mycoplasma with a DNA genome of 1.2 megabases.
Little is known about the new WU virus, except for its involvement in respiratory diseases. A new study has just reported it had been identified in respiratory secretions of patients with acute respiratory tract infection and had previously been detected in a nasopharyngeal aspirate in a child with pneumonia.
What do these viruses mean to adults? Are they going to be some of the viruses that are important in respiratory disease in adults, and what do they mean to adults who are immunocompromised?
Those questions are all up for grabs. They said that some of these viruses may be causing asthma to become severe in certain patients, but this too is uncertain.