Meet the new respiratory viruses

While these agents continue to cause disease each year, there remain many unanswered questions regarding their epidemiology, pathogenesis, diagnosis, prevention, and treatment. This review aims to familiarize pediatricians on all these aspects of this new-fangled breed of respiratory viruses.

Human metapneumovirus

An RNA virus and member of the Paramyxoviridae family, hMPV follows a seasonal distribution, peaking during the fall and winter, overlapping with respiratory syncytial virus (RSV). Roughly five to 10% of all lower respiratory tract infections in healthy children today are attributed to hMPV, making it the third most common cause of pediatric lower respiratory tract infection (LRTI), after RSV and influenza. ² Moreover, seroprevalence surveys indicate that virtually all children are infected by the ages of 5 to 10 years,³ and that reinfection throughout life is common.¹

The clinical manifestations of hMPV are indistinguishable from those caused by RSV, including both upper respiratory tract infections (URTIs) and LRTIs. As with RSV, clinical signs of hMPV infection may include rhinorrhea, cough, fever, otitis media, as well as acute wheezing and asthma exacerbation.⁴ Interestingly, data have shown that hMPV and RSV can co-infect patients. As a result, disease severity may be increased.⁵

The most common clinical diagnosis associated with hMPV infection is bronchiolitis-a property it shares with RSV. Both viruses can cause severe infections in immunocompromised hosts, including adult patients with hematological malignancies after stem cell transplant.⁶ Investigations into hMPV's role in adult lung transplant, for example, suggest that hMPV should be added to the list of pathogens associated with episodes of allograft rejection.⁷ However, the data in immunocompromised children are lacking. To date, there are only two reported cases; however, one resulted in death secondary to LRTI (retrospectively demonstrated to be caused by hMPV) in a child treated for acute lymphoblastic leukemia.^8,9

Another important population to consider is that of premature infants. While data on the effects of hMPV on preterm infants are accumulating, more are needed. Small-scale studies suggest that hMPV is not a frequent agent of lung disease in premature infants, but that it can elicit severe illness. Cases can require intubation for apnea and even measures such as extracorporeal membrane oxygenation support.^10,11

So when should you suspect hMPV? If your patient is acting as though he/she has RSV but the diagnostic studies are negative, there is a good chance he/she is infected with hMPV. Since hMPV grows poorly in cell culture, testing relies on genome amplification techniques such as polymerase chain reaction (PCR), immunofluorescence assays for the detection of viral antigen, or serology. Because infection is nearly universal by age 5, a definitive serologic diagnosis in older children and adults requires a fourfold rise in antibody titer or seroconversion. Therefore, serologic testing is not appropriate for rapid detection in the outpatient setting (see Diagnostic assays: Coming to your practice?).