What is Human metapneumovirus, particularly in relation to its effects on young children, older adults, and individuals with compromised immune systems?

What is Human Metapneumovirus (HMPV)?

Human metapneumovirus is a paramyxovirus that causes acute respiratory tract infections ranging from mild upper respiratory symptoms to severe pneumonia and bronchiolitis, particularly affecting young children, older adults, and immunocompromised individuals. 1, 2

Viral Classification and Discovery

• HMPV is a recently described paramyxovirus discovered in 2001, belonging to the Paramyxoviridae family 1, 3
• The virus is classified into two main genetic groups (A and B) with antigenic diversity that complicates vaccine development 4, 5
• HMPV is the pathogen most closely related to respiratory syncytial virus (RSV) in terms of clinical presentation and genomic organization 4

Clinical Manifestations by Population

Young Children and Infants

• HMPV accounts for approximately 5-10% of hospitalizations in children with acute respiratory tract infections 3, 5, 6
• Initial symptoms include rhinorrhea, nasal congestion, cough, and fever, which can progress to dyspnea, cyanosis, malaise, restlessness, poor feeding, and reduced activity 2, 5
• The virus causes severe bronchiolitis and pneumonia that are clinically indistinguishable from RSV infection 3, 6
• In severe pediatric cases, progression to respiratory failure unresponsive to conventional oxygen therapy, septic shock, metabolic acidosis, and coagulation dysfunction may occur 2
• Most commonly detected in children under 2 years of age, with symptoms ranging from mild to severe requiring hospitalization 1, 5

Older Adults

• HMPV causes significant morbidity in elderly patients, particularly those aged ≥65 years 1
• Patients with chronic cardiac or pulmonary diseases face substantially elevated risk of severe disease and mortality 1, 2
• In adults, symptoms often mimic influenza and can exacerbate chronic conditions like chronic obstructive pulmonary disease (COPD) or asthma 5

Immunocompromised Patients

• HMPV causes severe lower respiratory tract disease in immunocompromised individuals, including hematopoietic stem cell transplant (HSCT) recipients, with mortality rates of 10-30% 2
• Solid organ transplant recipients and patients with profound lymphopenia are at particularly high risk for progression to severe disease 2

Diagnostic Challenges

• No clinical or radiographic criteria reliably distinguish HMPV infection from bacterial infection, necessitating a low threshold for empirical antibiotics in severe cases 1, 2
• Molecular methods, particularly reverse transcriptase PCR (RT-PCR), are the preferred diagnostic modality due to the virus's slow and fastidious growth in cell culture 3, 4, 6
• Rapid antigen detection tests are not available for HMPV, unlike for influenza or RSV 1
• Asymptomatic and prolonged viral shedding can occur, complicating infection control measures 2
• Coinfection with other respiratory viruses (especially RSV) is common and obscures attributable morbidity 2

Laboratory Findings

• Laboratory abnormalities include leukocytosis, leukopenia, neutrophilia, lymphopenia, and elevated inflammatory markers 5
• These findings are nonspecific and do not distinguish HMPV from other respiratory viral infections 5

Treatment Limitations

• There is no antiviral agent with established efficacy for the treatment of adults with pulmonary infections involving metapneumovirus 1, 2
• Ribavirin has shown activity in vitro and in animal models but lacks clinical trial evidence for efficacy in humans 6
• No commercial vaccines are currently available, though several candidates are in development including subunit, live attenuated, vector-based, and mRNA platforms 7
• Management remains primarily supportive, focusing on adequate hydration, supplemental oxygen when oxygen saturation falls below 90%, and monitoring for bacterial superinfection 2

Epidemiology and Seasonality

• HMPV has a seasonal distribution in temperate countries, with most cases occurring in winter and spring 4
• Initial infection typically occurs during early childhood, but reinfections are common throughout life, especially in older adults 3, 6
• The virus has been recognized globally since its discovery, with significant clinical and socioeconomic impact 7

Key Clinical Pitfalls

• Failing to consider bacterial superinfection: Secondary bacterial infections are common in hospitalized adults with viral pneumonias, with S. pneumoniae being the most frequent pathogen 1
• Over-reliance on negative rapid tests: Unlike influenza, no rapid antigen tests exist for HMPV, requiring molecular diagnostics for confirmation 1
• Underestimating risk in elderly patients with comorbidities: The presence of cardiopulmonary disease substantially increases mortality risk and warrants aggressive supportive care 1, 2
• Inadequate infection control: Prolonged viral shedding and asymptomatic carriage require strict hand hygiene and droplet precautions to prevent nosocomial transmission 2