What are the clinical presentation, diagnostic approach, and management recommendations for human metapneumovirus infection in children, elderly adults, and immunocompromised patients?

Human Metapneumovirus (HMPV) Infection

Clinical Presentation

HMPV typically presents as an upper respiratory tract infection with flu-like symptoms including rhinorrhea, congestion, sore throat, sneezing, cough, and fever, though clinical features are less characteristic in children than adults and are indistinguishable from respiratory syncytial virus (RSV) infection. 1, 2

High-Risk Populations

• Children under 5 years: HMPV causes 5-10% of hospitalizations for acute respiratory tract infections, with severe bronchiolitis and pneumonia as potential complications 2, 3
• Elderly patients: Particularly vulnerable to severe disease and complications, similar to RSV 1, 4
• Immunocompromised patients: Hematopoietic stem cell transplant (HSCT) recipients face highest risk for progression to lower respiratory tract disease, especially early post-transplant 5
• Patients with chronic lung disease: Including COPD and asthma, at increased risk for exacerbations 1, 6

Risk Factors for Severe Disease in Immunocompromised Patients

• Higher corticosteroid exposure 5
• Neutropenia and lymphopenia 5
• Early post-transplant period in HSCT recipients 5

Diagnostic Approach

Molecular methods, specifically reverse transcriptase PCR (RT-PCR), are the preferred diagnostic modality due to fastidious growth in cell culture. 2, 3, 7

Specimen Collection

• Respiratory secretions or nasopharyngeal swabs in viral transport medium are specimens of choice 1
• Transport at room temperature within 2 hours 1
• In immunocompromised adults, lower respiratory tract specimens (BAL, endotracheal aspirate) are significantly more sensitive than upper respiratory specimens (BAL: 88.9% vs nasal wash: 15%) 1

Testing Platforms

• Multiplex PCR respiratory panels now commonly include HMPV detection alongside other respiratory viruses 1
• Immunofluorescent assays available but less sensitive than molecular methods 1
• Culture takes 4-7 days and is not recommended for routine diagnosis 1, 3

Diagnostic Pitfalls

• Upper respiratory tract testing may miss lower tract infections, particularly in critically ill and immunocompromised patients 1
• High rates of discordance (44%) between upper and lower respiratory specimens in HSCT recipients 1
• Co-detection with other pathogens is extremely common (2-33% in children under 2 years), making attribution of disease difficult 1, 8

Management Recommendations

Immunocompetent Patients

There is no antiviral agent with established efficacy for treatment of HMPV infection; supportive care is the only recommended approach. 1, 5

Supportive Care Measures

• Oxygen therapy titrated to maintain adequate saturation 5
• Monitoring of vital signs, oxygen saturation, and respiratory status 5
• Fluid and electrolyte management 5
• High-protein, high-vitamin, carbohydrate-containing diets for nutritional support 5
• Cool-mist humidification and elevation of head of bed 6

Respiratory Support Escalation

• High-flow nasal oxygen (HFNO) or non-invasive ventilation (NIV) for moderate to severe ARDS 5
• Invasive mechanical ventilation with prone positioning for severe ARDS 5

Hospitalization Criteria for Children

• Moderate to severe disease with respiratory distress and hypoxemia (SpO2 <90% at sea level) 1
• Infants less than 3-6 months with suspected bacterial CAP 1
• Inability of family to provide appropriate observation 8

Immunocompromised Patients

Consider treating HMPV lower respiratory tract disease with ribavirin and/or intravenous immunoglobulin in immunocompromised patients, particularly HSCT recipients and leukemia patients with pneumonia, despite lack of randomized controlled trial data. 5

Key Treatment Principles

• Upper respiratory tract infection alone does not typically warrant antiviral therapy 5
• Treatment decisions are complex due to frequent co-detection with other pathogens 5
• No general recommendation for treatment can be made based on available evidence 5
• Single cases of severe disease and fatal outcomes have been reported even with treatment attempts 5

Bacterial Superinfection Management

Empiric treatment of suspected bacterial superinfection should provide activity against S. pneumoniae, S. aureus, and H. influenzae with antibiotics such as amoxicillin-clavulanate, cefpodoxime, cefprozil, cefuroxime, or a respiratory fluoroquinolone. 1

When to Suspect Bacterial Coinfection

• Clinical deterioration after initial improvement 8
• Increased systemic inflammation 8
• Radiographic findings suggestive of bacterial pneumonia 8
• Secondary bacterial infections occur in 26-77% of hospitalized adults with viral pneumonia 1

Special Considerations for Asthma Patients

• Continue regular asthma controller medication without interruption 6
• Have albuterol readily available: 2 puffs every 4-6 hours as needed for wheezing 6
• If tachypnea, chest retractions, or moderate-to-severe respiratory distress develop, administer oral prednisolone 1-2 mg/kg/day for 3-5 days 6
• Avoid over-the-counter cough and cold medications 6

Infection Control

Implement standard and droplet precautions to prevent nosocomial transmission. 5

Key Infection Control Considerations

• Asymptomatic and prolonged shedding occurs, particularly in HSCT patients 5
• Estimated incubation period: 2.6 days 5
• High rates of asymptomatic shedding in immunocompromised populations 5
• Nosocomial outbreaks can occur 5

Return to School/Work

• Children may return when fever-free for 24 hours without fever-reducing medication and feeling well enough for normal activities 6
• Encourage frequent handwashing 6

Common Pitfalls to Avoid

• Do not request antibiotics for uncomplicated viral infection - HMPV does not respond to antibiotics 6
• Do not stop asthma controller medications during viral illness - increases exacerbation risk 6
• Do not rely on nasopharyngeal bacterial cultures for lower respiratory tract infection diagnosis - high false positive rate due to colonization 8
• Do not assume urinary pneumococcal antigen positivity indicates causation - poor specificity in children 8
• Do not assume all detected pathogens are clinically significant - PCR detects low levels that may not represent active infection 8