Differential Diagnosis for a 5-month-old with Viral Bronchitis and Moraxella Infection

• Single Most Likely Diagnosis:
  • Viral bronchitis with secondary bacterial infection: This is the most likely diagnosis given the initial positive test for viral bronchitis and the subsequent positive test for Moraxella, which is a common cause of bacterial infections in children, often following a viral respiratory infection.
• Other Likely Diagnoses:
  • Pneumonia: Given the symptoms of bronchitis and the presence of Moraxella, pneumonia is a possible diagnosis, as Moraxella catarrhalis can cause pneumonia, especially in young children.
  • Acute otitis media: Moraxella catarrhalis is also a common cause of acute otitis media (middle ear infection), which can occur concurrently with or following a respiratory infection.
• Do Not Miss Diagnoses:
  • Severe bacterial infection (e.g., sepsis): Although less likely, it's crucial not to miss a severe bacterial infection, which could have significant morbidity and mortality if not promptly treated.
  • Pertussis (whooping cough): Given the respiratory symptoms, pertussis should be considered, especially if the vaccination status is incomplete or if there's been exposure to someone with pertussis.
• Rare Diagnoses:
  • Congenital immunodeficiency: A rare but important consideration in a young child with recurrent or severe infections, as it could indicate an underlying immunodeficiency.
  • Cystic fibrosis: While rare, cystic fibrosis can present with recurrent respiratory infections, and Moraxella catarrhalis can be one of the pathogens involved. This diagnosis would be considered if there are other suggestive symptoms or findings.