Differential Diagnosis for a 5-Month-Old with Fever

Single Most Likely Diagnosis

• Viral upper respiratory tract infection: This is the most common cause of fever in infants, often due to viruses such as rhinovirus, respiratory syncytial virus (RSV), or influenza. The justification for this being the single most likely diagnosis is the high prevalence of viral infections in this age group and the typical presentation of fever with or without other symptoms like cough, runny nose, or irritability.

Other Likely Diagnoses

• Urinary tract infection (UTI): UTIs are more common in infants than often realized and can present with nonspecific symptoms such as fever alone. The lack of specific symptoms makes it an important consideration in the differential diagnosis.
• Otitis media: Ear infections are common in young children and can cause fever, irritability, and pulling at the ears, although these symptoms may not always be present.
• Gastroenteritis: Viral gastroenteritis can cause fever, vomiting, and diarrhea in infants. The presence of gastrointestinal symptoms would support this diagnosis.

Do Not Miss Diagnoses

• Bacterial meningitis: Although less common, bacterial meningitis is a life-threatening condition that requires immediate diagnosis and treatment. Fever, irritability, and signs of increased intracranial pressure or neck stiffness (if present) would suggest this diagnosis.
• Sepsis: Sepsis, particularly neonatal sepsis, is a critical condition that can present with fever, lethargy, and poor feeding. Early recognition is crucial for effective management.
• Pneumonia: Bacterial pneumonia can cause fever, cough, and difficulty breathing. It's a serious condition that needs prompt antibiotic treatment.

Rare Diagnoses

• Kawasaki disease: This is an acute febrile illness of childhood that can lead to coronary artery aneurysms if not treated. It's characterized by fever for more than five days, plus four of five criteria: bilateral conjunctival injection, cervical lymphadenopathy, polymorphous rash, oral mucosal changes, and extremity changes.
• Congenital or acquired immunodeficiency: Infants with immunodeficiencies may present with recurrent or severe infections, including those causing fever.
• Inborn errors of metabolism: Certain metabolic disorders can present with episodes of fever, among other symptoms, due to metabolic crises.

Each of these diagnoses has a different level of urgency and requires a tailored approach to diagnosis and management. The key to managing a febrile infant is a thorough history, physical examination, and judicious use of diagnostic tests to identify the cause of the fever and initiate appropriate treatment.