In a 23‑day‑old infant with cough and dyspnea, no fever, and a normal chest X‑ray, what is the most likely diagnosis and recommended immediate management?

Most Likely Diagnosis: Pertussis (Whooping Cough)

In a 23-day-old infant presenting with cough and dyspnea without fever and a normal chest X-ray, pertussis is the most likely diagnosis and requires immediate macrolide antibiotic therapy plus close monitoring for life-threatening complications including apnea, bradycardia, and respiratory failure. 1, 2

Clinical Reasoning for Pertussis

The constellation of findings strongly suggests pertussis rather than other respiratory conditions:

• Absence of fever is characteristic of pertussis in young infants, distinguishing it from bacterial pneumonia which typically presents with fever >38.5°C 1, 2
• Normal chest X-ray does not exclude pertussis, as radiographic changes are often absent early in the disease or may show only nonspecific findings 1
• Age 23 days places this infant in the highest-risk category for severe pertussis complications and mortality 3, 2
• Cough with dyspnea in an afebrile neonate is a classic presentation of early pertussis, which progresses from mild coryza and cough to paroxysmal episodes 4, 2

Why Not Other Diagnoses

Bronchiolitis is unlikely because while it can present with cough and dyspnea, it typically includes wheezing and follows a viral prodrome with rhinorrhea; the absence of fever and the infant's very young age (23 days) make bronchiolitis less probable 5, 6. Additionally, if wheeze were present, primary bacterial pneumonia would be very unlikely 1, 5.

Bacterial pneumonia is effectively ruled out by the combination of no fever and normal chest X-ray, as bacterial pneumonia in infants typically presents with fever >38.5°C, tachypnea >50/min, chest recession, and radiographic abnormalities 1.

Chlamydia trachomatis pneumonia could be considered given the age, but typically presents with a "staccato" cough, crackles more than dyspnea, and a history of conjunctivitis (sticky eye) in 50% of cases in the neonatal period—none of which are mentioned here 1.

Immediate Management Algorithm

Step 1: Confirm Diagnosis

• Obtain nasopharyngeal swab for B. pertussis PCR immediately (preferred test in infants <3 weeks of illness) 1, 4
• Check complete blood count looking for leukocytosis with lymphocytosis (characteristic finding in pertussis) 2
• Do NOT delay treatment while awaiting test results given the high-risk age 2

Step 2: Initiate Antibiotic Therapy

• Start azithromycin immediately (first-line macrolide for pertussis) 4, 2
• Dosing: 10 mg/kg once daily for 5 days in infants
• Alternative: Erythromycin if azithromycin unavailable, though it has more gastrointestinal side effects 4
• Antibiotics eradicate bacterial carriage and reduce transmission but do not significantly alter clinical course once paroxysmal stage begins 4, 7

Step 3: Hospitalize for Monitoring

• All infants <3 months with suspected pertussis require hospitalization for monitoring of life-threatening complications 2
• Monitor continuously for:
  • Apneic episodes (can occur suddenly and be fatal) 3, 2
  • Bradycardia and cyanosis during coughing paroxysms 2
  • Oxygen saturation (maintain >92%) 5
  • Feeding difficulties and hydration status 2

Step 4: Supportive Care

• Provide supplemental oxygen if saturation <92% 5
• Ensure adequate hydration (may require IV fluids if feeding poorly) 6
• Gentle nasal suctioning to clear secretions 6
• Prepare for potential intubation and mechanical ventilation if respiratory failure develops 2

Step 5: Infection Control and Contact Management

• Isolate the infant using droplet precautions 8
• Identify and treat all household contacts prophylactically with macrolide antibiotics regardless of vaccination status 4, 2
• The source is most often a family member with unrecognized cough illness 2

Critical Red Flags Requiring Immediate Escalation

• Apneic episodes (may be the only manifestation in very young infants) 3, 2
• Severe paroxysmal coughing with gasping, gagging, eye bulging 2
• Cyanosis or bradycardia during coughing episodes 2
• Leukocytosis >20,000-30,000/mm³ (associated with worse outcomes including pulmonary hypertension) 2
• Post-tussive vomiting (classic pertussis feature) 1

Common Pitfalls to Avoid

• Do not wait for the classic "whoop" sound—very young infants often do not develop the characteristic whoop and may present only with apnea or gagging 2
• Do not be reassured by the absence of fever—pertussis characteristically presents without fever in young infants 1, 2
• Do not be falsely reassured by a normal chest X-ray—pertussis does not require radiographic abnormalities for diagnosis 1
• Do not assume vaccination status protects the infant—at 23 days old, this infant has not yet received any pertussis vaccines (first dose given at 6 weeks) 2
• Do not discharge home without confirming adequate monitoring capability—the deceptively well appearance can rapidly deteriorate 2

Prognosis and Follow-Up

• Deaths in pertussis relate to leukocytosis, pulmonary hypertension, and pneumonia 2
• Young infants account for practically all pertussis hospitalizations and deaths 3
• Clinical course typically progresses through catarrhal, paroxysmal, and convalescent stages over weeks to months 4, 7
• Prevention of future cases requires maternal Tdap vaccination during pregnancy (second or third trimester) to provide passive immunity to newborns 3, 2