Diagnosis: Chlamydia trachomatis Pneumonia
The most likely diagnosis in this 3-month-old infant is Chlamydia trachomatis pneumonia (Option D), based on the distinctive combination of preceding conjunctivitis, staccato cough pattern, elevated eosinophils with normal white blood cell count, and absence of fever.
Clinical Reasoning
Key Distinguishing Features Present
The clinical presentation contains several pathognomonic features that point away from pertussis and toward Chlamydia trachomatis pneumonia:
Preceding conjunctivitis: This is the hallmark prodrome of C. trachomatis infection in infants, typically occurring 1-2 weeks before respiratory symptoms develop. Pertussis characteristically presents with conjunctivitis during the catarrhal phase alongside rhinorrhea and malaise, not as a distinct preceding event 1.
Staccato cough pattern: The description of "staccato" cough is clinically distinct from the paroxysmal cough of pertussis. Staccato cough consists of repetitive short bursts separated by brief inspirations, whereas pertussis features prolonged paroxysms with multiple cough bursts during a single expiratory phase without the ability to breathe during spells 1, 2.
Normal white blood cell count with eosinophilia: This laboratory pattern is characteristic of C. trachomatis pneumonia. In contrast, pertussis typically presents with leukocytosis and lymphocytosis in young infants, though these findings are not universally present 1, 3, 4.
Why Not Pertussis?
While pertussis remains a critical consideration in any infant with paroxysmal cough, several features argue against this diagnosis:
Absence of typical leukocytosis/lymphocytosis: Pertussis in infants under 12 months characteristically causes leukocytosis (≥16,000/mL) and lymphocytosis (≥11,000/mL), which are significantly more common in confirmed pertussis cases 3, 4.
Eosinophilia: Elevated eosinophils are not a feature of pertussis but are characteristic of C. trachomatis infection 1.
Cough pattern: While the question mentions "inspiration between each cough," true inspiratory whooping in pertussis consists of a continuous inspiratory sound with a whooping quality following multiple expiratory cough bursts 1, 2. The staccato pattern described is more consistent with Chlamydia.
Conjunctivitis timing: The conjunctivitis occurring "a few days ago" as a distinct preceding event is more consistent with C. trachomatis than the concurrent conjunctivitis seen in pertussis catarrhal stage 1.
Why Not Other Options?
Adenovirus pneumonia (Option B): While adenovirus can cause conjunctivitis and pneumonia, it typically presents with fever and does not characteristically cause eosinophilia or the specific staccato cough pattern described.
Mycoplasma pneumonia (Option C): This is extremely rare in infants under 6 months of age and does not explain the preceding conjunctivitis or eosinophilia pattern.
Clinical Implications
Diagnostic Approach
For this infant, the clinical diagnosis of C. trachomatis pneumonia should be confirmed with:
- Nasopharyngeal culture or PCR for C. trachomatis
- Chest radiography (typically shows bilateral interstitial infiltrates)
- Complete blood count confirming eosinophilia without leukocytosis
Treatment Considerations
If C. trachomatis is confirmed or strongly suspected, macrolide antibiotics (azithromycin or erythromycin) should be initiated 1, 5, 6. This is the same antibiotic class used for pertussis, which provides appropriate coverage if the diagnosis is uncertain.
Important Caveat
In any infant under 12 months presenting with severe cough, paroxysms, and respiratory distress, pertussis must always be considered and empiric treatment initiated while awaiting confirmatory testing 7, 3. The high morbidity and mortality of pertussis in this age group, combined with the need to prevent transmission, justifies empiric macrolide therapy even when the diagnosis is uncertain 1, 6. However, the specific constellation of findings in this case—particularly the preceding conjunctivitis and eosinophilia—makes C. trachomatis the most likely single diagnosis.