What is the recommended treatment for a 5-month-old infant with suspected pertussis (whooping cough) presenting with paroxysms of cough and post-tussive vomiting?

Treatment of Suspected Pertussis in a 5-Month-Old Infant

Start azithromycin immediately at 10 mg/kg/day for 5 days without waiting for laboratory confirmation, as early antibiotic treatment can reduce symptom severity and prevent transmission to vulnerable contacts. 1

Immediate Antibiotic Therapy

Azithromycin is the first-line agent for infants 1-5 months of age due to its effectiveness, superior safety profile, and significantly lower risk of infantile hypertrophic pyloric stenosis (IHPS) compared to erythromycin. 1 The recommended dosing is 10 mg/kg/day for 5 consecutive days. 1, 2

Alternative Macrolide Options

• Clarithromycin is also recommended as a first-line agent for this age group (1-5 months), with dosing at 7.5 mg/kg twice daily for 7 days. 1
• Avoid erythromycin in infants under 6 months whenever possible due to the association with IHPS; if it must be used, dose at 40-50 mg/kg/day divided for 14 days. 3, 1
• Trimethoprim-sulfamethoxazole can be used only if the infant is >2 months old and has a macrolide contraindication. 1

Critical Timing Considerations

Do not delay treatment waiting for culture confirmation—therapy should begin immediately upon clinical suspicion. 3, 1 Early treatment during the catarrhal phase (first 2 weeks) rapidly clears B. pertussis from the nasopharynx and decreases coughing paroxysms and complications. 3 While this 5-month-old with paroxysmal cough and post-tussive vomiting is likely in the paroxysmal phase, antibiotics are still indicated to prevent transmission even if clinical benefit is limited. 1

Isolation and Infection Control

Isolate the infant at home for 5 days after starting antibiotics to prevent spread to other vulnerable individuals. 3, 1 This is particularly critical given that pertussis has a secondary attack rate exceeding 80% among susceptible persons. 4

Household Contact Management

All household contacts require post-exposure prophylaxis (PEP) with the same antibiotic regimens used for treatment, administered within 21 days of exposure. 1, 4 This is strongly recommended for all household contacts, especially if there are other infants under 12 months or pregnant women in the third trimester in the home. 4

PEP Dosing for Contacts

• Infants <6 months: azithromycin 10 mg/kg/day for 5 days 1
• Children ≥6 months: azithromycin 10 mg/kg (max 500 mg) day 1, then 5 mg/kg (max 250 mg) days 2-5 1
• Adults: azithromycin 500 mg day 1, then 250 mg days 2-5 1

Diagnostic Confirmation

While treatment should not be delayed, obtain a nasopharyngeal aspirate or Dacron swab for culture to confirm B. pertussis, as isolation of the bacteria is the only certain way to make the diagnosis. 3 PCR is available but lacks universally accepted, validated techniques for routine clinical testing. 3

Supportive Care and Monitoring

Do not use β-agonists, antihistamines, corticosteroids, or pertussis immunoglobulin—these have no proven benefit in controlling coughing paroxysms. 3, 1 Monitor closely for complications including apnea (particularly dangerous in young infants), weight loss from post-tussive vomiting, secondary bacterial infections, and neurological complications. 2, 5

Medication Administration Precautions

• Do not administer azithromycin with aluminum- or magnesium-containing antacids as they reduce absorption. 1
• Use caution if hepatic impairment is present. 1
• Monitor for drug interactions with agents metabolized by cytochrome P450 (e.g., digoxin). 1

Vaccination Status Review

Verify and update the infant's vaccination status and ensure all household contacts are up to date with pertussis vaccination, as vaccine immunity wanes after 5-10 years. 4 The infant should continue the primary DTaP vaccination series as scheduled. 3