## Clinical Features of Pertussis **Key Point:** The 'whoop' is NOT pathognomonic and is absent in up to 50% of cases, particularly in vaccinated children, infants <6 months, and adolescents/adults. Its absence does not exclude pertussis. ### Stages of Pertussis | Stage | Duration | Features | |-------|----------|----------| | **Catarrhal** | 1–2 weeks | Mild URI symptoms, minimal cough, highly contagious | | **Paroxysmal** | 2–8 weeks (can extend to 12 weeks) | Severe paroxysmal cough, post-tussive vomiting, apnea in infants | | **Convalescent** | Weeks to months | Gradual decline in cough frequency | **High-Yield:** The whoop is more common in infants and young children; it is often absent in older children, adolescents, and vaccinated individuals. Diagnosis should NOT rely on the presence of a whoop. ### Diagnosis by Stage 1. **Catarrhal stage**: Culture and PCR are most sensitive (>90%) 2. **Early paroxysmal stage**: PCR remains sensitive; culture sensitivity declines 3. **Late paroxysmal stage**: Serology (IgA, IgG antibodies) is preferred; culture rarely positive **Clinical Pearl:** Lymphocytosis (absolute count >10,000/μL, often >20,000/μL) is characteristic of the paroxysmal stage and may be the only clue in older children or vaccinated individuals presenting atypically. **Mnemonic:** **WHOOP is NOT always present** — Remember that absence of whoop does NOT rule out pertussis, especially in vaccinated or partially vaccinated children. ### Why the Correct Answer is Wrong Option 0 states the whoop is "pathognomonic and present in all cases" — both claims are false. The whoop is neither pathognomonic (can occur in other conditions) nor universal (absent in >50% of cases overall).
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