## Distinguishing S. typhi from S. paratyphi Infection ### Key Microbiological Difference **Key Point:** Positive blood culture in the first week is the hallmark of S. typhi infection; S. paratyphi bacteraemia is less frequent and typically occurs later, making urine and stool cultures more common for paratyphi detection. ### Clinical and Laboratory Comparison | Feature | S. typhi | S. paratyphi | |---------|----------|---------------| | **Blood culture positivity** | High in week 1–2 (60–80%) | Low; often negative even in early illness | | **Urine culture** | Positive from week 2 onwards | Often positive; may be first positive culture | | **Stool culture** | Positive from week 3 onwards | Positive earlier than typhi | | **Rose spots** | Classic finding (5–10% of cases) | Rare or absent | | **Severity** | Generally more severe | Often milder, shorter duration | | **Splenomegaly** | Common | Common (not discriminating) | | **Relative bradycardia** | Characteristic | Present but less consistent | ### Clinical Pearl **Clinical Pearl:** The pattern of culture positivity is the most reliable discriminator. S. typhi shows early and sustained bacteraemia (blood culture positive in weeks 1–2), whereas S. paratyphi typically has weak or absent bacteraemia, making urine and stool cultures the diagnostic mainstay. This is why paratyphi infections are often diagnosed later or by culture of non-blood specimens. ### High-Yield Point **High-Yield:** In the first week of fever, a positive blood culture strongly suggests S. typhi; if blood culture is negative but urine or stool culture is positive, consider S. paratyphi. This distinction has epidemiological and prognostic implications — typhi is more likely to cause severe systemic disease and complications. [cite:Harrison 21e Ch 159]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.