In enteric fever (typhoid), the Widal test becomes positive in the second week of illness. The antigen used to detect Vi antibodies is clinically important because:
- A Vi antigen positivity distinguishes S. typhi from S. paratyphi A
- B Vi antigen tests replace blood culture for diagnosis in the first week
- C Anti-Vi antibody persists for years after infection or vaccination and can identify chronic carriers ✓
- D Vi antigen testing is used to assess vaccine efficacy of typhoid conjugate vaccine (TCV)
Explanation
The Vi polysaccharide capsular antigen is expressed by S. typhi and S. paratyphi C but not S. paratyphi A. Anti-Vi IgG antibody persists at high titres (≥1:40) for years in chronic typhoid carriers (bile reservoir), making it a useful screening tool to identify carriers in food handlers and suspected chronic carrier households. It is not used in acute diagnosis (blood culture is gold standard in week 1). Typhoid conjugate vaccine (TCV) produces anti-Vi antibody as a marker of vaccination but anti-Vi titres are not routinely used to evaluate individual vaccine response.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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Written and medically reviewed by the StethoPrep medical team.