Ans. (a) Vi. agglutination test Vi antigen - Polysaccharide antigen enveloping the 0 antigen because of which many strains of S. typhi fails to agglutinate with the '0' antiserum Antigens of Salmonella H. antigan O. antigan Vi. antigan Present flagalla Integral pa of cell wall Envelops 'O' antigan Heat protein Phospholipid - protein polysaccharide comlex Surface polysaccharide Strongly immunogenic ldentical with endotoxin so, isless immunogenic Acts as virulence factor and is poorly immunogenic Antibody formation is rapid and in high titre Antibody formation is slow andtitre is low Antibody production is slow and titres is low Normally Vi antibody disappears early in conoalescence, lts persistance indicates the deaelopment of can'ier state, Note: Total absence of Vi antibody in a proven case of typhoid indicates poor prognosis. Chronic carriers of S. typhi: Persons who excrete bacilli for more than a year after clinical attack. Average carrier rate is 3% (i.e. 3% cases become chronic carriers). Carrier state is more common in females (cases are more common in males). Fecal carriers are more frequent than urinary carriers but urinary carriers are more dangerous. Urinary carrier state is often associated with some abnormality of the urinary tract. Fecal carrier stage is more common in patients with biliary abnormality and GI malignancy. Diagnosis of carriers: - Demonstration of Vi antigen Isolation of S. typhi by sewer swab technique. Treatment: - Ampicilin (4-6 g a day) together with probenecid for 6 week. Cholecystectomy with concomitant ampilcillin therapy has been regarded as the MOST suscessfull approach to treatment of carriers.