Salmonella Typhi

Unlike the non-typhoidal salmonellae, S. Typhi has no animal reservoir; transmission is faecal-oral from human cases or chronic carriers, so it tracks with poor sanitation. It requires a relatively large inoculum and is acid-labile, so gastric acid is protective and proton pump inhibitors increase risk. After ingestion it invades M cells over Peyer patches, survives within macrophages, and disseminates via the reticuloendothelial system, producing a systemic rather than diarrhoeal illness. The Vi polysaccharide capsule is antiphagocytic and is the basis of the injectable vaccine; the alternative is a live attenuated oral Ty21a vaccine. Chronic carriage in the gallbladder, particularly with gallstones, allows continued shedding, immortalized by the case of Typhoid Mary; treatment of carriage may require prolonged antibiotics or cholecystectomy. Blood culture is the diagnostic test in the first week, with stool positive later; bone marrow culture is most sensitive. Fluoroquinolone resistance is now widespread, so ceftriaxone or azithromycin is generally used.