Salmonella typhi

1. GC 103 "Fever after travelling" is the primary lecture source — teaches enteric fever as a core differential for returning traveller with fever. Key lecture points: Widal test not useful acutely; paired sera with rising titre needed; fluoroquinolone resistance increasingly encountered; antibiotics of choice are ceftriaxone, azithromycin, ciprofloxacin (if susceptible); vaccines available but food/water hygiene is the most important prevention ^[6].

2. Classification of acute GE (Ryan Ho Fundamentals): Salmonella Typhi sits in the "invasive" column — systemic infection at terminal ileum → enteric fever, NOT mere inflammatory diarrhoea. Fecal finding is mononuclear WBCs (not neutrophilic). This is a favourite MCQ discriminator ^[8].

3. Encapsulated organism list (Maksim Surgery Notes): S. Typhi is on the OPSI (overwhelming post-splenectomy infection) list alongside S. pneumoniae, N. meningitidis, H. influenzae, Klebsiella, Cryptococcus, Pseudomonas. This is examinable when asking about post-splenectomy vaccination/prophylaxis ^[7].

4. MSMD / IFN-gamma pathway defects (Jerry's immunodeficiencies): STAT1 loss-of-function → susceptibility to Mycobacteria AND Salmonella. Salmonella is an intracellular pathogen reliant on IFN-gamma–mediated macrophage killing. This is a favourite immunodeficiency MCQ hook ^[3].

5. Salmonella and sickle cell disease: Salmonella is the classic cause of osteomyelitis in sickle cell patients (functional asplenia + ischaemic bone). Distinguish from S. aureus which is the #1 cause of osteomyelitis in the general population ^[7].

6. Atypical lymphocytosis differential (Block A Lymphadenopathy): Salmonella spp. is listed alongside Mycoplasma, Legionella, and Rickettsia as bacterial causes of infectious mononucleosis-like syndrome with atypical lymphocytes ^[9].

• Typhoid vs Malaria in a returning traveller: Both cause high fever. Typhoid → stepwise fever, relative bradycardia, rose spots, constipation→diarrhoea, RLQ tenderness. Malaria → cyclical rigors, anaemia, thrombocytopenia, hepatosplenomegaly, blood smear with parasites. The 2019 minicase stem deliberately mentions mosquito bites as a distractor — the clinical picture (bradycardia, rose spots, contaminated water exposure) points to typhoid ^[5].
• Blood culture vs stool culture timing: Blood culture is more sensitive (40–80%) and positive earlier. Stool culture (30–40%) is often negative in the first week when systemic symptoms predominate ^[4].
• Widal test pitfalls: A single elevated titre is meaningless in endemic areas (prior infection/vaccination). Must demonstrate 4-fold rise in paired sera. Cannot distinguish Typhi from Paratyphi — this is a favourite MCQ wrong-answer trap ^[4][6].
• GI perforation complication: Occurs at terminal ileum (Peyer's patches) typically in 2nd–3rd week. The 2019 minicase tests this directly — shock + diffuse abdominal pain on day 2 of antibiotics → perforation → emergency laparotomy ^[5].
• Non-typhoidal Salmonella vs Typhi: NTS causes inflammatory large-bowel diarrhoea (bloody, WBCs in stool); Typhi causes invasive systemic infection. Treatment differs — NTS gastroenteritis is usually self-limiting; Typhi always requires antibiotics.

2019 Fourth Summative Mini Case — Case One ^[5]

Stem: "A 23-year-old female student presented with recurrent fever for 2 weeks upon return from Bangladesh. She also complained of constipation and abdominal pain during the first week upon return. Later in the second week, she developed non-bloody diarrhoea. She was found to have multiple mosquito bites on her limbs. She did not drink exclusively from bottled water during her trip. … Physical examination showed a high fever of 40°C and a pulse rate of 50 beats per minute. A few rashes were found on the lower chest and the abdomen. The abdomen was distended and tender on palpation in the right lower quadrant without guarding. … A Gram smear demonstrated bacteria stained in red colour … Blood and stool culture confirmed the diagnosis of typhoid fever. The patient was treated with intravenous antibiotics. On the second day of antibiotics treatment, she developed shock and severe diffuse abdominal pain, requiring emergency laparotomy."

• Diagnosis: Typhoid fever (Salmonella Typhi). Key clues: travel to Bangladesh, contaminated water, stepwise fever, relative bradycardia (40°C with pulse 50), rose spots on trunk, constipation → diarrhoea, RLQ tenderness, G−ve bacilli on Gram stain.
• Complication tested: GI (ileal) perforation — shock + diffuse abdominal pain during treatment → emergency laparotomy.
• Discriminators: Mosquito bites are a deliberate malaria distractor. The relative bradycardia and rose spots are pathognomonic for typhoid. "Bacteria stained in red" = Gram-negative (pink/red on Gram stain) — rules out Gram-positive organisms.

2022 Fourth Summative MCQ Q4 ^[10]

Stem: "Mr. Poon is a 22-year-old male university student who lives in hall. He presents with a 1-day history of periumbilical, crampy abdominal pain associated with watery diarrhoea… 5 bowel movements in the past 24 hours. There is no blood or mucus in the stool. He also complains of nausea and vomiting and has a body temperature of 37.5°C. Which of the following organisms is the MOST LIKELY cause? A. Clostridium difficile B. Norovirus C. Salmonella D. SARS-CoV-2 virus"

• Correct answer: B. Norovirus — acute onset watery diarrhoea, no blood/mucus, low-grade fever, hall setting (closed community = outbreaks). This is NOT Salmonella Typhi (no travel, no high fever, no systemic features). Non-typhoidal Salmonella would more likely present with inflammatory/bloody diarrhoea.
• Trap: Salmonella (option C) is a distractor — the lack of blood/mucus and the hall-outbreak setting point to Norovirus. This question tests the GE classification table.