5.4 Tropical & Endemic Infections: Malaria, Typhoid & Dengue

Tropical & Endemic Infections: Malaria, Typhoid & Dengue

Tropical and endemic infectious diseases constitute a major proportion of general medicine questions in UPSC CMS. Mastery of species-specific parasite life cycles, diagnostic timelines (e.g., blood culture vs Widal timing in typhoid), clinical staging of dengue plasma leakage, and national guidelines (NVBDCP) for antimicrobial therapy is required.

1. Malaria

Malaria is caused by intraerythrocytic protozoa of the genus Plasmodium (P. falciparum, P. vivax, P. ovale, P. malariae, P. knowlesi), transmitted by the bite of infected female Anopheles mosquitoes.

Clinical Paroxysms & Species Differentiation

Classic malarial paroxysms consist of a Cold stage (chills, severe rigor, 15-60 min) → Hot stage (high fever >40°C, intense headache, 2-6 hrs) → Sweating stage (profuse diaphoresis, defervescence, 2-4 hrs).

Pathophysiology of Severe P. falciparum Malaria

P. falciparum erythrocyte membrane protein 1 (PfEMP-1) expressed on infected RBC surfaces binds to endothelial receptors (CD36, ICAM-1, VCAM-1), causing cytoadherence, rosetting, and microvascular sequestration. This blocks microvascular blood flow, causing tissue hypoxia and organ dysfunction:

1. Cerebral Malaria: Sequestration in cerebral capillaries causing unrousable coma (Glasgow Coma Scale <11), convulsions, and symmetric upper motor neuron signs.
2. Blackwater Fever: Massive intravascular hemolysis resulting in severe hemoglobinuria (dark brown/black urine), jaundice, and acute tubular necrosis / acute kidney injury.
3. Metabolic Complications: Severe hypoglycemia (exacerbated by quinine-stimulated insulin secretion) and lactic acidosis.

National NVBDCP Treatment Guidelines

• Uncomplicated P. falciparum Malaria:
  • North-Eastern States: Artemether-Lumefantrine (AL) for 3 days + Single dose Primaquine 0.75 mg/kg on Day 2.
  • Rest of India: Artesunate + Sulfadoxine-Pyrimethamine (AS+SP) for 3 days + Single dose Primaquine 0.75 mg/kg on Day 2.
• Uncomplicated P. vivax Malaria:
  • Chloroquine: 25 mg/kg total dose over 3 days (10 mg/kg Day 1, 10 mg/kg Day 2, 5 mg/kg Day 3).
  • Primaquine: 0.25 mg/kg daily for 14 days to eradicate liver hypnozoites and prevent relapse.
  • MANDATORY PRECAUTION: Screen for G6PD deficiency prior to initiating 14-day Primaquine therapy! If G6PD deficient, give Primaquine 0.75 mg/kg weekly for 8 weeks under close supervision.
• Severe / Complicated Malaria (Any Species):
  • Drug of Choice: IV Artesunate 2.4 mg/kg at 0 hours, 12 hours, and 24 hours, then once daily for at least 24 hours until oral therapy can be tolerated.

2. Typhoid (Enteric) Fever

Typhoid fever is a systemic infection caused by Salmonella enterica serovar Typhi (and Paratyphi A, B, C), transmitted via the fecal-oral route through contaminated food and water.

Clinical Progression by Week

• Week 1: Remittent step-ladder fever pattern (rising each day), severe headache, malaise, abdominal pain, constipation (common in adults). Key sign: Relative Bradycardia (Faget Sign) — heart rate inappropriately low for the level of high fever.
• Week 2: Continuous high fever, Rose spots (faint, salmon-pink, evanescent macules on upper abdomen/chest that blanch on pressure), hepatosplenomegaly, abdominal distension, and "pea-soup" diarrhea.
• Week 3: "Typhoid State" (apathy, delirium, coma vigil). High risk of life-threatening complications due to necrosis of hyperplastic Peyer's patches in the terminal ileum:
  1. Intestinal Hemorrhage (sudden drop in temperature and blood pressure, melena).
  2. Ileal Perforation (sudden severe right lower quadrant pain, guarding, rigidity, and pneumoperitoneum / free gas under diaphragm on erect X-ray abdomen).
• Week 4: Gradual resolution or chronic gallbladder carrier state (defined as excretion of S. typhi in stool/urine for >1 year; associated with gallstones).

Diagnostic Workup & Antimicrobial Management

Remember the BASU mnemonic for optimal culture diagnostic yield:

• Blood culture: Positive in Week 1 (85-90% sensitive).
• Antibody / Widal test: Positive in Week 2 onwards (Significant titers: TO > 1:160 indicates active acute infection; TH > 1:200 indicates past infection or vaccination).
• Stool culture: Positive in Week 3.
• Urine culture: Positive in Week 4.

Antimicrobial Protocol

• First-line Choice (Empiric / Uncomplicated): Ceftriaxone 2 g IV once daily for 10-14 days OR Oral Azithromycin 500 mg daily for 7 days (due to widespread Nalidixic Acid Resistant S. typhi / NARST and fluoroquinolone resistance).
• Chronic Carrier State: Oral Ciprofloxacin 750 mg twice daily for 4 weeks OR Ampicillin for 6 weeks; Cholecystectomy if gallstones present.

3. Dengue Virus Infection

Dengue is caused by an arbovirus (Flaviviridae family, 4 serotypes: DEN-1, DEN-2, DEN-3, DEN-4) transmitted by the daytime-biting Aedes aegypti mosquito. Primary infection confers lifelong immunity to that specific serotype. Secondary infection with a different serotype leads to Antibody-Dependent Enhancement (ADE), markedly increasing the risk of severe dengue.

Phases of Dengue & Clinical Case Definitions

1. Febrile Phase (Days 1 - 3): Sudden high-grade fever, retro-orbital pain, severe myalgia/arthralgia ("breakbone fever"), facial flushing, positive Tourniquet Test (>20 petechiae per square inch following cuff inflation halfway between SBP and DBP for 5 minutes).
2. Critical Phase (Days 3 - 7, coinciding with Defervescence):
   • Characterized by systemic vascular endothelial leakage (plasma leakage).
   • Warning Signs: Persistent vomiting, severe abdominal pain, mucosal bleeding, lethargy, fluid accumulation (pleural effusion, ascites), hepatomegaly >2 cm.
   • Laboratory Hallmark: Rapid rise in Hematocrit (HCT) ≥20% above baseline (indicating hemoconcentration) accompanied by rapid drop in platelet count (<100,000/mm³).
   • Dengue Shock Syndrome (DSS): Severe plasma leakage leading to circulatory collapse, narrow pulse pressure (≤20 mmHg), cold clammy extremities, and hypotension.
3. Recovery Phase (Days 7 - 10): Plasma leakage ceases, fluid is reabsorbed. Classic rash appears: "islands of white in a sea of red" (confluent erythematous petechial rash with spared normal skin pockets), sinus bradycardia.

Diagnostic Tests & Fluid Resuscitation

• NS1 Antigen ELISA: Positive during early febrile phase (Days 1 to 5).
• Dengue IgM ELISA: Becomes positive from Day 5 onwards (indicates acute/recent infection).
• Dengue IgG ELISA: High titers within first 1-2 days indicate secondary dengue infection.
• Fluid Management Protocol: Judicious IV crystalloid therapy (0.9% Normal Saline or Ringer's Lactate). In critical phase without shock, start at 5-7 mL/kg/hr, titrating down based on HCT and urine output (>0.5 mL/kg/hr).
• Platelet Transfusion Guidelines: Platelet transfusion is NOT routinely recommended based solely on low platelet numbers. Indications for platelet transfusion are strictly limited to:
  • Presence of significant active systemic bleeding.
  • Emergency surgical procedures.
  • Severe thrombocytopenia (<10,000/mm³) with high risk of bleeding.