2.4 Enteric & Neglected Tropical Infections

Enteric & Neglected Tropical Infections

Enteric infections and Neglected Tropical Diseases (NTDs) impose a substantial health burden in Kenya, particularly in arid/semi-arid counties, informal settlements, and agricultural zones with limited water, sanitation, and hygiene (WASH) infrastructure. For the Clinical Officers Council examinations, clinicians must understand the pathogenesis, classical clinical presentation, diagnostic limitations, emergency resuscitation protocols, and pharmacological regimens for Typhoid Fever, Cholera, Protozoan Enteric Diseases, and Visceral Leishmaniasis (Kala-Azar).

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1. Typhoid Fever (Enteric Fever)

Etiology & Pathophysiology

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

Following ingestion, the organisms survive gastric acidity, invade the mucosa of the terminal ileum through microfold (M) cells overlying Peyer's patches, and are engulfed by mononuclear phagocytes. Instead of being destroyed, S. Typhi survives and replicates within macrophage phagosomes. The bacteria travel through mesenteric lymph nodes into the thoracic duct, generating a transient, asymptomatic primary bacteremia that seeds the reticuloendothelial system (liver, spleen, bone marrow, and gallbladder).

Following an incubation period of 7 to 14 days, secondary sustained bacteremia develops, marking the onset of clinical disease and seeding the gallbladder. Bacteria shed continuously from the infected gallbladder back into the bile duct and small intestine, re-invading necrotic Peyer's patches.

Classic Clinical Progression by Week

NATURAL TIMELINE OF UNTREATED TYPHOID FEVER:

Week 1: Insidious Onset
  • "Step-ladder" remittent fever: Temperature rises incrementally each evening, reaching 39–40°C by day 7.
  • Dull frontal headache, generalized malaise, dry cough.
  • Faget's sign (relative bradycardia): Inappropriately low heart rate relative to body temperature.
  • "Typhoid tongue": Centrally coated tongue with red margins and tip. Constipation common.

Week 2: Sustained Bacteremia & Toxemia
  • Sustained plateau fever (39.5–40.5°C).
  • "Typhoid state": Stupor, apathy, muttering delirium, vacant stare.
  • Marked abdominal distension, splenomegaly, hepatomegaly.
  • "Rose spots": Faint, blanching, erythematous maculopapular lesions on abdomen/chest (in 10–30%).
  • Diarrhea ("pea-soup" consistency) begins in a subset of patients.

Week 3: Complication Stage (Severe Intestinal Ulceration)
  • Hyperplastic Peyer's patches undergo ischemic necrosis and ulcerate parallel to bowel long axis.
  • Massive Lower Gastrointestinal Hemorrhage: Sudden melena, drop in blood pressure and hematocrit.
  • Intestinal Perforation: Occurs at the antimesenteric border of the terminal ileum. Manifests as
    abrupt peritonitis (sudden severe abdominal pain, board-like rigidity, guarding, absent bowel sounds,
    subdiaphragmatic crescentic free gas on erect chest/abdominal radiograph).

Week 4: Convalescence or Relapse / Chronic Carriage
  • Gradual defervescence; 5–10% suffer clinical relapse 1–2 weeks later.
  • 2–5% become chronic biliary carriers (persistently shedding S. Typhi in feces >1 year).

Diagnostic Strategy & The Widal Test Pitfall

• Blood Culture: The diagnostic gold standard during Week 1 (sensitivity 70% to 85% prior to antibiotics).
• Bone Marrow Culture: The most sensitive diagnostic investigation overall (>90% sensitivity), remaining culture-positive even after several days of prior antibiotic therapy.
• Stool & Urine Culture: Positive predominantly during Weeks 2 and 3, reflecting biliary shedding and intestinal mucosal sloughing.
• The Widal Agglutination Test (Diagnostic Limitations):
  • Measures agglutinating antibodies against S. Typhi O (somatic lipopolysaccharide) and H (flagellar protein) antigens.
  • Major Pitfalls: The Widal test suffers from high false-positive rates due to cross-reactivity with non-typhoidal Salmonella, malaria, enteroviruses, and prior TAB immunization. False negatives occur in early illness. A single isolated titer is non-diagnostic. Only a four-fold rise in paired titers taken 10 to 14 days apart has diagnostic value. The Widal test should never be used as a standalone confirmation for clinical decisions.

Pharmacological Management

• Uncomplicated Typhoid:
  • First-line: Oral Azithromycin (1 g on Day 1, followed by 500 mg daily for 6 days, total 7 days) or oral Ciprofloxacin (500 mg BD for 7 to 10 days, where fluoroquinolone resistance is low) or oral Cefixime (20 mg/kg/day divided BD for 10–14 days).
• Severe / Complicated Typhoid:
  • First-line: Intravenous Ceftriaxone (2 g once daily or 1 g BD for 10 to 14 days).
  • Adjunctive Dexamethasone: For patients presenting with delirium, stupor, coma, or shock: Intravenous Dexamethasone (initial loading dose 3 mg/kg, followed by 1 mg/kg every 6 hours for 48 hours). This high-dose steroid protocol reduces mortality by >70% in severe typhoid encephalopathy without increasing perforation risk.
  • Surgical Laparotomy: Mandatory emergency intervention for ileal perforation (peritoneal lavage and primary two-layer closure or wedge resection / temporary ileostomy).

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2. Cholera

Etiology & Pathogenesis

Cholera is an acute, fulminant diarrheal illness caused by toxigenic strains of Vibrio cholerae (serogroups O1 [biotypes Classical and El Tor] and O139 Bengal). It is a comma-shaped, highly motile Gram-negative rod propelled by a single polar flagellum. Transmission occurs via ingestion of water or food contaminated directly or indirectly with feces from infected individuals.

• Choleragen Enterotoxin Mechanism:
  1. V. cholerae colonizes the small intestinal enterocytes using Toxin-Coregulated Pili (TCP).
  2. The bacterium secretes Cholera Toxin (choleragen), an $AB_5$ subunit enterotoxin.
  3. Five B subunits bind specifically to GM1 ganglioside receptors on enterocyte brush border membranes.
  4. The enzymatic A1 subunit enters the cell cytoplasm and catalyzes the irreversible ADP-ribosylation of the stimulatory G-protein (Gs alpha), freezing it in the active state.
  5. Adenylate cyclase is continuously stimulated, driving massive production of cyclic AMP (cAMP).
  6. Elevated cAMP hyperactivates the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) chloride channels, producing rapid, massive secretion of chloride, bicarbonate, sodium, and water into the gut lumen, overwhelming colonic absorptive capacity.

Clinical Manifestations

Following an incubation period of 2 hours to 5 days, the patient experiences sudden, painless, voluminous watery diarrhea accompanied by effortless vomiting.

• "Rice-Water Stool": Stool rapidly loses fecal odor and color, becoming a translucent, greyish-white, watery fluid with flecks of mucus and epithelial cells, with a faint, inoffensive fishy odor. Stool output can reach 500 to 1,000 mL per hour.
• Rapid Dehydration: Loss of 10% or more of total body weight within hours leads to sunken eyes, dry mucosal membranes, "washerwoman's hands" (shriveled skin over digits), profound hypotension, hypovolemic shock, anuria, severe metabolic acidosis (loss of bicarbonate), and hypokalemic muscle cramps.

WHO Rehydration Protocols

Immediate fluid replacement is the single life-saving priority. Mortality can exceed 50% without rehydration, but drops below 1% with rapid, appropriate fluid replacement.

WHO PLAN C INTRAVENOUS RESUSCITATION PROTOCOL (Total 100 mL/kg Ringer's Lactate):

Adults & Children ≥ 1 Year:
  • First 30 mL/kg: Infuse rapidly over the first 30 MINUTES.
  • Remaining 70 mL/kg: Infuse over the next 2.5 HOURS (Total duration: 3 Hours).

Infants (< 1 Year Old):
  • First 30 mL/kg: Infuse over the first 1 HOUR.
  • Remaining 70 mL/kg: Infuse over the next 5 HOURS (Total duration: 6 Hours).

*Continuous Monitoring: Check radial pulse and skin pinch continuously. As soon as the patient is
conscious and can swallow, provide oral ORS (5 mL/kg/h) concurrently to replace dynamic stool losses.

Adjunctive Antibiotic Therapy

Antibiotics shorten diarrhea duration, decrease fluid requirements by 50%, and eliminate vibrio excretion within 48 hours. They are administered once vomiting is controlled.

• First-Line for Adults: Oral Doxycycline 300 mg as a single dose.
• Preferred for Children & Pregnant Women: Oral Azithromycin 1 g single dose (or 20 mg/kg single dose in children).
• Alternative: Oral Ciprofloxacin 1 g single dose.
• Zinc Supplementation: Children aged 6 months to 5 years must receive 20 mg elemental zinc daily for 10 to 14 days (10 mg daily for infants <6 months) to facilitate mucosal repair.

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3. Protozoan Enteric Infections: Amoebiasis vs. Giardiasis

Clinical Pearl: Metronidazole clears tissue trophozoites in the bowel wall and liver, but is poorly active against cysts in the colonic lumen. In amoebiasis, a course of a luminal amoebicide (Diloxanide furoate or Paromomycin) is mandatory following metronidazole to prevent relapse and break the transmission cycle.

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4. Visceral Leishmaniasis (Kala-Azar)

Etiology & Vector Ecology in Kenya

Visceral Leishmaniasis (VL), or Kala-Azar ("black fever"), is a systemic protozoan disease caused by the Leishmania donovani complex. In Kenya, it is transmitted by female sandflies of the genus Phlebotomus (Phlebotomus martini and Phlebotomus orientalis). Transmission is endemic in arid and semi-arid lowlands of the Rift Valley and Eastern/Northern Kenya: Baringo, West Pokot, Turkana, Isiolo, Marsabit, Wajir, and Garissa counties. Sandflies breed in ventilation shafts of termite mounds (Macrotermes) and deep fissures of dry, cracked black cotton soils.

Pathophysiology

Flagellated promastigotes inoculated into the dermis during a sandfly bite are engulfed by host tissue macrophages. Within macrophage phagolysosomes, they transform into aflagellated amastigotes (Leishman-Donovan [LD] bodies). Amastigotes divide by binary fission, burst the host cell, and infect neighboring mononuclear cells, disseminating throughout the reticuloendothelial system: the spleen, liver, bone marrow, and lymph nodes.

• Spleen: Marked histiocytic hyperplasia causes massive splenomegaly. Hypersplenism accelerates clearance of peripheral blood cells.
• Bone Marrow: Extensive reticuloendothelial infiltration suppresses normal erythropoiesis, granulopoiesis, and megakaryopoiesis, producing pancytopenia.
• Immune Dysregulation: Marked polyclonal B-cell activation leads to hypergammaglobulinemia, whereas cell-mediated Th1 immunity is profoundly suppressed.

Classic Clinical Triad

1. Prolonged, Irregular, Undulating Fever: Persisting for weeks or months, characteristically displaying a "double daily temperature spike" (two distinct fever peaks within a 24-hour period).
2. Massive Splenomegaly & Hepatomegaly: The spleen enlarges progressively (often extending into the right iliac fossa), with a firm, smooth, non-tender edge. Contrasts sharply with severe temporal, thoracic, and proximal limb muscle wasting.
3. Pancytopenia:
   • Severe Anemia: Marked pallor, tachycardia, fatigue.
   • Leukopenia / Neutropenia (WBC <4.0 × 10⁹/L): Predisposes to secondary, fatal bacterial infections (severe pneumonia, septicemia, cancrum oris/noma).
   • Thrombocytopenia (Platelets <100 × 10⁹/L): Epistaxis, gingival bleeding, petechiae.

Diagnostic Evaluation

• Serological Screening: rK39 Rapid Diagnostic Test:
  • An immunochromatographic dipstick detecting serum IgG antibodies against the 39-amino-acid repeat of the recombinant kinesin protein of L. donovani.
  • High sensitivity (>95%) and specificity in immunocompetent East African patients.
  • Limitation: Cannot differentiate between active disease and previous cured infection because antibodies persist for years. Unreliable in HIV-coinfected individuals.
• Parasitological Confirmation (Gold Standard):
  • Direct microscopic visualization of intracellular amastigotes (LD bodies: round/oval cells, 2–4 μm, with a blue cytoplasm, distinct nucleus, and rod-shaped kinetoplast) on Giemsa-stained tissue aspirate smears.
  • Splenic Aspirate: Highest sensitivity (>95–98%). Critical Hazard: Carries a risk of splenic capsule laceration, massive intra-abdominal hemorrhage, and death. Strictly contraindicated if Prothrombin Time is prolonged (>5 seconds above control) or Platelet count is <40,000/μL.
  • Bone Marrow Aspirate: Sensitivity 60% to 85%; substantially safer than splenic aspiration.
  • Lymph Node Aspirate: Sensitivity 50% to 60%; safe and quick in patients with prominent peripheral lymphadenopathy.

Treatment Protocols (Kenya National Guidelines)

FIRST-LINE COMBINATION THERAPY (Total 17 Days):
  • Sodium Stibogluconate (SSG): 20 mg Sb5+/kg/day IM or slow IV injection
    PLUS
  • Paromomycin: 15 mg paromomycin base/kg/day IM injection
  • Both administered concurrently once daily for 17 CONSECUTIVE DAYS.
  • Benefits: Achieves >95% cure rates, shortens duration (vs. 30 days SSG monotherapy),
    and reduces cumulative antimony cardiotoxicity and nephrotoxicity.

SECOND-LINE / SPECIAL POPULATION REGIMEN:
  • Liposomal Amphotericin B (AmBisome):
    - Total cumulative dose: 20 to 30 mg/kg administered by slow IV infusions.
    - Mandatory for: Pregnant women, patients >45 years, severe baseline cardiac or renal disease,
      children <2 years, HIV co-infection, or patients failing SSG+PM therapy.

• Post-Kala-Azar Dermal Leishmaniasis (PKDL): A dermatological condition developing months or years after successful treatment of visceral leishmaniasis. Characterized by a macular, papular, or nodular rash predominantly on the face and trunk. Although patients are systemically well, the skin lesions harbor abundant amastigotes, serving as a silent, potent anthroponotic reservoir for ongoing sandfly transmission.