Makindo Medical Notes

💉 Immunisation targets the diphtheria toxin rather than the bacterium itself. The toxin gene is carried by a bacteriophage, not bacterial DNA.

📖 About

• Caused by Corynebacterium diphtheriae (and occasionally C. ulcerans).
• Fatality rate: 5–10%.
• The bacterium releases an exotoxin that blocks protein synthesis.
• Toxin gene is encoded by a bacteriophage (virus infecting the bacterium).
• Mild cases may go undiagnosed, making outbreaks difficult to trace.

🦠 Microbiology

• Gram-positive, club-shaped, non-motile rods.
• Classically arranged at angles forming “Chinese letter” shapes (V, L, W).
• Grow best at 37 °C in oxygen.
• Only phage-infected strains produce exotoxin.
• Exotoxin has two subunits:
  • A component: Blocks protein synthesis (RNA translation inhibition via ADP-ribosylation of EF-2).
  • B component: Targets oropharynx, heart, and nerves.
• Exotoxin is a 58 kDa A–B polypeptide linked by disulphide bond.
• Effects: local necrosis, systemic toxaemia, myocarditis, and demyelinating neuritis.

⚡ Virulence

• Corynephage β-infected strains release exotoxin.
• A subunit: ADP-ribosyl transferase → inhibits elongation factor-2.
• B subunit: Guides toxin to target tissues.

📤 Transmission

• Direct person-to-person contact.
• Inhalation of aerosolised droplets from infected individuals.

🧑‍⚕️ Clinical Presentation (Incubation: 2–4 days)

⚠️ Suspect toxin-induced myocarditis if tachycardia is out of proportion to fever — perform frequent ECGs.

• Pharyngitis: Grey “dirty” pseudomembrane (fibrin + dead cells) in throat.
• Bull neck: Cervical lymphadenopathy + swelling.
• Affected sites: nose, pharynx, larynx.
• Airway obstruction risk with stridor.
• Symptoms: sore throat, fever, malaise.
• Complications:
  • Myocarditis → arrhythmias, heart block, heart failure.
  • Cranial nerve palsies (diplopia, dysphagia, dysarthria).
  • Laryngeal nerve palsy → hoarseness.

🔬 Investigations

• Swab culture below pseudomembrane + PCR.
• Culture: Hoyle’s medium (blood tellurite) → black colonies; Tinsdale’s medium is an alternative.
• Strain types: gravis (severe), intermedius, mitis (milder) — based on colony/haemolysis pattern.
• Biochemical tests: Catalase, urease, nitrate reduction, cystinase, pyrazinamidase.
• Toxin detection: Elek test (precipitin line with antitoxin).
• Extra: Serotyping, phage typing, cardiac monitoring (ECG, echo).

💉 Mass vaccination in the mid-20th century led to near-eradication of diphtheria in developed countries.

🩺 Management

• Supportive: ABCs, isolation, cardiac monitoring, bed rest.
• Diphtheria Antitoxin: 10,000–30,000U IM (any age; higher if severe). Neutralises unbound toxin. ⚠️ Risk of anaphylaxis (horse serum origin).
• Antibiotics: IV/PO Erythromycin. Close contacts: 7 days of erythromycin (dosing by age).
• Airway: Laryngoscopy/bronchoscopy for pseudomembrane removal if obstruction risk.
• Contacts: Vaccinate or give booster; routine booster every 10 years for ongoing immunity.