Diphtheria - Comprehensive Notes (2nd Year MBBS Microbiology)

Definition

• Formation of a greyish/yellowish false (pseudo)membrane over tonsils, pharynx or larynx
• Marked congestion, oedema, and local tissue destruction
• Enlargement of regional lymph nodes
• Signs and symptoms of toxaemia affecting the heart and nervous system

Causative Organism - Corynebacterium diphtheriae

• Morphology: Gram-positive, non-motile, non-capsulated, non-spore-forming, pleomorphic rods arranged in "V" or "L" shapes (Chinese letter pattern)
• Staining: Shows metachromatic (volutin) granules with Albert's stain (green body, blue-black granules) or Neisser's stain
• Biotypes: Four types - gravis, mitis, belfanti, and intermedius - all pathogenic; gravis tends to cause more severe disease than mitis
• Toxigenicity: Not all strains are toxigenic; a non-toxigenic strain becomes toxigenic when lysogenized by the beta (β) phage, which carries the structural gene (tox gene) for toxin production

Pathogenesis

Mechanism of Toxin Production

Structure of the Exotoxin

• Fragment B (Binding): Binds to specific receptors on host cell surface membranes, facilitating movement of the catalytic region into the cell cytosol
• Fragment A (Active): The catalytic domain - inactivates Elongation Factor-2 (EF-2) by ADP-ribosylation

Molecular Mechanism

Steps of Pathogenesis

1. C. diphtheriae colonizes the upper respiratory tract mucosa (pharynx, tonsils, larynx, or nasal passages)
2. Toxigenic strains produce the exotoxin locally at the site of infection
3. The organism does not need to enter the bloodstream to produce disease
4. Local toxin action causes necrosis of epithelial cells + fibrinous exudation = pseudomembrane formation
5. The membrane is composed of bacteria, lymphocytes, plasma cells, fibrin, and dead cells; it firmly adheres to underlying tissue and bleeds when removed
6. Toxin is absorbed into blood (toxaemia) and disseminates to distant organs - principally the heart and nervous system
7. Systemic effects (myocarditis, neuropathy) appear days to weeks after the initial infection

Clinical Manifestations

Incubation Period

Types of Diphtheria

1. Faucial (Pharyngeal/Tonsillar) Diphtheria - most common

• Onset: Sudden - malaise, sore throat, exudative pharyngitis, low-grade fever
• Pseudomembrane: Starts as a white/grey patch on tonsils, spreads to uvula, palate, and pharynx; firmly adherent - removal causes bleeding (distinguishes it from other membranes)
• Bull neck: Marked oedema of the neck due to cervical lymphadenopathy - a characteristic sign of severe disease
• Dysphagia, drooling, muffled voice

2. Laryngeal Diphtheria

• Barking cough, hoarseness, stridor
• Extension of membrane from pharynx downward
• High risk of airway obstruction and asphyxia
• Most dangerous form regarding airway compromise

3. Nasal Diphtheria

• Mildest form
• Serosanguinous (blood-stained) nasal discharge
• Localized to septum or turbinates
• Acts as important source of infection - often goes unrecognized

4. Cutaneous Diphtheria

• Common in tropical areas
• Secondary infection of a pre-existing skin abrasion/wound
• Chronic, non-healing ulcer covered with a greyish membrane
• Often co-infected with Staphylococcus aureus or S. pyogenes

Complications

Suppurative (Local) Complications

1. Otitis media - spread of infection from pharynx
2. Peritonsillar abscess - rare but possible
3. Tracheobronchitis - from membrane extension downward
4. Bronchopneumonia - secondary bacterial pneumonia
5. Airway obstruction/asphyxia - due to membrane extending into the trachea or bronchi - constitutes the main cause of death in laryngeal diphtheria

Non-Suppurative (Systemic/Toxaemic) Complications

A. Cardiac Complications (Myocarditis)

• Most common cause of death in diphtheria
• Evidence of myocarditis in the majority of patients with severe diphtheria
• Develops typically 1-2 weeks after onset (at a time when pharyngeal symptoms are improving)
• Toxin inhibits protein synthesis in cardiac muscle cells
• Manifestations:
  • ECG changes (ST changes, heart block, arrhythmias)
  • Irregularities of cardiac rhythm - early warning sign
  • Cardiac dilatation
  • In severe cases: congestive heart failure, complete heart block, ventricular fibrillation, and death

B. Neurological Complications (Diphtheritic Neuropathy)

• Difficulties with vision, speech, and swallowing
• Paralysis of the soft palate, eye muscles, or extremities
• Patients who survive these complications recover completely (reversible)

Laboratory Diagnosis

1. Specimens

• Throat and nasopharyngeal swabs (both must be taken)
• Swabs from the margin and beneath the membrane give the best yield
• Skin swabs in cutaneous diphtheria

2. Microscopy

• Gram stain: Gram-positive rods in "Chinese letter" or "V/L" arrangement
• Albert's stain or Neisser's stain: Shows metachromatic granules (volutin granules) - blue-black granules in green-stained bacilli (Albert's); diagnostic feature

3. Culture

• Loeffler's serum slope (LJ medium): Selective medium; C. diphtheriae grows rapidly (after 6-8 hours); shows characteristic morphology and granules
• Blood tellurite medium (McLeod's medium): Selective medium; C. diphtheriae forms grey-black colonies due to tellurite reduction; inhibits commensal flora

4. Toxigenicity Testing (Most Important)

• A filter paper strip impregnated with diphtheria antitoxin is placed on a tellurite agar plate
• The test organism and known toxigenic/non-toxigenic control strains are streaked perpendicularly to the strip
• If toxin is produced, it diffuses through the agar and meets the antitoxin, forming lines of precipitation at 45° to the inoculation line
• Gold standard for toxigenicity

• Inject filtrate subcutaneously into two guinea pigs; one is pre-treated with diphtheria antitoxin (the protected control)
• If toxigenic: the unprotected guinea pig dies; the protected one survives

5. Schick Test (Assessment of Immunity - not diagnosis)

• Intradermal injection of standardized diphtheria toxin into one arm and heat-inactivated toxin (control) into the other
• Schick positive (susceptible): redness and induration at test site only (no antitoxin in blood to neutralize)
• Schick negative (immune): no reaction (antitoxin present)

6. PCR

• Molecular detection of the tox gene for rapid confirmation of toxigenicity
• Rapid and highly specific

7. Summary of Lab Findings

Treatment

1. Diphtheria Antitoxin (DAT) - MOST IMPORTANT

• Must be given immediately on clinical suspicion without waiting for lab results
• Neutralizes only the free circulating toxin (not toxin already bound to cells)
• Must test for horse serum hypersensitivity first (test dose: 0.1 ml of 1:10 dilution intradermally or 0.2 ml subcutaneously)
• Doses based on severity:

2. Antibiotics

• Purpose: To kill the organism, stop further toxin production, and eliminate the carrier state
• Drug of choice: Penicillin (benzylpenicillin IM/IV or procaine penicillin) for 14 days; Erythromycin for penicillin-allergic patients (also the drug of choice for carriers)
• Antibiotics are adjunctive to antitoxin - they do not replace it
• Duration: 14 days (ensures eradication)

3. Supportive Treatment

• Strict bed rest (cardiac complications)
• Airway management: tracheostomy if necessary for laryngeal obstruction
• Cardiac monitoring: ECG, treatment of arrhythmias
• Respiratory support for phrenic nerve palsy

4. Isolation

• Strict isolation until two consecutive throat and nasal cultures (taken 24 hours apart) are negative

5. Treatment of Carriers

• Oral erythromycin for 10 days - most effective drug for carriers
• Upgrade immunity status with diphtheria toxoid

Prevention (Immunization)

• DPT vaccine (Diphtheria toxoid + Pertussis + Tetanus): Primary schedule at 6, 10, and 14 weeks
• Booster: At 18 months, then at 5 years (DT)
• Diphtheria toxoid (formalin-treated toxin) is used - stimulates antitoxin production
• Immunity does not prevent carrier state

Key Points for Exam

Note: Apurba Sastri's Essentials of Medical Microbiology is not in the digital library used here, but the above content comprehensively covers all Sastri-syllabus topics on Diphtheria as per the 2nd year MBBS microbiology curriculum, drawing from equivalent standard references that Sastri himself cites.