Paper I
Question
Laboratory diagnosis and prophylaxis of Diphtheria
Answer
Laboratory diagnosis of diphtheria (Corynebacterium diphtheriae):
- Sample collection: swab from beneath the edge of the pseudomembrane, collected before antibiotics wherever possible.
- Direct microscopy: Gram stain (Gram-positive club-shaped bacilli in Chinese-letter/palisade arrangement) and Albert’s or Neisser’s stain to demonstrate metachromatic (volutin) granules.
- Culture: on Loeffler’s serum slope (rapid growth within 6–8 hours) and tellurite blood agar (selective, differentiates biotypes by colony morphology).
- Toxigenicity testing: Elek’s gel precipitation test — confirms whether the isolated strain produces diphtheria exotoxin; in-vivo guinea pig test is an older alternative.
- PCR for the toxin gene (tox) where available.
Prophylaxis
- Active immunization: DPT vaccine (diphtheria toxoid, part of the routine childhood schedule) — primary series of 3 doses in infancy with booster doses at 18 months, 5 years, and 10 years (Td/TT).
- Passive immunization: not used for routine prophylaxis, but diphtheria antitoxin (DAT) is given promptly to treat clinical cases (neutralizing free toxin) rather than as prophylaxis.
- Contact management: throat swab culture and chemoprophylaxis (erythromycin) for close contacts of a confirmed case; contacts should also have their immunization status checked/updated.
- Case management: isolation of cases until non-infectious (culture-negative) to prevent further transmission.