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Gram-negativeBacilliRespiratory

Bordetella pertussis

Gram-negative coccobacillus that causes pertussis (whooping cough), characterized by severe paroxysmal coughing with inspiratory whoop, primarily in unvaccinated children.

Organism Card

DomainMust know
Identity
  • G−ve coccobacillus (small, pleomorphic) [1]
  • Strictly aerobic; fastidious; non-motile
  • Genus Bordetella; species pertussis (human-only pathogen)
  • Closely related: B. parapertussis (milder disease), B. bronchiseptica (animal pathogen)
Lab discriminator
  • Nasopharyngeal swab/aspirate is the optimal specimen (NOT throat swab in viral transport medium) [1]
  • Culture on Bordet-Gengou (potato-glycerol-blood agar) or Regan-Lowe (charcoal agar) → small, mercury-drop "bisected-pearl" colonies
  • Multiplex RT-PCR on NPA is most sensitive [1]
  • Swabs for B. pertussis culture should NOT be refrigerated/frozen — plate immediately (unlike most other bacterial swabs) [1]
  • Oxidase +, catalase +, urease − (vs B. parapertussis urease +)
Reservoir / transmission
  • Humans are the only reservoir (no animal reservoir)
  • Transmitted by respiratory droplets (highly contagious, attack rate ~80% in susceptible household contacts)
  • Infectivity highest during catarrhal stage (before the classic cough)
Key virulence
  • Pertussis toxin (PT): ADP-ribosylates Gi protein → ↑cAMP → lymphocytosis, impaired immune cell chemotaxis; causes marked lymphocytosis on CBC
  • Filamentous haemagglutinin (FHA): adhesin to ciliated respiratory epithelium
  • Tracheal cytotoxin: damages ciliated cells → impaired mucociliary clearance → paroxysmal cough
  • Adenylate cyclase toxin: inhibits phagocyte function
  • Pertactin & fimbriae: additional adhesins (vaccine targets)
Clinical syndromes
  • Classical whooping cough — 3 stages [2]:
  • Catarrhal (1-2 wk): URTI symptoms, most contagious
  • Paroxysmal (2-8 wk): spasmodic cough bursts → inspiratory "whoop"; post-tussive vomiting; cough worse at night
  • Convalescent (wks-months): gradual ↓ cough ("100-day cough")
  • Infants < 6 mo: may present with apnoea / cyanosis without classic whoop → high mortality
  • Adolescents/adults (waned immunity): prolonged non-specific cough, often undiagnosed
  • B. pertussis can cause bronchiolitis in children < 2 y [2]
  • Complications: pneumonia, seizures, encephalopathy, subconjunctival haemorrhage, rectal prolapse
Diagnosis
  • Best specimen: NPA or nasopharyngeal swab (NOT oropharyngeal) [1]
  • PCR is most sensitive diagnostic test [1]
  • Culture on Bordet-Gengou / Regan-Lowe agar (low sensitivity, especially after antibiotics started)
  • Serology (anti-PT IgG): useful in later stages when culture/PCR may be negative
  • CBC: marked lymphocytosis (WCC can be > 50 × 10⁹/L) — classic lab clue
  • Pitfall: lymphocytosis in infants can mimic leukaemia (→ d/dx with ALL in haematology context) [3]
Treatment
  • Macrolides are first-line: azithromycin (preferred, short course) or erythromycin or clarithromycin
  • Antibiotics shorten infectious period but have limited effect on cough once paroxysmal stage established
  • TMP-SMX as alternative if macrolide-intolerant
  • Supportive care for infants: O₂, suction, monitoring for apnoea
Prevention
  • DTaP-IPV vaccine: 3 doses at 2nd, 4th, 6th months (HK childhood immunisation programme) [2][4]
  • D = diphtheria toxoid, T = tetanus toxoid, aP = acellular pertussis, IPV = inactivated poliovirus [2][4]
  • Booster: Tdap (reduced-antigen) for adolescents / adults / pregnant women (cocooning strategy)
  • Post-exposure prophylaxis: macrolides for close contacts (especially household contacts of infants)
  • Notifiable disease in Hong Kong
  • Droplet precautions for hospitalised cases until 5 days of effective antibiotic therapy
Classic traps
  • Catarrhal stage most contagious but least recognisable → diagnosis often delayed
  • Marked lymphocytosis can mimic ALL in infants — check PBS for blasts [3]
  • B. pertussis swab: use bacterial transport medium, NOT viral transport medium (contains antibiotics that kill Bordetella) [1]
  • Acellular vaccine (aP) does NOT prevent colonisation/transmission as effectively as it prevents disease
  • B. parapertussis: urease +, milder illness, no pertussis toxin gene expression

Exam Intelligence

  1. GC 101 (Diagnosis of Infections) — NPA/throat swab specimen rules: "Throat swabs in bacterial transport medium (NOT viral transport medium containing antibiotics) are most important for…Bordetella pertussis." Furthermore, "With the exception of Bordetella pertussis and Neisseria gonorrhoeae, swabs for culture of other pathogens should be refrigerated." This means B. pertussis swabs must be plated immediately — a classic MCQ trap [1].

  2. GC 101 — PCR as most sensitive test: "Multiplex RT-PCR (most sensitive)" is emphasised across respiratory pathogen diagnosis. For B. pertussis, PCR on nasopharyngeal specimens is the gold standard diagnostic modality [1].

  3. Felix Lai Paediatrics & Medicine Notes — DTaP-IPV vaccine schedule: "3 doses required at 2nd, 4th and 6th months respectively; Pertussis (aP) = Acellular" [2][4]. Know that the "a" in DTaP stands for acellular pertussis — contrasted with older whole-cell vaccines (wP) that had more side effects.

  4. Felix Lai Paediatrics Notes — B. pertussis as a cause of bronchiolitis: "Occasionally a bacterial infection such as Mycoplasma pneumoniae or Bordetella pertussis" can cause bronchiolitis in children < 2 years [2]. This is a discriminator in a paediatric LRTI vignette — if the child has paroxysmal cough, apnoea, and marked lymphocytosis, think pertussis rather than RSV.

  5. Ryan Ho Haematology — Pertussis in the d/dx of reactive lymphocytosis: Listed alongside infectious mononucleosis, HIV, and TB as a cause of reactive lymphocytosis that must be distinguished from ALL [3]. In an exam stem showing a child with extremely high WCC and lymphocyte-predominant picture, pertussis should be in the differential before jumping to leukaemia.

  • Pertussis vs RSV bronchiolitis: Both present in infants with cough and respiratory distress. Pertussis → paroxysmal cough, inspiratory whoop, post-tussive vomiting, marked lymphocytosis; RSV → wheeze/crackles, seasonal (winter), antigen-detectable on NPA RV panel.
  • Pertussis vs Chlamydia trachomatis afebrile pneumonia of infancy: Both cause cough in young infants. C. trachomatis → staccato cough (individual coughs separated by inspiration), age 2-4 months, conjunctivitis, eosinophilia. Pertussis → paroxysmal cough with whoop, lymphocytosis, no conjunctivitis.
  • Lymphocytosis trap: Pertussis causes the highest WCC among infectious causes of lymphocytosis (can exceed 100 × 10⁹/L in infants). If a stem shows extreme lymphocytosis in an infant with cough, do NOT default to ALL — check for blasts on PBS.
  • Specimen transport trap: B. pertussis is killed by antibiotics in viral transport medium and by cold temperatures — always use bacterial transport medium and plate promptly [1].
  • Vaccine does not eliminate carriage: Acellular pertussis vaccine prevents severe disease but does not fully prevent nasopharyngeal colonisation or transmission — hence outbreaks can still occur in vaccinated populations.

No directly relevant past paper questions specifically testing Bordetella pertussis were identified in the indexed Fourth Summative past paper stems (2016–2025). The organism appears indirectly in differential diagnosis lists (e.g., reactive lymphocytosis d/dx in haematology contexts [3]) but no standalone pertussis-focused MCQ, SAQ, or minicase stem was retrieved from the provided past paper text.

[1] Lecture slides: GC 101. Diagnosis of infections [Handouts].pdf [2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf [3] Senior notes: Ryan Ho Haemtology.pdf [4] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf

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