Pertussis is one of those Step 1 bugs that shows up everywhere: microbiology, immunology, pediatrics, and even pharmacology (macrolides, vaccines). If you can explain why Bordetella pertussis causes the classic “whooping cough,” you’ll also understand its toxins, lab diagnosis, complications, and the key management/vaccine pearls that USMLE loves.
Quick ID: What is Bordetella pertussis?
Bordetella pertussis is a small, gram-negative coccobacillus that causes pertussis (whooping cough), a toxin-mediated respiratory infection.
High-yield characteristics (Step-ready)
- Gram stain: Gram-negative coccobacillus
- Oxygen: Obligate aerobe
- Motility: Nonmotile
- Reservoir: Humans only
- Transmission: Respiratory droplets
- Key virulence tools:
- Pertussis toxin (PT)
- Adenylate cyclase toxin
- Tracheal cytotoxin
- Filamentous hemagglutinin (adhesion)
First Aid cross-reference: Microbiology → Gram-negative bacteria → Bordetella (look for “pertussis toxin ADP-ribosylates → ↑cAMP” and “whooping cough,” plus “Regan-Lowe/Bordet-Gengou media”).
Pathophysiology: How it causes disease (the toxin story)
Pertussis is largely toxin-mediated, and it stays on the respiratory epithelium (it’s not a classic invasive bloodstream pathogen).
Step 1 core mechanism: Pertussis toxin
Pertussis toxin ADP-ribosylates → inactivates it → disinhibits adenylate cyclase → ↑cAMP.
Consequences of ↑cAMP (high yield):
- Lymphocytosis (classically absolute lymphocytosis)
- Impaired immune cell function (e.g., neutrophil/macrophage activity)
- Increased insulin release has been described historically, but lymphocytosis is the big testable clue
Other important virulence factors (often tested together)
- Adenylate cyclase toxin: directly increases cAMP inside phagocytes → impairs killing
- Tracheal cytotoxin: damages ciliated epithelium → loss of mucociliary clearance
- Filamentous hemagglutinin (FHA): promotes adherence to ciliated respiratory cells
Why the cough is so intense
Cilia damage + thick secretions + inability to clear debris → paroxysms of coughing. The “whoop” is often from a forceful inspiratory effort against a narrowed glottis after prolonged coughing.
Clinical presentation: Stages you should recognize instantly
Pertussis has a classic staged illness. USMLE questions often give a timeline plus a key symptom.
1) Catarrhal stage (≈ 1–2 weeks)
- Mild URI symptoms: rhinorrhea, sneezing, low-grade fever
- Mild cough that progressively worsens
- Most contagious stage
Exam trap: early pertussis can look like a common cold—this is why outbreaks happen.
2) Paroxysmal stage (≈ 2–6 weeks)
- Paroxysms of cough (sudden, violent coughing fits)
- Inspiratory “whoop” (more classic in children than adults)
- Posttussive emesis
- Apnea/cyanosis can occur in infants
High-yield complications
- Subconjunctival hemorrhages
- Rib fractures (from severe coughing)
- Pneumothorax
- Secondary bacterial pneumonia (major cause of death in infants)
- Seizures/encephalopathy (less common but testable, often related to hypoxia)
3) Convalescent stage (weeks to months)
- Gradual resolution
- Cough can persist (“100-day cough” concept)
Adults vs infants (common vignette patterns)
- Adults/teens: prolonged cough, may lack classic whoop; often source of spread to infants
- Infants: may present with apnea rather than a whoop; high risk for severe disease
Diagnosis: What to order and what you’ll see
Best initial practical test (board-style and real life)
- Nasopharyngeal swab for PCR (fast, sensitive)
Culture (classic Step fact)
- Culture on Bordet-Gengou agar or Regan-Lowe medium
- Regan-Lowe is commonly emphasized in clinical settings; FA often lists both.
Classic lab clue
- Marked lymphocytosis on CBC (due to pertussis toxin)
When to suspect pertussis (high-yield trigger)
- Paroxysmal cough + posttussive vomiting
- Cough illness >2 weeks, especially with household exposure
- Infant with apneic episodes
Treatment: What to give and why
Antibiotics (mainstay)
- Macrolides: azithromycin, clarithromycin, erythromycin
- Alternative: TMP-SMX (if macrolide intolerance)
Important concept: Antibiotics are best early (catarrhal stage) to reduce symptoms, but later they mainly:
- Reduce transmission
- May not dramatically change cough duration once paroxysmal stage is established
Supportive care (especially in infants)
- Oxygen, suctioning, hydration
- Monitor for apnea and complications
Post-exposure prophylaxis (PEP)
- Macrolide prophylaxis for close contacts (especially if high-risk contacts: infants, pregnant people, immunocompromised)
First Aid cross-reference: Pharm/micro crossover—macrolides for respiratory pathogens; pertussis management often appears in vaccine/immunization contexts.
Prevention: Vaccines & immunity (USMLE loves this)
DTaP/Tdap overview
Pertussis prevention uses acellular vaccine:
- DTaP (children): diphtheria, tetanus, acellular pertussis
- Tdap (adolescents/adults): booster with pertussis component
What’s in the “acellular” pertussis vaccine?
- Inactivated components such as:
- Pertussis toxoid
- Filamentous hemagglutinin, pertactin, fimbriae (varies by formulation)
High-yield vaccine pearls
- Immunity wanes over time → adolescents/adults can get mild disease and transmit to infants
- Tdap in pregnancy (each pregnancy, typically 27–36 weeks) to protect newborn via maternal antibodies
- Herd protection is imperfect because acellular vaccines reduce disease severity but don’t fully block colonization/transmission as well as older whole-cell vaccines (concept sometimes referenced in explanations)
High-yield associations & “favorite” exam hooks
The toxin mechanism (memorize this line)
- Pertussis toxin ADP-ribosylates → ↑cAMP → lymphocytosis
Symptom triad you should be able to recall instantly
- Paroxysmal cough
- Inspiratory whoop
- Posttussive vomiting
Commonly tested differentiators (mini table)
| Feature | Pertussis | Viral URI | Croup (Parainfluenza) |
|---|---|---|---|
| Cough quality | Paroxysmal fits, “whoop,” posttussive emesis | Mild/moderate, non-paroxysmal | Barking cough, inspiratory stridor |
| Fever | Low/absent | Variable | Usually low-grade |
| CBC | Lymphocytosis | Usually normal/mild changes | No classic lymphocytosis |
| Best test | PCR from nasopharyngeal swab | Clinical | Clinical ± viral testing |
“Buzzwords” that point to B. pertussis
- Unvaccinated child or waning immunity in teen/adult
- Apnea in an infant
- Household outbreak
- Cough so severe it causes vomiting or hemorrhages
First Aid-style micro summary (rapid review)
- Gram-negative coccobacillus, obligate aerobe
- Pertussis toxin: ADP-ribosylates → ↑cAMP → lymphocytosis
- Adenylate cyclase toxin: ↑cAMP in immune cells
- Tracheal cytotoxin: damages cilia → cough
- Diagnosis: PCR; culture on Bordet-Gengou / Regan-Lowe
- Tx: macrolides (treat + prophylaxis)
- Prevention: DTaP/Tdap (acellular; immunity wanes)
USMLE-style checkpoints (self-test)
- If a question says “ADP-ribosylates ”, you should think: pertussis toxin → ↑cAMP → lymphocytosis.
- If a question says posttussive emesis or apnea in an infant, think pertussis and choose PCR nasopharyngeal swab + azithromycin.
- If a question is about preventing newborn disease, think Tdap during pregnancy + ensure household contacts are vaccinated.