Pertussis

Organism

Bordetella pertussis. Small fastidious Gram-negative coccobacillus. OBLIGATE human pathogen (no animal reservoir/environmental persistence — depends entirely on human-human transmission). “Whooping cough.” VERY transmissible — secondary attack rate 80-90% susceptible household contacts.

Virulence Factors

Pertussis toxin: AB exotoxin, analogous to cholera toxin (ADP-ribosylates G-protein, but INHIBITS regulation here) → lymphocytosis + disrupts immune clearance. Target of acellular vaccine toxoid. FHA (filamentous hemagglutinin) + Pertactin: attachment to ciliated respiratory epithelium. Tracheal cytotoxin: DESTROYS ciliated cells → paralyzes mucociliary clearance → EXPLAINS paroxysmal cough (only way to clear secretions is violent coughing).

Clinical Course — 3 Stages

Catarrhal (1-2wk): mild nonspecific URI (rhinorrhea, mild cough, low fever) — indistinguishable from viral URI. PEAK INFECTIVITY (diagnosis never suspected here, unchecked transmission).

Paroxysmal (2-6wk): DEFINING phase. Severe repetitive coughing fits → INSPIRATORY “WHOOP” (forced high-pitched gasp against partial glottis closure). Often + POST-TUSSIVE VOMITING. Patient appears WELL between paroxysms — episodic pattern.

Convalescent (weeks-months): gradual resolution, cough persists long time — “100-DAY COUGH.”

Clinical Manifestations by Age

Classic whoop: older children, previously healthy unvaccinated/waning-immunity.

INFANTS <6 MONTHS (HIGHEST risk severe disease/death): OFTEN NO CLASSIC WHOOP — APNEA = dominant/most dangerous manifestation (immature respiratory reserve, smaller airways). IMPORTANT: relying on classic whoop would MISS the highest-risk group.

Adults/adolescents (waning immunity): prolonged nonspecific cough, no classic paroxysmal pattern. IMPORTANT UNDERAPPRECIATED TRANSMISSION RESERVOIR to unvaccinated household infants — rationale for adult booster/pregnancy vaccination.

Laboratory Diagnosis

Culture (Bordet-Gengou/Regan-Lowe agar): highly specific, INSENSITIVE/SLOW, yield drops sharply as illness progresses past catarrhal stage (mismatch — exactly when clinically suspected/tested). PCR: better sensitivity/faster, PREFERRED method now. Serology: role later in illness (culture/PCR sensitivity declined further by then). Marked LYMPHOCYTOSIS on CBC: nonspecific but useful, striking in infants (pertussis toxin’s direct effect).

Treatment

Macrolides (azithromycin, or erythromycin/clarithromycin) = DOC. MOST effective for ↓TRANSMISSIBILITY/shortening infectious period if EARLY (catarrhal stage). MODEST effect on clinical course once paroxysmal-stage ciliary damage already done — antibiotics don’t meaningfully shorten established cough (important patient expectation point). Supportive care: apnea/respiratory monitoring in infants (may need hospitalization) = essential highest-risk group.

Prevention

Vaccination: acellular pertussis component of DPT/DTaP (purified toxoid+FHA, REPLACED older whole-cell reactogenic vaccine). Highly effective BUT IMMUNITY WANES over years → booster doses needed (Tdap adolescents/adults) → outbreaks continue even well-vaccinated populations (waning-immunity older individuals transmit to unprotected infants).

Maternal Tdap in pregnancy (3rd trimester): maternal Ab crosses placenta → passive newborn protection during vulnerable window before infant’s own vaccination series (see NIS, Vaccines topic).

Post-exposure macrolide prophylaxis: close contacts, esp. unvaccinated infants/household — standard given high transmissibility + infant risk.

The Organism

Bordetella pertussis is a small, fastidious, Gram-negative coccobacillus, an obligate human pathogen (no animal reservoir or environmental persistence, meaning the organism depends entirely on continuous human-to-human transmission to survive as a species) and the cause of pertussis (“whooping cough”) — genuinely one of the most transmissible respiratory infections in this curriculum, with secondary attack rates in susceptible household contacts reported as high as 80–90%.

Virulence Factors

B. pertussis produces several coordinated virulence factors that together explain its characteristic disease course: pertussis toxin — an AB exotoxin, structurally analogous to cholera toxin (ADP-ribosylating a G-protein, though here inhibiting rather than activating adenylate cyclase regulation, producing lymphocytosis and disrupting normal immune cell signalling and clearance) — is the organism’s central toxin, and is specifically what’s targeted by the toxoid component of the acellular vaccine (below). Filamentous haemagglutinin (FHA) and pertactin mediate tight attachment to ciliated respiratory epithelium. Tracheal cytotoxin directly damages and destroys the ciliated cells the organism has attached to, paralyzing mucociliary clearance in the airway — a genuinely direct mechanistic explanation for the disease’s hallmark paroxysmal cough, since with normal ciliary clearance destroyed, the only way to expel accumulating airway secretions is violent, repetitive coughing.

Clinical Course — Three Stages

Pertussis runs through a genuinely distinctive three-stage course over several weeks, worth understanding as a sequence since each stage differs in both symptoms and infectivity/diagnostic yield:

Catarrhal stage (1–2 weeks): mild, nonspecific upper respiratory symptoms — rhinorrhoea, mild cough, low-grade fever — clinically indistinguishable from an ordinary viral URI, but genuinely important as the stage of peak infectivity, since the diagnosis is essentially never suspected or made at this point, letting transmission continue unchecked before anyone realizes what’s actually circulating.
Paroxysmal stage (2–6 weeks): the disease’s defining phase — severe, repetitive coughing fits (paroxysms), each ending in a characteristic inspiratory “whoop” (a forced, high-pitched inspiratory gasp against a partially closed glottis, following the exhausting coughing fit) — and often followed by post-tussive vomiting, a genuinely useful, specific clinical clue in the right context. Between paroxysms, the patient can appear entirely well, which is itself a distinctive pattern (episodic severe symptoms punctuating an otherwise normal-appearing patient) worth recognizing.
Convalescent stage (weeks to months): gradual symptom resolution, though the cough can persist, diminishing gradually, for a genuinely prolonged period — pertussis is sometimes informally called the “100-day cough” for exactly this reason.

Clinical Manifestations by Age

The classic paroxysmal-whoop presentation is most typical in older children and previously healthy, unvaccinated or waning-immunity individuals. Infants under 6 months — the age group at genuinely highest risk of severe disease and death — often present atypically, without the classic whoop at all, instead showing apnoea as the dominant, most dangerous manifestation, reflecting their immature respiratory reserve and smaller airway calibre; this atypical, whoop-free presentation in the most vulnerable age group is a genuinely important, specifically testable point, since relying on the “classic” whoop to trigger suspicion would systematically miss the infants most at risk. Adults and adolescents, particularly those with waning vaccine-derived immunity (below), often present with a prolonged, non-specific cough without the classic paroxysmal pattern at all, and represent a genuinely important, underappreciated transmission reservoir to vulnerable, unvaccinated infants within the same household — a real public-health rationale behind the adult/adolescent booster and pregnancy-vaccination recommendations covered under Prevention.

Laboratory Diagnosis

Culture on specialized media (Bordet-Gengou or Regan-Lowe agar) is highly specific but genuinely insensitive and slow, and its yield drops sharply as illness progresses past the catarrhal stage (exactly when the disease actually becomes clinically recognizable and testing is actually ordered) — a real, practical diagnostic mismatch worth understanding. PCR offers substantially better sensitivity and faster turnaround, and has become the preferred diagnostic method in most modern clinical settings, particularly useful given how the culture-yield/clinical-suspicion timing mismatch above limits culture’s practical usefulness. Serology has a role later in illness, once culture/PCR sensitivity has further declined. Marked lymphocytosis on a routine complete blood count is a nonspecific but genuinely useful supportive clue, particularly striking in infants, reflecting pertussis toxin’s direct lymphocyte-mobilizing effect described above.

Treatment

Macrolides (azithromycin, or erythromycin/clarithromycin as alternatives) are the treatment of choice — genuinely most effective at reducing transmissibility and shortening the infectious period when given early, in the catarrhal stage, but with only modest effect on the actual clinical course once the paroxysmal stage’s tracheal cytotoxin-mediated ciliary damage has already occurred, since the damage driving the cough is already done by that point regardless of whether the organism itself is subsequently cleared by antibiotics — a genuinely important treatment-expectations point to convey, since patients/families are often disappointed that antibiotics don’t meaningfully shorten the cough once well established. Supportive care (particularly close monitoring for apnoea/respiratory compromise in young infants, who may need hospitalization) is essential in the highest-risk age group.

Prevention

Vaccination — the acellular pertussis component of DPT/DTaP (containing purified pertussis toxoid plus other antigens like FHA, replacing the older, more reactogenic whole-cell pertussis vaccine in most current schedules) — is highly effective but, genuinely importantly, immunity wanes over years, which is exactly why booster doses are recommended (Tdap for adolescents/adults) and why pertussis outbreaks continue to occur even in well-vaccinated populations, disproportionately affecting waning-immunity older children/adults who then transmit to unprotected infants. Maternal Tdap vaccination during pregnancy (typically third trimester) is a genuinely important, specifically targeted strategy — maternal antibody crosses the placenta, providing the newborn with passive protection during the vulnerable window before the infant’s own vaccination series (starting per the National Immunization Schedule, see Vaccines and Immunoprophylaxis) can generate active immunity. Post-exposure macrolide prophylaxis for close contacts, particularly unvaccinated infants and household members, is standard practice given pertussis’s high transmissibility and the severe risk to unprotected infants specifically.