Pertussis

01Pathophysiology

Bordetella pertussis colonizes the ciliated respiratory epithelium of the upper airways. The organism does not invade the bloodstream; instead, it produces a series of toxins and adhesins that cause localized damage and systemic immune dysregulation.

Pertussis toxin (PT) is the most important virulence factor. It is an ADP-ribosylating toxin that inactivates the inhibitory G-protein ($G_{i}G\_i$) of host cells. This leads to two testable consequences. First, in pancreatic islet cells, loss of $G_{i}G\_i$ signaling causes unopposed insulin secretion, which can produce hypoglycemia, particularly in neonates. Second, PT causes lymphocytosis by preventing lymphocyte migration from the bloodstream into lymph nodes, a phenomenon called "lymphocyte trapping." This is why the CBC classically shows a striking leukocytosis with lymphocyte predominance, not neutrophilia.

Filamentous hemagglutinin (FHA) and fimbriae mediate attachment to ciliated epithelial cells. Once attached, the organism releases tracheal cytotoxin, which directly destroys ciliated cells and paralyzes the mucociliary escalator. The loss of cilia results in mucus accumulation and impaired clearance, which is the direct cause of the prolonged, violent cough. Because the damage is structural and takes weeks to regenerate, the cough persists long after the bacteria are eradicated by antibiotics. This explains why macrolide therapy shortens infectivity but does not necessarily shorten the duration of cough if started late.

Adenylate cyclase toxin (ACT) impairs neutrophil and macrophage function, allowing the organism to evade early innate immunity. This is why the infection establishes itself during the catarrhal phase when it clinically resembles a benign upper respiratory infection, making early diagnosis difficult.

In young infants (under 6 months), the immune response to pertussis toxin can trigger severe pulmonary hypertension driven by leukocyte aggregation in the pulmonary vasculature. This is the mechanism behind fulminant or "malignant" pertussis, which carries a high mortality rate and may require exchange transfusion to reduce the white cell burden.

02Clinical Manifestation and Classification

Pertussis follows a three-stage clinical course. Recognizing the stage at presentation determines both the diagnostic approach and the expected benefit of antibiotic therapy.

Atypical Presentations

03Diagnostic Workup

The diagnostic approach to pertussis depends heavily on how many weeks the patient has been coughing.

For patients presenting within the first 3 weeks of cough onset, nasopharyngeal PCR is the first-line test. The swab must be obtained from the posterior nasopharynx (not the throat or anterior nares) using a Dacron or calcium alginate swab. PCR has high sensitivity during this window because bacterial load in the nasopharynx is still significant. Culture can be obtained simultaneously from the same swab, but because B. pertussis is extremely fastidious and growth takes over a week, culture alone should not delay clinical decision-making.

For patients presenting after 3 to 4 weeks of cough, the organism is largely cleared from the nasopharynx, and both PCR and culture sensitivity drop substantially. In this window, serology becomes the most useful tool. A single elevated anti-pertussis toxin IgG level in a patient who has not received a pertussis-containing vaccine in the prior 12 months is considered diagnostic. This scenario is commonly tested: a teenager or adult with a cough lasting 6 or more weeks, negative PCR, but elevated anti-PT IgG.

The CBC finding of lymphocytosis is not diagnostic on its own but is a powerful clue, particularly in infants. If you see a vignette describing a young infant with paroxysmal cough, apnea, and a WBC of 60,000 with 80% lymphocytes, pertussis should be at the top of your differential. Notably, this lymphocytosis is composed of mature, normal-appearing small lymphocytes (not blasts), which distinguishes it from leukemia.

04Management and Treatment

Acute management centers on two goals: reducing transmission and preventing complications.

Antibiotics are most effective when started during the catarrhal stage, where they can shorten the duration and severity of symptoms. Once the paroxysmal stage is well established, antibiotics still reduce contagiousness (the patient is considered non-infectious after 5 days of appropriate therapy) but have minimal impact on symptom duration because the cough is driven by epithelial damage, not ongoing infection.

Azithromycin is universally preferred across all age groups, including neonates. The critical teaching point is why erythromycin is avoided in infants under 1 month old: it is associated with infantile hypertrophic pyloric stenosis. This is a commonly tested association. If a vignette describes a 2-week-old infant with pertussis exposure, the correct prophylactic agent is azithromycin, not erythromycin.

Post-exposure prophylaxis (PEP) is given to all close contacts (household members, childcare contacts, healthcare workers who had unprotected face-to-face exposure). The regimen is identical to treatment. PEP is given regardless of vaccination status because vaccine-induced immunity wanes over time and does not guarantee protection against colonization.

For hospitalized infants, supportive care is paramount. This includes continuous cardiorespiratory monitoring, gentle nasopharyngeal suctioning, supplemental oxygen, and IV fluids if post-tussive emesis impairs oral intake. In fulminant cases with hyperleukocytosis (WBC >100,000) and evolving pulmonary hypertension, exchange transfusion or leukapheresis may be life-saving by reducing the leukocyte burden in the pulmonary vasculature.

Vaccination is the cornerstone of long-term prevention. The DTaP series is given at 2, 4, 6, and 15 to 18 months, with a booster at 4 to 6 years. Tdap is given as a single booster at age 11 to 12 and is recommended for all pregnant women during each pregnancy (optimally between 27 and 36 weeks of gestation) to allow passive transfer of maternal antibodies to the newborn. This maternal vaccination strategy is the primary method of protecting infants too young to be vaccinated.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap on pertussis questions involves age-dependent presentation. Students are conditioned to look for the classic triad of paroxysmal cough, inspiratory whoop, and post-tussive vomiting. However, the highest-risk group (infants under 6 months) often does not whoop. Instead, they present with apnea, cyanosis, or feeding difficulty. If a vignette describes a 6-week-old infant with episodic apnea, cyanosis, and a WBC of 50,000 with lymphocyte predominance, the answer is pertussis, not sepsis or seizure disorder.

A second trap involves antibiotic selection in neonates. Erythromycin is a correct treatment for pertussis in older patients, but it is the wrong answer for infants under 1 month because of the pyloric stenosis risk. The correct choice is azithromycin. Expect a question where erythromycin is listed as a distractor for a newborn with pertussis exposure.

A third testing angle involves the timing of antibiotic benefit. A common vignette presents an adolescent who has been coughing for 4 weeks. Students may think antibiotics are no longer indicated. However, antibiotics are still given to reduce transmission to vulnerable contacts, even though they will not shorten the patient's own symptoms at this stage. The question tests whether you understand the public health rationale for treatment.

Finally, be prepared to distinguish pertussis lymphocytosis from leukemia. Both can produce WBC counts above 50,000. The discriminator is that pertussis produces mature, morphologically normal lymphocytes on the peripheral smear, whereas ALL produces immature lymphoblasts with associated cytopenias (anemia and thrombocytopenia). If the vignette includes a normal hemoglobin and platelet count with mature lymphocytes, the answer is pertussis.

The core competency being tested across pertussis questions is your ability to recognize atypical presentations in high-risk populations, sequence the correct diagnostic test based on duration of illness, and select the appropriate antibiotic based on age.