Pertussis (Whooping Cough)

Pertussis classically progresses through three stages:

Catarrhal stage (1–2 weeks): Resembles a mild cold — runny nose, sneezing, mild cough, low-grade fever. Highly contagious at this stage.

Paroxysmal stage (1–6 weeks):

• Sudden, severe coughing paroxysms — multiple rapid coughs in a single expiration

• "Whoop" — characteristic high-pitched inspiratory sound as patient gasps for air

• Posttussive vomiting

• Cyanosis during coughing fits (especially in infants)

• Apnea without coughing in infants <3 months (atypical presentation)

Convalescent stage (weeks to months): Gradual decrease in coughing frequency; residual cough may persist months ("100-day cough").

Risk factors for pertussis infection and severe outcomes are primarily related to age, vaccination status, and proximity to infectious contacts.

Risk factors for infection:

• Waning immunity: Pertussis immunity is not lifelong, whether acquired through vaccination or natural infection. Acellular vaccine protection wanes after 4–6 years; whole-cell vaccine protection after 4–15 years; natural immunity after 7–20 years. This creates large pools of susceptible adolescents and adults who drive ongoing transmission.

• Household exposure: Secondary attack rates among susceptible household contacts are 80–90% — among the highest of any infectious disease. The primary source of infant infection is usually a household member (parent, sibling, or grandparent) with unrecognized pertussis.

• Incomplete vaccination: Children who have not completed the primary series (3 doses) have substantially higher risk. Even a partial series provides some protection, but full protection requires ≥3 doses.

• Crowded settings: Schools, daycare centers, military barracks, dormitories — pertussis outbreaks are frequently school-based.

• Epidemic cycles: Pertussis epidemics occur every 3–5 years even in countries with >90% DTP3 coverage, reflecting the accumulation of susceptible individuals as vaccine immunity wanes.

Risk factors for severe disease and death:

• Age <6 months: The highest-risk group for severe pertussis, hospitalization, and death. Infants this age have not yet completed the primary vaccine series and may lack protective maternal antibodies (especially if the mother was not vaccinated with Tdap during pregnancy). Apnea, pneumonia, pulmonary hypertension, and encephalopathy are disproportionately common.

• Prematurity and low birth weight: Increased risk of severe pertussis due to smaller airways, immature immune function, and lower transplacental antibody transfer.

• Absence of maternal Tdap vaccination: Infants born to unvaccinated mothers have 3–5× higher risk of pertussis in the first 2 months of life.

• Comorbid respiratory disease: Asthma, bronchopulmonary dysplasia, and other chronic lung conditions may exacerbate the severity and duration of pertussis.

• Extreme leukocytosis: WBC >100,000/µL in infants is an independent predictor of mortality, associated with leukostasis, pulmonary hypertension, and refractory hypoxemia.

Risk factors for prolonged cough in adults:

• Asthma and chronic respiratory conditions

• Active smoking

• Advanced age

• Immunocompromised status

Infants <6 months:

• Apnea (life-threatening; leading cause of death)

• Hypoxia and cyanosis during paroxysms

• Pneumonia (secondary bacterial infection — complicates ~25% of infant cases)

• Seizures from hypoxia/encephalopathy

• Pulmonary hypertension (severe, often fatal)

• ICU admission required in up to 25% of hospitalized infants

Older children and adults:

• Urinary incontinence from coughing pressure

• Rib fractures from severe cough

• Hernia, rectal prolapse

• Pneumothorax, epistaxis

• Syncope during paroxysms

• Encephalopathy (rare, possibly from hypoxia)