Pertussis (Whooping Cough)

Introduction And Etiology

Pertussis, commonly known as whooping cough, constitutes a highly contagious acute respiratory tract infection.
The term signifies an intense cough, representing the hallmark clinical feature of the disease.
The primary etiologic agent is Bordetella pertussis, a fastidious, gram-negative, aerobic, pleomorphic coccobacillus acting as an exclusive human pathogen.
Other species include Bordetella parapertussis, causing 5–20% of milder clinical cases as it does not produce pertussis toxin.

Bordetella pertussis produces multiple biologically active components serving as immunogens and virulence factors.

Virulence Factor	Mechanism And Clinical Impact
Pertussis Toxin (PT)	An A-B toxin that ADP-ribosylates G proteins, causing absolute lymphocytosis, insulin secretion, and histamine sensitivity.
Filamentous Hemagglutinin (FHA)	A large surface protein that facilitates bacterial attachment to the ciliated respiratory epithelium.
Pertactin (PRN)	An outer membrane protein promoting adhesion and resisting neutrophil-mediated clearance.
Fimbriae (FIM)	Types 2 and 3 act as the main agglutinogens involved in mucosal attachment.
Adenylate Cyclase Toxin (ACT)	Enters phagocytes to inhibit chemotaxis and killing functions by elevating cyclic AMP levels.
Tracheal Cytotoxin (TCT)	Causes ciliostasis and the extrusion of ciliated epithelial cells, damaging the local respiratory tract.

The incubation period spans 7 to 10 days.
Attachment: Infection initiates via inhalation of aerosol droplets, with bacteria attaching specifically to cilia of the nasopharynx, trachea, and bronchi using FHA, FIM, PRN, and PT.
Host Evasion: ACT and PT paralyze host neutrophils and macrophages. PT additionally impairs lymphocyte recruitment to lymphoid tissues, trapping them in the vasculature to create extreme leukocytosis with absolute lymphocytosis.
Local Tissue Damage: TCT induces direct damage to ciliated epithelial cells, resulting in ciliostasis. Loss of ciliary function and thick mucus production obstruct small airways, predisposing patients to atelectasis and secondary bacterial pneumonia.
Systemic Effects: Absorbed PT causes hyperinsulinemia (triggering hypoglycemia) and severe pulmonary hypertension in young infants driven by extreme leukocytosis.

Category	Specific Manifestations
Respiratory	Pneumonia represents the most common cause of infant mortality. Apnea frequently occurs in infants under 6 months. Life-threatening pulmonary hypertension can cause cardiovascular collapse.
Neurologic	Seizures manifest in 1-2% of infants secondary to hypoxia or hyponatremia. Encephalopathy and cerebral hemorrhage result from hypoxia or increased intracranial pressure during paroxysms.
Mechanical	Increased intrathoracic and intra-abdominal pressure causes subconjunctival hemorrhages, epistaxis, hernias, rectal prolapse, and pneumothorax.
Nutritional	Severe weight loss and dehydration emerge due to intractable post-tussive vomiting.

Primary Series: Administer three doses of DTP-containing vaccine starting at 6 weeks of age.
Boosters: Recommended at 15–18 months, 4–6 years, and 11–12 years using Tdap.
Maternal Immunization: Administer Tdap during every pregnancy between 27 and 36 weeks. This facilitates transplacental antibody transfer, offering 80-91% efficacy in protecting infants under 2 months.

Administer chemoprophylaxis to all household and high-risk close contacts regardless of their immunization status.
Azithromycin given at 10 mg/kg/day for 5 days or Erythromycin given at 40–50 mg/kg/day for 14 days serves as the drug of choice.
Prophylaxis must commence within 21 days of cough onset in the index case.
Isolate patients using strict droplet precautions for 5 days following the initiation of effective antibiotic therapy. Untreated patients remain contagious for 21 days after paroxysmal cough onset.

Feature	Whole Cell Vaccine (wP)	Acellular Vaccine (aP)
Composition	Killed whole bacteria containing thousands of antigens.	Purified antigens including 1 to 5 components.
Immune Response	Induces robust Th1 and Th17 cellular and humoral response.	Induces predominantly a Th2 humoral response.
Mucosal Immunity	Effectively prevents colonization and transmission.	Protects against clinical disease but fails to prevent colonization and transmission.
Protection Duration	Long-lasting protection waning slowly over 6 to 12 years.	Short-lived protection dropping to 34% within 2 to 4 years.
Reactogenicity	High reactogenicity with common fever and injection site pain; rare Hypotonic-Hyporesponsive Episodes.	Low reactogenicity; associated with extensive limb swelling in later doses.