RespiratoryTier 2Disease (DEADMAN)

Pertussis

Description

"Whooping cough" - Bordetella pertussis, a fastidious Gram-negative coccobacillus
*Notifiable* in every Australian jurisdiction

Three clinical phases

Phase	Duration	Features
Catarrhal	1-2 weeks	Coryza, mild cough, low-grade fever. *Most infectious phase*
Paroxysmal	2-8 weeks	Paroxysms of coughing, inspiratory "whoop", post-tussive vomiting, cough syncope, subconjunctival haemorrhage
Convalescent	Weeks-months	Gradually resolving cough. "The 100-day cough"

Adults and vaccinated children often have none of the classic features - just a persistent dry cough
- -> consider pertussis in any cough lasting >2 weeks, especially if paroxysmal, worse at night, or with post-tussive vomiting
B. parapertussis causes a milder, similar illness

Endemic with epidemic peaks every 3-5 years in Australia
Vaccination has not eliminated it: immunity from acellular vaccine wanes within 4-12 years, and from natural infection within 4-20 years
- -> adolescents and adults become the reservoir
Highest morbidity and mortality in infants <6 months, particularly <3 months (too young to be fully vaccinated)
- Source of infection for an infant is usually a parent or older sibling**
Aboriginal and Torres Strait Islander infants: higher hospitalisation rates
Attack rate in susceptible household contacts up to 80-90%

Transmission: respiratory droplets; incubation 7-10 days (range 4-21)
Infectious from the catarrhal phase until 21 days after paroxysms begin, or until 5 days of appropriate antibiotic

Virulence factors -> the clinical picture

Filamentous haemagglutinin and pertactin - adhere to ciliated respiratory epithelium
Tracheal cytotoxin - kills ciliated cells -> impaired mucociliary clearance -> paroxysmal cough
Pertussis toxin - ADP-ribosylates Gi -> disinhibits adenylate cyclase -> inc cAMP
- -> lymphocytosis (inhibits lymphocyte migration into tissues - they stay in the circulation)
- -> insulin secretion, histamine sensitisation
Adenylate cyclase toxin - impairs phagocyte function
There is no bacteraemia - the illness is toxin-mediated and confined to the airway. This explains why antibiotics do not shorten symptoms once paroxysms have begun

Clinical case definition

Cough >=2 weeks plus one of: paroxysms, inspiratory whoop, post-tussive vomiting, apnoea (infants)

Laboratory

*Nasopharyngeal swab or aspirate for PCR - the investigation of choice*
- Highest yield in the first 3 weeks of cough; nasopharyngeal (not throat) sampling
Culture - specific but insensitive (Regan-Lowe or Bordet-Gengou medium), useful for public health typing
Serology (anti-pertussis toxin IgA/IgG) - for late presentation (>2-3 weeks of cough) when PCR yield has fallen
- Interpret against recent vaccination - vaccine induces the same antibodies
FBE: marked leukocytosis with an absolute lymphocytosis - characteristic; in infants a WCC >50 x10^9/L predicts severe disease and pulmonary hypertension
CXR - usually normal; "shaggy" right heart border, perihilar infiltrate, or evidence of secondary pneumonia

Differential for a chronic paroxysmal cough

Post-viral / upper airway cough syndrome, asthma and cough-variant asthma, GORD, ACE inhibitor, Mycoplasma or Chlamydophila, adenovirus, RSV, TB, foreign body (children), bronchiectasis, eosinophilic bronchitis

A. Antibiotics

Purpose: to reduce transmission, NOT to shorten the illness

Effective at improving symptoms only if started in the catarrhal phase or the first week of paroxysms. After that, treat to stop spread
Indicated if within 3 weeks of cough onset (or within 6 weeks in infants <6 months, pregnant women near term, or contacts of high-risk individuals)

Agent	Dose (adult)	Notes
Azithromycin	500 mg day 1, then 250 mg daily for 4 days	Preferred - shortest course; first-line in infants <1 month (lowest hypertrophic pyloric stenosis risk)
Clarithromycin	500 mg bd for 7 days	Alternative
Trimethoprim-sulfamethoxazole	160/800 mg bd for 7 days	Macrolide allergy or intolerance. Avoid in infants <1 month (kernicterus)

Isolate / exclude from work, school and childcare until 5 days of antibiotic completed (or 21 days from paroxysm onset if untreated)

B. Post-exposure prophylaxis - targeted, not universal

Give the same antibiotic regimen to close contacts within 14-21 days of exposure, where the contact is or lives with:
- Infants <6 months
- Women in the last month of pregnancy
- Childcare workers, healthcare workers caring for infants or pregnant women
- Anyone in a household with a high-risk person
Blanket prophylaxis of all contacts is not recommended - it drives resistance without preventing much disease

C. Supportive

Adults: reassurance about the duration; cough suppressants, bronchodilators, antihistamines and corticosteroids have no proven benefit**
Infants: hospitalise if <6 months, apnoea, cyanosis, feeding difficulty, or severe paroxysms
- Cardiorespiratory monitoring, oxygen, gentle suction, small frequent feeds or NG feeding
- Severe pertussis in young infants: hyperleukocytosis -> leukostasis and pulmonary hypertension -> exchange transfusion or leukodepletion considered; ECMO in refractory cases; mortality high

D. Prevention - the real intervention

Acellular vaccine (DTPa / dTpa) - Australian NIP schedule:
- 6 weeks, 4 months, 6 months, 18 months, 4 years, then dTpa at ~12-13 years (school program)
*Maternal dTpa in every pregnancy, ideally at 20-32 weeks* - transplacental antibody protects the infant in the vulnerable first months
- ~90% effective at preventing infant pertussis - the single most effective measure
"Cocooning" - dTpa booster for parents, grandparents and other carers of a new infant if not vaccinated in the last 10 years
Healthcare and childcare workers: 10-yearly dTpa booster
Acellular vaccines protect against disease better than against infection and transmission - hence ongoing circulation

E. Public health

Notify the public health unit - this is the doctor's responsibility, not the laboratory's alone
Contact tracing, exclusion, targeted prophylaxis, outbreak management in schools and childcare

Pregnancy - the key intervention point (maternal vaccination)
Waning immunity in adolescents and adults - the reservoir
Prematurity, congenital heart disease, chronic lung disease in infants - severe disease
Co-infection with RSV or other respiratory viruses - worse outcomes in infants
Immunodeficiency - consider in unusually severe or prolonged disease
Chronic cough clinic populations - pertussis accounts for a meaningful proportion of adult chronic cough

Self-limiting in immunocompetent adults, but the cough commonly persists 2-3 months
Reinfection is possible - immunity is not lifelong

Complications

Adults - mostly mechanical, from the force of coughing

Rib fractures, cough syncope, pneumothorax, subconjunctival haemorrhage, epistaxis
Urinary incontinence, inguinal hernia, sleep disruption, weight loss
Secondary bacterial pneumonia

Infants - the dangerous group

Apnoea and bradycardia (may be the only presentation in neonates - a whoop may be entirely absent)
Secondary pneumonia - the commonest cause of death
Seizures and hypoxic encephalopathy
Pulmonary hypertension from leukostasis - associated with hyperleukocytosis and very high mortality
Dehydration and malnutrition from post-tussive vomiting
Case fatality ~1% in infants <2 months

Long-term

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