Pertussis

Epidemiology

Pertussis is the least well-controlled vaccine-preventable disease. It is highly infectious — over 90% of household contacts contract it. Incubation period 7–20 days. Estimated to account for ~7% of coughing illnesses in adults. The most common source of infant infections is an adult with waning immunity — parents being the most common source.

Greatest risk of severe infection is in infants, particularly before they have received 2 vaccine doses (i.e. before 4 months). Mortality in unvaccinated infants is around 8 per 1000 cases.

Clinical features

• Persistent cough (>2 weeks) with one or more of: paroxysms of coughing, inspiratory whoop, post-tussive vomiting.
• Cough + coryza for around 1 week, then paroxysmal coughing.
• Fever is uncommon.
• Adults and older children typically do not have the classic whoop.

Diagnosis

Nasopharyngeal swab PCR — most sensitive in the first 3 weeks of illness.

Treatment

• Reduces infectiousness; effect on symptoms is not well supported by evidence.
• Must be started within the first 3 weeks of symptom onset for benefit.
• Azithromycin 500 mg PO day 1, then 250 mg PO for a further 4 days.
• Child >6 months: 10 mg/kg (up to 500 mg) day 1, then 5 mg/kg for 4 days.
• Child <6 months: 10 mg/kg for 5 days — consider hospitalisation.
• Notify public health.
• Advise the patient to isolate, particularly from young children, for 5 days after starting antibiotics.

Contact management

Assess the need for treatment of contacts (liaise with the public health unit to determine risk) — a contact is defined as someone within 1 m of the patient for >1 hour. Only treat high-risk contacts:

• Infants <6 months
• Women in the last month of pregnancy
• Anyone who may transmit to an infant <6 months or a woman in the last month of pregnancy (i.e. treat all household contacts if there’s a small baby in the house)

Benefits of prophylaxis outweigh harms if started within the first 14 days of exposure.