Pertussis

• Pertussis, or whooping cough, is an acute infection of the respiratory tract caused by the bacterium Bordetella pertussis, which lives only in humans.
• It passes from person to person in respiratory droplets and is one of the most infectious infections known: more than 80% of susceptible household contacts catch it.
• The illness runs in three stages over weeks: a catarrhal phase like a cold, a paroxysmal phase of violent coughing fits with a whooping inspiration and post-tussive vomiting, and a slow convalescent recovery.
• It is most dangerous for infants, especially those under 6 months, in whom it can present as apnoea rather than cough and (among reported cases in the United States between 2000 and 2017) accounted for 84.0% of pertussis deaths.^{[8] [2]}
• Diagnosis rests on a nasopharyngeal swab for polymerase chain reaction (PCR) and culture; treatment is a macrolide given early; prevention is vaccination, including a dose of Tdap (tetanus, diphtheria and acellular pertussis) in every pregnancy.

By the end of this page you should be able to:

1. Describe the organism, how it spreads, and why pertussis remains common despite high vaccination coverage.
2. Explain the three clinical stages and link the symptoms (paroxysms, whoop, vomiting, apnoea) to the underlying inflammation and toxin effects.
3. Recognise the atypical infant presentation and the raised white cell count with lymphocytosis.
4. Choose the right test for the time since cough onset and interpret it sensibly.
5. Select an antibiotic by age, with correct dose, route and duration, and explain the cautions in neonates.
6. Apply the vaccination schedule, including maternal Tdap in pregnancy, and outline postexposure prophylaxis and notification.

Pertussis is classified as a vaccine-preventable bacterial infection. It should be distinguished from parapertussis, which is caused by Bordetella parapertussis and can produce a similar cough illness, and from infection with other Bordetella species such as B. bronchiseptica and B. holmseii, which are rare causes of pertussis-like illness.^[7] In practice, when a clinician says "pertussis" they mean disease due to B. pertussis.

B. pertussis is a small, aerobic, gram-negative rod that is fastidious and needs special culture media for isolation.^[1][8] It produces several antigenic and biologically active products that are responsible for the clinical illness: pertussis toxin, filamentous haemagglutinin, agglutinogens, adenylate cyclase, pertactin and tracheal cytotoxin.^[1][8]

The organism spreads from person to person through respiratory droplets or contact with airborne droplets of respiratory secretions; transmission by freshly contaminated objects is less common.^[1][8] Humans are the only reservoir, and an infected adolescent or adult is often the source of infection for a young infant.^[8]

Risk factors for severe disease are driven by age and immune status rather than by lifestyle:

• Age under 12 months, especially under 4 months, carries the highest risk of severe, and sometimes fatal, complications.^[5][7]
• Incomplete or absent vaccination in an infant younger than 12 months matches the group at highest risk for severe complications and death.^[2]
• Maternal vaccination status matters: pertussis can be more severe in infants under 2 months of age if their mother did not receive Tdap during pregnancy.^[2]
• Immunocompromise, moderate to severe medically treated asthma, and other conditions that pertussis could worsen also place a patient in the high-risk group for prophylaxis.^[5]
• Adolescents and adults who were vaccinated in childhood become susceptible again because immunity wanes: a macrolide-treated, vaccinated case is often mild enough to be missed, yet still infectious.^[7]

Natural infection provides immunity for roughly 4 to 20 years but is not lifelong.^[1]

Pertussis is primarily a toxin-mediated disease. The bacteria attach to the cilia of the respiratory epithelial cells, then release toxins that paralyse the cilia and cause inflammation of the respiratory tract; this interferes with clearance of pulmonary secretions.^[8] Pertussis antigens also let the organism evade host defences in an unusual way: lymphocytosis is promoted while chemotaxis is impaired.^[8] That is why the white cell count can be strikingly high even though the infection is not cleared quickly.

The clinical picture follows from these mechanisms:

• Catarrhal inflammation produces coryza, sneezing and a mild cough indistinguishable from a cold.
• Thick, poorly cleared secretions in the trachea and bronchi trigger the paroxysms of cough as the airway tries to force them out.
• Ciliary paralysis prolongs the illness for weeks even after the bacteria are cleared.
• Coughing paroxysms generate high pressure, explaining complications such as pneumothorax, epistaxis, subdural haematoma, hernia, rectal prolapse, urinary incontinence and rib fractures.^[7][8]
• Pertussis toxin is thought to contribute to neurologic complications such as seizures and encephalopathy, either directly or through hypoxia from coughing.^[8]

Apnoea in infants is an atypical presentation thought to arise from the same airway inflammation and toxin effects, possibly with central effects; many young infants are not strong enough to produce a whoop at all.^[2][8]

Pertussis occurs worldwide.^[8] It is a human disease with no animal or insect source or vector, and adolescents, adults and older school-age children are an important reservoir.^[8]

A modelling study estimated that more than 24 million new pertussis cases occurred globally among children younger than 5 years in 2014, causing an estimated 160,700 deaths.^[8] Most deaths occur in low- and middle-income settings and in the youngest infants.

Communicability. Pertussis is highly communicable, with secondary attack rates of 80% among susceptible household contacts.^[8] A patient is infectious from the beginning of the catarrhal stage through the third week after the onset of paroxysms, or until 5 days after the start of effective antimicrobial treatment.^[8]

Temporal pattern. There is no distinct seasonal pattern, although cases may increase in the summer and autumn.^[8]

United States illustration. Before the vaccine era there were an average of 175,000 reported cases per year (about 150 per 100,000 population); reported incidence fell, then has risen again since the late 1980s, with large epidemic peaks since the mid-2000s.^[8] In 2018 the incidence per 100,000 was 72.3 in infants younger than 6 months and 32.7 in infants aged 6 to 12 months, compared with 1.4 in persons aged 20 years or older.^[8] This pattern, common to many countries that switched to acellular vaccines, illustrates waning immunity rather than a new organism.

Pertussis classically moves through three stages; not every patient shows all of them, and previously vaccinated people may have a much milder illness.^[2][7][8]

Specific points worth knowing:

• Paroxysmal attacks average 15 per 24 hours, are more frequent at night, increase in frequency during the first 1 to 2 weeks, stay at that level for 2 to 3 weeks, then gradually decrease.^{[2][7][8] [3]}
• The "whoop" is a high-pitched sound on the long inspiratory effort that ends a paroxysm. It can be absent in adolescents, adults, vaccinated people and infants who lack the strength to make it.^[2][7][8]
• Post-tussive vomiting is a hallmark and may be the only symptom in older children and adults.^[7][8]
• Fever is minimal throughout the illness; a high fever points to a complication or another diagnosis.^[7][8]
• Infants may have apnoea as the only symptom, with little or no cough.^[2][7]
• The illness can be asymptomatic or present merely as a persistent cough lasting more than a week in adolescents and adults.^[8]

Between paroxysms the patient often looks surprisingly well. During a paroxysm there may be cyanosis, choking and vomiting. Useful findings and pointers include:

• Eyes and face: subconjunctival haemorrhage, facial petechiae or puffiness from the pressure of coughing.^[7][8]
• Chest: signs of pneumonia (the most common complication) or, rarely, pneumothorax or rib tenderness.^[7][8]
• Weight and hydration: patients often lose weight and are dehydrated from vomiting.^[7][8]
• Neurology: in severe cases, seizures or altered consciousness from hypoxia or toxin.^[7][8]

Elicit the diagnosis by asking specifically about paroxysms, whoop, vomiting and the exact week-by-week course of the cough, and by obtaining a contact history: an adult with a prolonged cough and a new baby in the home is highly suggestive.^[7][8]

The clinical picture must be separated from other causes of prolonged cough. The classic differential for pertussis-like illness includes adenoviruses, respiratory syncytial virus, Mycoplasma pneumoniae, Chlamydia pneumoniae and other Bordetella species.^[7] In an infant, bronchiolitis and pneumonia from other organisms also enter the differential, while in adults a post-viral cough can mimic the convalescent stage. A non-paroxysmal cough continuing for 2 to 6 weeks or longer after any respiratory infection is common, so the time course and the night-time paroxysms matter more than the label "cough".

The diagnosis is supported clinically but confirmed in the laboratory. Everyone with suspected pertussis should have a nasopharyngeal (NP) swab or aspirate taken.^[1][6] The same specimen can be used for both culture and PCR.^[6]

• Obtain an NP specimen, not a throat swab, with a suitable swab (polyester, rayon, nylon or calcium alginate; not cotton).^[8]
• If culture is planned, plate the specimen or place it in transport medium immediately, and plate within 24 hours of collection.^[6]
• In an outbreak, culture should confirm at least one case while PCR can be used on the rest.^[6]

A chest radiograph is useful when pneumonia is suspected, a complication that is the commonest cause of death in pertussis.^[8] No blood test makes the diagnosis; the raised white cell count with lymphocytosis is a clue that is usually present in classic infant disease but can be normal in modified cases.^[8]

There is no single, universally applied scoring system for pertussis. The diagnosis rests on a compatible clinical picture, usually with laboratory confirmation by culture or PCR, interpreted alongside symptoms and epidemiology.^[6][8] Case definitions for public health purposes exist, and these guide surveillance and notification, but bedside diagnosis still depends on recognising the stage-wise illness.

Severity is determined mainly by age and vaccination status: infants under 12 months, especially under 4 months, are at highest risk; older and previously vaccinated patients usually have milder disease.^[2][5][7] Clinical red flags that warrant admission include apnoea, cyanosis, feeding difficulty, dehydration, pneumonia or suspected encephalopathy.^[2][7]

Most care is supportive: fluids, nutrition, oxygen, airway clearance, rest and observation.^[8] Infants warrant a particularly low threshold for admission because about one third of infants under 12 months with pertussis need hospital treatment, most commonly those under 6 months of age.^[2]

Supportive measures that matter in practice:

• Hydration and feeding. Vomiting limits intake; small frequent feeds and anti-emetic strategies are useful, and intravenous fluids may be needed.
• Oxygen and monitoring. Infants should be monitored for apnoea and for desaturation during paroxysms.
• Minimise triggers. Keep the environment calm and avoid unnecessary procedures that provoke coughing.
• Hospitalise any infant under 6 months with confirmed or strongly suspected pertussis, and any older patient with red flags.

Antibiotics are of most value when given early, ideally in the first 1 to 2 weeks before paroxysms begin; they eradicate the organism from the nasopharynx, shorten the period of communicability and may lessen severity.^[3][7][8] When started later they do not alter the course of illness or prevent transmission, although they are still given to limit spread.^[1][3]

A reasonable treatment window is: people 1 year and older within 3 weeks of cough onset; infants under 1 year and pregnant women (especially near term) within 6 weeks of cough onset.^[3] Because of the severity of infant disease, many clinicians treat infants up to 6 weeks after onset out of caution.^[3]

Healthcare providers should strongly consider treating before test results if the clinical picture is strongly suggestive of pertussis, if the person is at high risk, or if they will have contact with someone at high risk.^[3] Infants are high risk, as are pregnant women in the third trimester who will soon have contact with their newborn.^[3]

Macrolides are the preferred agents. The choice between them depends on age, tolerability, interactions and cost.^[3][7][8]

Important cautions:

• Infants under 1 month receiving a macrolide must be monitored for infantile hypertrophic pyloric stenosis and other serious adverse events.^[3][7]
• Azithromycin carries a warning about possible abnormal cardiac electrical activity and fatal arrhythmia in susceptible patients; it remains recommended but an alternative should be considered in those with known cardiovascular disease, including prolonged QT interval.^[3]
• Trimethoprim-sulfamethoxazole is used from 2 months of age when macrolides are contraindicated or a macrolide-resistant strain is suspected or confirmed; macrolide resistance in B. pertussis is rare.^[3][7]
• Other antibiotics, including ampicillin, amoxicillin, cephalosporins, tetracyclines and fluoroquinolones, are not recommended for pertussis, even though some have in-vitro activity.^[7]
• Macrolides interact with drugs metabolised by the cytochrome P450 enzyme system, especially erythromycin and clarithromycin; azithromycin interacts less. Preparations taken with food or in enteric-coated form can reduce stomach upset.^[7]

• Isolation and hygiene. A treated patient becomes non-infectious 5 days after starting effective antibiotics; untreated patients are infectious much longer. Patients and contacts should cover coughs, wash hands and avoid contact with infants until treatment is complete.^[3][8]
• Household management. Assess vaccination status of close contacts and complete any missing infant doses. Close contacts under 7 years who have not finished the primary series and first booster should complete it; those aged 4 to 6 years who have not had the second booster should receive it.^[8]
• Counselling. Explain that the cough often persists for weeks even after successful treatment, and that a later respiratory infection may trigger a recurrence.^[4][8]

• Refer urgently any infant under 6 months with suspected or confirmed pertussis, and any patient with red flags such as apnoea, cyanosis, feeding failure, dehydration, pneumonia or neurologic signs.^[2][7]
• Refer for admission when the diagnosis is likely in a neonate or young infant, or when social circumstances prevent safe observation at home.
• Follow up to ensure completion of antibiotics and of any unfinished vaccination course.

Complications are the main source of morbidity and mortality and can occur at any age, although they are far more common and more severe in infants.

Minor complications: anorexia, dehydration, difficulty sleeping, epistaxis, hernias, otitis media, syncope, weight loss and urinary incontinence.^[2]

Major complications in all ages: pneumonia (the commonest serious complication and the cause of most deaths), seizures or encephalopathy from hypoxia or toxin, pneumothorax, rib fractures from coughing, subdural haematoma, rectal prolapse and death.^[2][7][8]

Infants: apnoea occurred in 68% and pneumonia in 22% of reported complications in infants under 12 months of age.^[2] In the United States, between 2000 and 2017, pneumonia occurred in 13.2% of all reported pertussis cases and in 18.6% of infants under 6 months, with 307 deaths from pertussis over those years, of which 84.0% were in children under 2 months.^[8] Between 2000 and 2017 the annual mean among infants was 2,957 cases (range 1,803 to 4,994), 1,122 hospitalisations (range 544 to 1,938) and 15 deaths (range 3 to 35).^[8] Refractory pulmonary hypertension is a particularly dangerous complication in infants.^[2]

Adolescents and adults: complications include difficulty sleeping, urinary incontinence, pneumonia, rib fracture, syncope and weight loss; they are usually less severe, especially in vaccinated people.^[2][8] Complications in all age groups are usually less severe in people who have previously been vaccinated.^[2]

Untreated pertussis clears from the nasopharynx spontaneously in 80 to 90% of patients within 3 to 4 weeks of cough onset, although infants and unvaccinated patients can remain culture-positive for more than 6 weeks.^[7] The cough often continues for 2 to 6 weeks after the paroxysmal stage, and occasionally longer, and paroxysms can recur with subsequent respiratory infections for many months.^[7][8] Death is rare overall but concentrates in infants under 4 months, especially those whose mother was not vaccinated in pregnancy.^[2][5] Recovery from one episode does not give lifelong immunity.^{[1][8] [6]}

Vaccination of susceptible people is the most important preventive strategy.^[7] The United States schedule, which is representative of many acellular-pertussis programmes, is summarised below; all countries use their own schedule and it should be checked locally.^[4][8]

Notes on vaccination:

• DTaP (diphtheria, tetanus and acellular pertussis) is the vaccine for children under 7; Tdap contains a reduced dose of some antigens and is for those aged 7 and older.^[8]
• Neither vaccine gives complete or lifelong immunity; immunity wanes after about 5 to 10 years from the last dose.^[7]
• A dose of Tdap every 10 years is not recommended against pertussis specifically, because studies show antibody levels fall substantially during the first year after vaccination. A dose during each pregnancy is therefore needed to protect each baby.^[4][8]
• Contraindications to pertussis-containing vaccines are a severe allergic reaction to a previous dose or component, or encephalopathy of unknown cause within 7 days of vaccination. Precautions include a progressive or unstable neurologic disorder, uncontrolled seizures, Guillain-Barré syndrome within 6 weeks of a previous tetanus-containing dose and (for adolescents and adults) an Arthus-type reaction to a previous dose.^[8]
• Adverse effects are mainly local (pain, redness or swelling in 20% to 40% of children after the first three DTaP doses, and in 66% to 75% after Tdap), with low-grade fever in 3% to 5% after DTaP and moderate or severe systemic reactions in fewer than 1 in 10,000 doses.^[8]

PEP is an antibiotic course given to contacts to prevent disease and, most importantly, to protect those who could die from pertussis. The regimens are the same as for treatment, and the drug of choice in a neonate is azithromycin.^[5][7] Timing windows from current US public health guidance are: ^[3]

• Household contacts: treat all asymptomatic household contacts within 21 days of onset of cough in the index patient; symptomatic contacts are treated as cases.^[5][7]
• People at high risk of severe infection (infants under 12 months, especially under 4 months; people with immunocompromising or exacerbatable conditions): within 21 days of exposure to an infectious case.^[5]
• People who will have contact with high-risk individuals (pregnant women in the third trimester; people in neonatal intensive care, childcare and maternity settings): within 21 days of exposure.^[5]
• In closed settings with a small number of cases and no community-wide outbreak, broader use of PEP may be reasonable; multiple rounds of antibiotics are not recommended once transmission is established, and contacts should simply be monitored for symptoms for 21 days.^[5]

A PEP decision balances benefit against antibiotic side effects, resistance and the burden of monitoring.^[5]

Practising good respiratory hygiene, covering coughs and sneezes, and washing hands remain part of pertussis prevention alongside antibiotics and vaccines.^[1]

Pregnancy. Every pregnancy should receive one dose of Tdap, preferably between 27 and 36 weeks of gestation, because maternal antibodies pass to the infant and the antibody level wanes substantially in the first year after vaccination, so each pregnancy needs its own dose. Women not vaccinated during pregnancy should receive Tdap immediately postpartum, and previously Tdap-naïve women in the postpartum period should receive one dose.^[4][8] Pertussis in a pregnant woman is itself a reason to treat her, particularly if she is near term, within 6 weeks of cough onset.^[3]

Infants under 6 months. These patients most often lack the whoop, present with apnoea and have the highest risk of severe disease and death. They should be treated with azithromycin, monitored for pyloric stenosis, and usually admitted.^[2][3][7]

Adolescents and adults. Disease is often mild or atypical and easily missed; diagnosis is rarely confirmed by culture because presentation is late. They are the usual source of infection for infants, which makes vaccination and early treatment of household coughs important.^[7][8]

Older adults. Antibiotics, especially macrolides, must be used with renal and hepatic caution; trimethoprim-sulfamethoxazole is relatively contraindicated in renal impairment. Vaccination advice mirrors that for all adults, with Tdap for anyone who has never had it.^[3][4]

Immunocompromised patients and those with chronic respiratory disease. They count as high-risk for PEP purposes and are a reason to treat early.^[5]

Healthcare personnel. They should follow the general adult guidance and receive one dose of Tdap if they have never had it, regardless of when their last Td dose was, ideally before working with infants.^[4][8]

• Clinical suspicion beats laboratory confirmation. In the absence of PCR, diagnose clinically in the right epidemiological setting, particularly a prolonged cough with paroxysms or post-tussive vomiting, or unexplained apnoea in an infant.^[2][7]
• Azithromycin is preferred where it can be obtained, because a 5-day single-daily-dose regimen is easier to complete than 14 days of four-times-daily erythromycin; erythromycin is an acceptable and less expensive alternative with the same dosing by age.^[7]
• One NP swab can serve both tests, and, if transport or delay is likely, use the fastest available means. If no laboratory is available, treat empirically on clinical grounds for any ill infant.^[6]
• Sink the burden of disease in infants. Admit any sick infant, especially under 6 months, for feeding support and observation for apnoea.^[2][7]
• Vaccination is the long game. Maintaining high three-dose infant coverage and adding maternal Tdap where possible reduces infant deaths more than any single treatment measure.^[7][8]
• Notify every suspected case to the local health authority using whatever reporting channel exists; pertussis is a notifiable disease in many jurisdictions.^[1]

• Pertussis is a strictly human, toxin-mediated respiratory infection caused by Bordetella pertussis, spread in respiratory droplets.
• Immunity after infection or vaccination wanes after about 5 to 10 years, so adolescents and adults remain the reservoir and often bring infection to unvaccinated infants.
• The illness has three stages: catarrhal (1 to 2 weeks), paroxysmal (1 to 6 weeks) and convalescent (2 to 3 weeks or longer); about 15 paroxysmal attacks occur per 24 hours.
• The whoop is often absent in infants, vaccinated and adult patients; post-tussive vomiting and an apnoea in an infant can be the only clues.
• Fever is usually minimal; a high fever or pneumonia points to a complication.
• An elevated white cell count with an absolute lymphocyte count of 20,000 or greater is characteristic of classic infant disease but may be absent in modified cases.
• Nasopharyngeal PCR is the most useful test up to 3 to 4 weeks after cough onset; culture is the gold standard in the first 2 weeks; serology helps later. ^[6]
• Macrolides (azithromycin preferred under 1 month) are first-line; ampicillin, amoxicillin and related antibiotics are not effective. Treatment is most useful in the first 1 to 2 weeks. ^[7]
• Infants under 6 months account for most deaths; about one third of infants under 12 months need hospital care; the key complication in infants is apnoea, followed by pneumonia.
• Every pregnancy should receive Tdap at 27 to 36 weeks, and there is no routine pertussis booster in adulthood beyond one Tdap dose and tetanus-diphtheria boosters every 10 years.
• Postexposure prophylaxis is an antibiotic course given within 21 days to household and high-risk contacts; notification to public health is required in many jurisdictions.

This page sits alongside topics on immunisation, respiratory infection in infants, apnoea of the newborn, bronchiolitis, pneumonia and neurologic complications; cross-link them once their MedGrasp topics exist. Pertussis is most usefully read with diphtheria and tetanus, because the vaccines are combined. Plain text only is used for links because the cross-topics may not yet be verified.