Whooping cough is a highly contagious disease, notifiable on suspected clinical grounds, caused by a Gram-negative coccobacillus called Bordetella pertussis that is found on the pharynx.
Its incidence, morbidity, mortality and severity are highest in infants. More than 50 per cent of those under six months of age with whooping cough are admitted to hospital and one in every 500 infants under one year old dies from the infection. Over the past 10 years there has been an overall downward trend in incidence of notifications with a superimposed four-year cycle of peaks, the last being in 2008.
The incidence in adults has been rising but this is due to the introduction of serological testing and hence correct diagnosis.
Transmission of pertussis occurs via respiratory droplets. It has an incubation period of seven to 10 days and is considered to be infectious from one week after exposure until three weeks after the onset of paroxysms or seven days after antibiotics have been started.
1. Clinical features
In infants younger than three months the disease can be severe and potentially fatal, but it is often milder in adults. Only one in 15,000-20,000 older children or adults have severe illness.
The classic symptoms of pertussis are a paroxysmal cough, inspiratory whoop and post-tussive vomiting. It usually starts with a catarrh phase of mild coryzal-like symptoms of runny nose, sneezing and mild cough that last for about two weeks.
This is followed by the paroxysmal stage where a dry hacking cough occurs in spasms of forceful uncontrollable coughs many times an hour. These paroxysms may be followed by an inspiratory 'whoop sound' as the patient breathes in through partially closed vocal cords.
The whoop is often absent in adults and older children as they can inspire between coughs without whooping and younger infants often have cyanosis or apnoeas instead. This stage typically lasts for four to eight weeks but can last up to four months; long after the infection has passed. For this reason it is sometimes known as the 100-day cough. Vomiting may occur after a paroxysm and the patient is left exhausted. Between the spasms the patient is often well.
Finally the convalescence stage is entered, lasting about two weeks, as the frequency and severity of coughing and post-tussive vomiting reduces.
On examination the chest is clear and petechiae may be present around the face as a result of coughing. If there is clinical doubt and coughing does not occur spontaneously it may be possible to trigger a spasm by touching the pharynx with a tongue depressor.
2. Differential diagnosis
It is important to rule out bacterial pneumonia. Symptoms include cough, fever and pleuritic chest pain. On examination there are signs of consolidation, which are not present in pertussis. Adenoviruses and respiratory syncytial virus can cause viral pneumonia and bronchiolitis. They also present with upper and lower respiratory tract symptoms often indistinguishable from bacterial pneumonias. Again, on examination, chest signs are normally present.
Diagnosing whooping cough
Suspect whooping cough if:
Plus any of:
Notification should occur on clinical suspicion, so laboratory confirmation is not required.
The criteria for classical severe pertussis defined by WHO are at least 21 days of a cough illness with paroxysms associated with whoops or post- tussive vomiting and culture confirmation.
In patients older than one year, a nasopharyngeal swab is the gold standard. It can be cultured if taken within two weeks of catarrhal stage onset, one week of cough onset or up to 48 hours after commencing antibiotics. If positive, it is diagnostic of whooping cough but if negative it does not rule it out as false negatives can be up to 20 per cent.
If the cough has been present for more than two weeks or if more than 48 hours have passed since the start of antibiotics plus no immunisation has been given in the last year, serology should be taken. Serology tests for the presence of antipertussis toxin IgG, IgM and IgA antibodies.
In those less than one year of age who are acutely unwell in hospital, PCR from nasal swabs can be taken.
Management is mainly supportive. Antibiotics are indicated and work best when commenced within the first three weeks. They do not alter the clinical course but may reduce the period of infectivity. Erythromycin for seven days, clarithromycin for seven to 14 days or azithromycin for seven to 14 days show best clearance.
Prophylactic antibiotics are only indicated in situations where the onset of disease was in the past 21 days and there is a vulnerable close contact present, for example, an infant less than one year old, a woman in her last month of pregnancy, chronic illness or a partially or unvaccinated person. In these cases all household contacts should be given chemoprophylaxis with erythromycin.
Severe complications and death mainly occur in those less than six months old.
Complications include pneumonia, encephalitis and meningoencephalitis. Adults may also suffer from fractured ribs, pneumothorax, hernias, incontinence and aspirations.
Most patients recover after infection and have no lasting complications. The mortality rate is about 0.3 per cent.
An inactivated pertussis vaccination was introduced in the 1950s when the annual UK notifications were >120,000. Vaccination coverage is now 94 per cent and the vaccine gives 95 per cent protection.
Children under 10 are vaccinated under the UK national immunisation programme with three doses one month apart from two months old, followed by a booster at three years and four months to five years. Currently, immunisataion is not recommended in those older than 10. However in some countries, such as Australia, vaccination is offered to adults in contact with newborns because vaccine immunity wanes with time and the infection is most commonly spread from adults to children.
- Dr Taylor-Walker is a locum GP in Leicestershire
HPA guidelines for the public health management of pertussis. www.hpa.org.uk/web/HPAwebFile/HPAweb_C/1287142671506