Infectious diseasesepidemiology and surveillance

Page content: Victorian statutory requirement | Infectious agent | Identification | Incubation period | Public health significance & occurrence | Reservoir | Mode of transmission | Period of communicability | Susceptibility & resistance | Control measures | Outbreak measures | Additional sources of information

Victorian statutory requirement

Pertussis infection (Group B disease) requires written notification within five days of diagnosis.

School exclusion for cases and contacts is:

• cases should be excluded for five days after commencing antibiotic treatment
• unimmunised sibling contacts under seven years of age and unimmunised close child care contacts must be excluded from school and children’s services centres for 14 days from the last exposure to infection, or until they have taken five days of a ten day course of antibiotics.

Infectious agent

Bordetella pertussis.

Identification

Clinical features
The catarrhal state may be indistinguishable from a viral upper respiratory tract infection. The infection damages respiratory epithelium, producing respiratory obstruction and paroxysmal coughing. There is often a characteristic whoop. This is a crowing sound during inspiration preceding a bout of coughing.

There is little fever. Apnoea, seizures and encephalopathy may occur in very severe cases. Infants aged less than six months and adults often do not have the characteristic whoop. Paroxysms frequently end with the expulsion of clear, tenacious mucus. This is often followed by vomiting.

Pneumonia is the most common cause of death. Fatal encephalopathy, which is probably hypoxic, and severe weakness from repeated vomiting, occasionally occur.

Method of diagnosis
Pertussis can be diagnosed on a clinical basis if the patient has an acute illness lasting more than 14 days without another apparent cause, a classical paroxysmal cough with whooping and post-tussive vomiting. However bouts of coughing may occur without whoops or vomiting and the disease may only be suspected if the patient is a contact of a known case. Apnoea may be the only manifestation in infants. Laboratory confirmation can be problematic but should be sought where possible. A nasopharyngeal aspirate or swab is the best specimen to obtain to culture the bacterium. The likelihood of such cultures being positive is reduced 21 days after the cough onset or if effective antimicrobial therapy has commenced against B. pertussis. Serology using B. pertussis specific IgA may be falsely negative but a positive result is highly reliable in the presence of appropriate symptoms.

Incubation period

The incubation period is usually between six and 20 days. It is most commonly about 14 days.

Public health significance & occurrence

It is a distressing and often serious illness particularly in children under one year of age. The mortality rate is 0.5% in infants under six months. High immunisation levels reduce the number of cases and good nutrition and medical care reduce case fatality. Many vaccinated adults may have mild infection and act as a source of infection for younger children. Australia experiences an epidemic of whooping cough about every three or four years.

As it is not possible to completely control pertussis with the current vaccine, the highest priority should be given to protecting infants under 12 months of age.

The World Health Organization (WHO) estimates there were 40 million cases of pertussis in 1994 and 360 000 deaths. WHO believes only one to two per cent of cases are reported. In industrialised countries four children out of every 10 000 infected die from pertussis and its complications. In Australia the first pertussis vaccine was manufactured in the 1920s. There is a clear seasonal pattern with 65% of notifications occurring over the spring and summer months.

Reservoir

Humans are the only known natural reservoir of B. pertussis.

Mode of transmission

B. pertussis is highly infectious. It may be spread from person to person by close contact, usually by respiratory aerosols, infecting 70–100% of household contacts.

Period of communicability

It is highly communicable in the early catarrhal stage before the onset of paroxysmal cough. Thereafter communicability decreases and becomes negligible in about three weeks. When treated with a macrolide antibiotic the period of infectivity usually lasts five days or less after commencement of therapy.

Susceptibility & resistance

Maternal antibodies do not protect newborns against infection. Severity is greatest in young infants while milder and atypical cases occur in all age groups. Incomplete immunisation, waning immunity and the fact that vaccine efficacy is 70–80%, results in cases occurring in older children and adults. Lifelong immunity is not guaranteed, even after clinical disease.

Control measures

Preventive measures
Educate the public to the dangers of whooping cough and the advantages of initiating immunisation at two months of age and adhering to the immunisation schedule (DTPa at two, four and six months and four and fifteen years of age). Delay immunisation only for significant intercurrent infection or an evolving neurological disorder. Minor respiratory infections are not a contra-indication for immunisation.

Control of case
Antibiotics will have little effect on the clinical course of disease but can reduce the risk of transmission if commenced within 21 days of cough onset. Treatment generally consists of erythromycin or clarithromycin. If it is not tolerated alternative macrolides with fewer side effects may be considered. A patient who has been coughing for more than 21 days is no longer infectious and antibiotic treatment and school exclusion are not needed. Antibiotic treatment is required if there is complicating pneumonia. Consult the current version of Therapeutic guidelines: antibiotic (Therapeutic Guidelines Limited).

Control of contacts
Erythromycin should not be given if more than 14 days have elapsed since the first contact with the infectious case (doses and duration as for cases). In special circumstances, such as a high risk exposure for an infant contact, antibiotics may be given within 21 days of first contact with an infectious case. Antibiotics rarely prevent secondary transmission and should be limited to household or child care contacts at high risk of severe complications that have had direct contact with an infectious case:

• infants <12 months of age regardless of vaccination status
• any child aged between 12 and 24 months who has received less than three doses of pertussis vaccine
• any women in the last month of pregnancy
• any child or adult who attends or works at a child care facility.

Control of environment
Not applicable.

Outbreak measures

See Control of contacts, above.

Clusters of infection are managed on a case-by-case basis. Contact the Department of Health for further advice.

Additional sources of information

• Communicable Diseases Network Australia 1997, Guidelines for the control of pertussis in Australia, Communicable Diseases Intelligence Technical Report Series
  www.health.gov.au
• PHLS Communicable Disease Surveillance Centre 2002, ‘UK guidelines for use of erythromycin chemoprophylaxis in persons exposed to pertussis’, Journal of Public Health Medicine, vol. 24, pp. 200–206.