Diphtheria

Infectious agent:

Corynebacterium diphtheriae of the gravis, mitis or intermedius biotypes is an aerobic gram-positive bacillus. Toxin production results when the bacteria are infected by a bacteriophage containing

the diphtheria toxin gene tox.

Identification:

Clinical features :

Diphtheria is an acute bacterial infection caused by toxigenic strains of Corynebacterium diphtheriae. It primarily affects the tonsils, pharynx, nose and larynx. Other mucous membranes, skin, and rarely the vagina or conjunctivae can also be involved. The toxin causes local tissue destruction and membrane

formation. The characteristic lesion in the throat is an adherent greyish-white membrane that first occurs on the tonsils, but may spread up onto the palate and involve the pharynx and result in respiratory obstruction. The onset is insidious with early symptoms of malaise, sore throat, anorexia and low-grade fever. Patients with severe pharyngeal disease may develop neck swelling giving a characteristic ‘bull neck appearance’. Systemic absorption of the toxin can result in neuropathy and cardiomyopathy,

resulting in early death or later neurological complications. Laryngeal diphtheria can present as a slowly progressive croup which can result in death if the airway obstruction is not relieved. Non-toxigenic strains of C. diphtheria rarely cause local lesions but may cause infective endocarditis. Cutaneous diphtheria presents with lesions of variable appearance but which may resemble impetigo. Non-cutaneous diphtheria has a case
fatality rate of 5–10% with higher rates in children under five years and adults over 40 years of age.

Method of diagnosis:

Diagnosis is usually based on observation of the classical greyish-white membrane overlying the tonsils or pharynx. Specimens for C. diphtheriae culture should be obtained from the nose and throat and from any other suspicious lesions. Swabs should be obtained from the pharyngeal membrane, or a portion of the membrane itself could be submitted for culture. Selective medium is required to culture C. diphtheriae so the testing laboratory should be notified that the disease is clinically suspected. All isolates should be sent to a public health reference laboratory for C. diphtheriae toxin detection by polymerase chain reaction (PCR).

Incubation period:

The incubation period is two to five days but occasionally longer.

Public health significance and occurrence:

Diphtheria occurs worldwide and is more prevalent in winter months in temperate zones. Illness is now rare in highly immunised communities. An epidemic began in the Russian Federation in 1990 involving all of the countries of the former Soviet Union and Mongolia. The epidemic has declined after a peak in 1995 but was responsible for over 140 000 cases and over 4000 deaths. Over 70% of cases were aged 15 years or older. Diphtheria is now a very rare infection in Australia but may occur in unimmunized people who are recent travellers, or their contacts. The last reported case in Victoria occurred in 1991. Importation of the infection from other affected countries remains a concern in Australia with the potential to affect unimmunized children and adults as well as adults with waning immunity post-vaccination..

Reservoir:

Humans are the usual reservoir and carriers are usually asymptomatic.

Mode of transmission:

Transmission is droplet spread from the respiratory tract. More rarely transmission can occur from contact with articles soiled with discharges from infected lesions.

Period of communicability:

Transmission may occur as long as virulent bacilli are present in discharges and lesions. The time is variable but is usually two weeks or less and seldom more than four weeks without antibiotics. Appropriate antibiotic therapy promptly terminates shedding. The rare chronic carrier may shed organisms for six months or more.

Susceptibility and resistance:

Infants born of immune mothers are relatively immune, but passive immunity is usually lost by six months of age. Lifelong immunity is usually, but not always, acquired after disease or inapparent infection. A primary course of toxoid vaccination provides long lasting but not lifelong immunity. Vaccinated individuals may become colonised by C. diphtheriae in the nasopharynx while still being protected from clinical disease.

Control measures:

Preventive measures:

Diphtheria vaccination is part of the Australian Standard Vaccination Schedule. Primary vaccination is

achieved with three doses of a diphtheria-toxoid containing vaccine at two, four and six months of age. Booster doses are currently recommended. They are given as DTPa at four years of age and as adult/adolescent formulation DTPa at 15 to 17 years of age. Prior to the 8th birthday, DTP-containing vaccines should be used. After the eighth birthday, smaller doses of toxoid (adult/adolescent formulation DTPa or DT-containing vaccines) should be given. Refer to the current edition of The Australian immunisation handbook (National Health and Medical Research Council). Adults who have been fully vaccinated in the past should receive a booster dose of adult tetanus-diphtheria vaccine (Td, ‘ADT’) at the age of 50 years unless a booster dose has been documented in the previous ten years. For ‘catch-up’ diphtheria immunization schedules for children and adults refer to the current edition of The Australian immunisation handbook (National Health and Medical Research Council).

Control of case:

The management of cases involves diphtheria antitoxin, antibiotic therapy and infection control. Consult the current version of Therapeutic guidelines: antibiotic (Therapeutic Guidelines Limited). Specialist infectious diseases advice should always be sought on clinical suspicion of a case of diphtheria. Diphtheria antitoxin should be given if there is strong clinical suspicion, immediately after specimens are taken and without waiting for laboratory confirmation. The dosage will depend on severity which is assessed by the extent of the pharyngeal membrane and duration of disease. Antitoxin can be obtained from CSL Limited. Patients must be tested for hypersensitivity prior to administration. Co-administration of

antitoxin with corticosteroids may be recommended for patients with hypersensitivity to antitoxin. Parenteral antibiotic treatment is usually required initially and can be either erythromycin or benzathine penicillin. These can be substituted for by equivalent oral formulations once the patient can swallow comfortably. These should be continued to complete a total of 14 days of treatment. Natural infection with diphtheria does not guarantee ongoing immunity. The patient should begin or complete active immunisation with an age-appropriate diphtheria toxoid-containing vaccine during convalescence.

Use standard precautions with additional respiratory precautions for pharyngeal diphtheria and standard precautions with additional contact precautions for cutaneous diphtheria, until the case is shown to be clear of carriage. The disease is usually not highly contagious after 48 hours of antibiotic

therapy.

Control of contacts:

All close contacts of the case including all household contacts and other persons directly exposed to oral secretions from the case, should have swabs taken for culture from their throat and nose. A prophylactic course of seven days of oral erythromycin or a single dose of procaine penicillin IM is recommended for close contacts. Such contacts should also be kept under surveillance for seven days, regardless of their immunization status. Contacts are excluded from childcare and school until cleared by the Department of Human Services. Other contacts are advised to exclude themselves from work and particularly food handling, until bacteriologic examination shows they are not carriers of the organism.Unvaccinated contacts should be commenced on their primary course of vaccine. Vaccinated contacts should be given a booster injection of vaccine if more than five years have elapsed since their last dose. For close contacts identified as diphtheria carriers:

• ensure that prophylactic antibiotic therapy has been given (as above for close contacts)

• exclude until two negative swabs, the first not less than 24 hours after finishing the antibiotics and the other 48 hours later. If either of the repeat cultures is positive then an additional ten day course of erythromycin or penicillin is recommended, followed by two repeat cultures. Extensive swabbing to detect diphtheria carriers apart from close contacts is not recommended.

Control of environment:

Outbreak measures:

Outbreaks of diphtheria require immunising the largest possible proportion of the population involved, emphasising the need for protection of infants and preschool children. In outbreaks amongst adults immunize groups that are most affected and at high risk. Repeat immunisations may be recommended after one month. Outbreak investigations involve enhanced case surveillance with laboratory confirmation of all suspected cases, as well as the identification and appropriate management of close contacts and asymptomatic carriers (see Control of contacts, above).

sources of information

• Gidding, HF, Burgess, MA, Gilbert, GL 2000, ‘Diphtheria in Australia, recent

trends and future prevention strategies’, Communicable Diseases Intelligence, vol. 24, no. 6.

·         The blue book: Guidelines for the control of infectious diseases