Diphtheria

Diphtheria caused by Corynebacterium diphtheriae occurs world-wide. Its incidence in the West has fallen dramatically following widespread active immunization, but it is epidemic in Russia and Eastern Europe. Transmission is mainly through airborne droplet infection and rarely through fomites.

C. diphtheriae is a Gram-positive bacillus. Only strains which carry the tox+ gene, are capable of toxin production. The toxin has two subunits, A and B. Subunit A is responsible for clinical toxicity. Subunit B serves only to transport the toxin component to specific receptors, present chiefly on the myocardium and in the peripheral nervous system. Humans are the only natural hosts.

Clinical features

Diphtheria was formerly a disease of childhood but is increasingly affecting adults in countries where childhood immunization has been interrupted as in Russia and Eastern Europe. The incubation period is 2-7 days. The manifestations may be regarded as local (due to the membrane) or systemic (due to exotoxin). The presence of a membrane, however, is not essential to the diagnosis. The illness is insidious in onset and is associated with tachycardia but only low-grade fever. If complicated by infection with other bacteria such as Strep. pyogenes, fever is high and spiking.

Nasal diphtheria is characterized by the presence of a unilateral, serosanguineous nasal discharge that crusts around the external nares.

Pharyngeal diphtheria is associated with the greatest toxicity and is characterized by marked tonsillar and pharyngeal inflammation and the presence of a membrane. This tough greyish yellow membrane is formed by fibrin, bacteria, epithelial cells, mononuclear cells and polymorphs, and is firmly adherent to the underlying tissue. Regional lymphadenopathy, often tender, is prominent and produces the so-called 'bull-neck'.

Laryngeal diphtheria is usually a result of extension of the membrane from the pharynx. A husky voice, a brassy cough, and later dyspnoea and cyanosis due to respiratory obstruction are common features.

Clinically evident myocarditis occurs, often weeks later, in patients with pharyngeal or laryngeal diphtheria. Acute circulatory failure due to myocarditis may occur in convalescent individuals around the tenth day of illness and is usually fatal. Neurological manifestations occur either early in the disease (palatal and pharyngeal wall paralysis) or several weeks after its onset (cranial nerve palsies, paraesthesiae, polyneuropathy or, rarely, encephalitis).

Cutaneous diphtheria is increasingly being seen in association with burns and in individuals with poor personal hygiene. Typically the ulcer is punched-out with undermined edges and is covered with a greyish white to brownish adherent membrane. Constitutional symptoms are uncommon.

Diagnosis

This must be made on clinical grounds since therapy is usually urgent and bacteriological results of culture studies and toxin production cannot be awaited.

Treatment

The patient should be isolated and bed rest advised. Antitoxin therapy is the only specific treatment. It must be given promptly to prevent further fixation of toxin to tissue receptors, since fixed toxin is not neutralized by antitoxin. Depending on the severity, 20 000-100 000 units of horse-serum antitoxin should be administered intramuscularly after an initial test dose to exclude any allergic reaction. Intravenous therapy may be required in a very severe case. There is a risk of acute anaphylaxis after antitoxin administration and of serum sickness 2-3 weeks later.However, the risk of death outweighs the problems of anaphylaxis. Antibiotics should be administered concurrently to eliminate the organisms and thereby remove the source of toxin production. Benzylpenicillin 1.2 g four times daily is given for 1 week.

The cardiac and neurological complications need intensive therapy.

Prevention

Diphtheria is prevented by active immunization in childhood. Booster doses should be given to those travelling to endemic areas if more than 10 years has elapsed following their primary course of immunization. All contacts of the patient should have throat swabs sent for culture; those with a positive result should be treated with penicillin or erythromycin and active immunization or a booster dose of toxoid given.

Diphtheria

by Matt Loran.

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