diphtheria

diphtheria, acute infectious disease caused by the bacillus Corynebacterium diphtheriae and characterized by a primary lesion, usually in the upper respiratory tract, and more generalized symptoms resulting from the spread of the bacterial toxin throughout the body.

Diphtheria was a serious contagious disease throughout much of the world until the late 19th century, when its incidence in Europe and North America began to decline and was eventually reduced even further by immunization measures. Before then, various other measures were tried—doctor and American statesman Josiah Bartlett (1729–95), for instance, was one of the first physicians to use quinine, extracted from Peruvian bark, to treat diphtheria. The disease still occurs mainly in the temperate regions of the world, being more common during the colder months of the year and most often affecting children under age 10.

The diphtheria bacillus was discovered and identified by German bacteriologists Edwin Klebs and Friedrich Löffler. In most cases the bacillus is transmitted in droplets of respiratory secretions expelled by active cases or carriers during speaking or coughing. The most common portals of entry of the diphtheria bacillus are the tonsils, nose, and throat. The bacillus usually remains and propagates in that region, producing a powerful toxin that spreads throughout the body via the bloodstream and lymph vessels and damages the heart and the nervous system.

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Pathophysiology and symptoms

Once inside the upper respiratory tract, diphtheria bacilli produce an exotoxin, known as diphtheria toxin, which blocks protein synthesis in host cells, causing cell death and damage to local tissues. As diphtheria bacillus propagate, a thick, leathery, grayish pseudomembrane forms that is composed of bacteria, dead cells from the mucous membranes, and fibrin (the fibrous protein associated with blood clotting). This membrane firmly adheres to the underlying tissues of the mouth, tonsils, pharynx, or other site of localization. Increasing exotoxin production can result in the spread of the toxin, via the bloodstream, to other organs, including the heart and kidney. Thus, although the pseudomembrane separates in 7 to 10 days, toxic complications can occur later, especially in severe cases.

The symptoms of diphtheria include moderate fever, fatigue, chills, and a mild sore throat. The heart is affected first, often in the second or third week. The patient develops toxic myocarditis (inflammation of the heart muscle), which can be fatal. If the person survives this dangerous period, the heart will recover completely and the patient will appear to be well. However, this appearance is deceptive and is indeed one of the most treacherous aspects of the disease, because paralysis caused by the action of diphtheria toxin on the nervous system often strikes when the patient seems to have recovered.

Paralysis of the palate and some eye muscles develops in about the third week; this is usually transient and not severe. As late as the fifth to eighth week, however, paralysis affecting swallowing and breathing develops in severe cases, and the patient may die after weeks of apparent well-being. Later still, paralysis of the limbs may occur, though it is not life-threatening. If the patient can be supported through this critical phase, recovery will be complete.

Types of diphtheria

There are several types of diphtheria, depending in large part on the anatomic location of the primary lesion. The membrane appears inside the nostrils in anterior nasal diphtheria; almost no toxin is absorbed from this site, so there is little danger to life, and complications are rare. In faucial diphtheria, the most common type, the infection is limited mostly to the tonsillar region; most patients recover if properly treated with diphtheria antitoxin. In the most fatal form, nasopharyngeal diphtheria, the tonsillar infection spreads to the nose and throat structures, sometimes completely covering them with the membrane and causing septicemia (blood poisoning). Laryngeal diphtheria usually results from the spread of the infection downward from the nasopharynx to the larynx; the airway may become blocked and must be restored by inserting a tube or cutting an opening in the trachea (tracheotomy). Cutaneous diphtheria affects parts of the body other than the respiratory tract, notably the skin, following a wound or sore.

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Treatment

In response to the presence of diphtheria toxin, the body makes a neutralizing substance called antitoxin, which enables the affected person to recover from the disease if the antitoxin is produced fast enough and in sufficient quantities. The only effective treatment of diphtheria is in fact the prompt administration of this antitoxin, which is obtained from the blood of horses that have been injected with exotoxin and have responded by producing antitoxin. Antitoxin does not neutralize toxin that has already been bound to tissue and that has produced tissue damage. The antitoxin may be lifesaving if given early enough, but the body eventually eliminates it as a foreign substance, and it does not give any permanent protection against the disease. Antibiotics can destroy the diphtheria bacillus in the throat and also are given to every patient.

Prevention

To prevent diphtheria, the body must produce its own antitoxin in response to active immunization with diphtheria toxin. Active immunization has become a routine measure in many countries through immunization with diphtheria toxoid, a form of the exotoxin that has been rendered nontoxic but that has retained its capacity to induce antitoxin formation once injected into the body. The diphtheria toxoid is usually first given in several successive doses during the first few months of life, with booster doses within one or two years and again at five or six years of age. Modern diphtheria vaccination is part of routine childhood immunization schedules, often given as part of the DTaP vaccine (diphtheria, tetanus, and acellular pertussis).