Diphtheria

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Diphtheria

Diphtheria is a serious disease caused by the bacterium Corynebacterium diptheriae. Usually, the bacteria initially infect the throat and pharynx. During the course of the infection, a membranelike growth appearing on the throat can obstruct breathing. Some strains of this bacterium release a toxin, a substance that acts as a poison in the body. This toxin, when released into the bloodstream, travels to other organs of the body and can cause severe damage.

Diphtheria was first formally described as a disease in 1826. In 1888, Corynebacterium diptheriae was identified as the cause of the disease. A few years later, researchers discovered the antitoxin, or antidote, to the diphtheria toxin. If the antitoxin is given to a person with diphtheria in the early stages of the infection, the antitoxin neutralizes the toxin. This treatment, along with an aggressive vaccination program, has virtually eliminated the disease in the United States. Other countries that do not have an aggressive vaccination program see numerous cases of diphtheria, many of which end in death.

Incidence of diphtheria

Since most children in the United States are vaccinated against diphtheria, the domestic incidence of the disease is very low. When diphtheria does occur, it tends to strike adults, because fewer adults than children have been immunized against the disease. In developing countries, where fewer than 10% of the children are vaccinated against diphtheria, about five thousand deaths are still caused each year by this disease. Diphtheria is highly contagious. The disease is prevalent in densely-populated areas, especially during the winter months when more people crowd together indoors. Transmission of the bacteria occurs when an infected person sneezes or coughs and a susceptible person breathes in the saliva or mucus droplets form the air.

Diphtheria toxin

Interestingly, diphtheria toxin is produced by strains of Corynebacterium diptheriae that have themselves been infected with a special type of virus called a bacteriophage. The particular bacteriophage that infects C. diptheriae carries with it the gene that produces the diphtheria toxin. Strains of C. diptheriae without the bacteriophage do not produce the toxin.

The diphtheria toxin consists of two subunits, A and B. The B subunit binds to the plasmamembrane of a cell. Once it is bound to the membrane, it pulls the A subunit into the cell. The A subunit is the active segment of the toxin, producing most of the effects. Once inside the cell, the A subunit disrupts protein synthesis; once this mechanism is disrupted, the cell cannot survive for long. Diphtheria toxin thus kills cells. Cells in the throat and respiratory tract are killed first; if the toxin spreads in the bloodstream to other organs— such as the heart, kidney, and brain—severe and even fatal damage can result.

Symptoms

The incubation period—the time from exposure to the bacteria to the first symptoms—is one to seven days. The first symptoms of diphtheria are fatigue, a low-grade fever, and a sore throat. As the disease progresses, the throat swells, sometimes so much that the patient has noticeable neck swelling. The bacteria infect the throat first before spreading to the larynx (voice box) and trachea (windpipe). At the site of infection, the throat is red and sore. In reaction to the infection, the throat tissues release a discharge containing fibrous material and immune cells. This discharge covers the throat tissues and appears as a grayish, membranelike material. The throat and trachea continue to swell; if not relieved, the swelling may obstruct the airway, leading to death by suffocation.

Sometimes diphtheria bacteria infect the skin first. When this type of infection occurs, skin lesions appear. For reasons that are not clear, the diphtheria characterized by infection is more contagious than the disease characterized by respiratory infection. The skin-type of diphtheria is more common in tropical and sub-tropical countries.

Treatment

Diphtheria is treated with antibiotics and an antitoxin that can neutralize the toxin that has not yet bound to a cell membranes; it cannot neutralize the toxin that

KEY TERMS

Antitoxin— A antidote to a toxin that neutralizes its poisonous effects.

Bacteriophage— A virus that infects bacteria. When a bacteriophage that carries the diphtheria toxin gene infects diphtheria bacteria, the bacteria produce diphtheria toxin.

Schick test— A test that checks for the presence of diphtheria antitoxin in the body.

Toxin— A poisonous substance.

has already bound to and penetrated a cell. For this reason, antitoxin must be administered early in infection. In fact, some experts recommend giving doses of antitoxin if diphtheria is even suspected, since the additional time spent waiting for confirming lab results allows for more of the toxin to spread and penetrate the cells.

Vaccine

The diphtheria vaccine consists of a small amount of the toxin that has been altered so as not to cause toxic effects. The vaccine works by prompting the body’s immune system to make antitoxin against the altered vaccine toxin. The diphtheria toxin is combined with the tetanus toxin and the pertussis (whooping cough) toxin in one vaccine, abbreviated DPT. The DPT is given in four doses. In the United States, infants are given their first DPT dose at about six to eight weeks of age. If all four doses are administered before age four, the child should have a DPT “booster” before beginning kindergarten. This shot “boosts” the immunity to the disease.

A person can be tested for their immunity to diphtheria by the Schick test, which demonstrates the presence of antitoxin within the body. In this test, a small amount of diphtheria toxin is placed under the skin of the forearm. If the site develops a reaction— such as redness or swelling—the person has not developed the antitoxin from a previous infection or a vaccine, and is therefore susceptible to diphtheria. If no reaction is present, the person had already developed the antitoxin. The Schick test is useful for adults who cannot find their immunization records or do not know if they had diphtheria in childhood.

See also Childhood diseases.

Resources

BOOKS

Hammonds, Evelyn Maxine. Childhood’s Deadly Scourge: The Campaign to Control Diphtheria in New York City 1880–1930. Baltimore: Johns Hopkins, 2002.

Salisbury, Gay, and Salisbury, Lanie. The Cruelest Miles: The Heroic Story of Dogs And Men in a Race Against an Epidemic. New York: W.W. Norton, 2005.