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Merck Manual

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Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Last full review/revision Jun 2019| Content last modified Jun 2019
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Diphtheria is a contagious, sometimes fatal infection of the upper respiratory tract caused by the gram-positive, rod-shaped bacteria (see figure How Bacteria Shape Up) Corynebacterium diphtheriae. Some types of Corynebacterium diphtheriae release a potent toxin that can damage the heart, kidneys, and nervous system.

  • Diphtheria is caused by a bacterial infection that is now rare in developed countries.

  • Typical symptoms include a sore throat, general feeling of illness, and fever, sometimes with swollen lymph nodes, and a tough, gray pseudomembrane forms in the throat.

  • The diagnosis is based on symptoms, particularly a sore throat and the pseudomembrane, and the results of a culture.

  • Vaccination can help prevent this infection.

  • People are hospitalized and given antibiotics to eliminate the infection.

(See also Overview of Bacteria.)

Years ago, diphtheria was one of the leading causes of death among children. Today, diphtheria is rare in developed countries, primarily because of widespread vaccination. Fewer than five cases occur in the United States each year, but diphtheria bacteria still exist in the world and can cause outbreaks if vaccination is inadequate. Diphtheria is common in many countries in Asia, the South Pacific, the Middle East, and Eastern Europe and in Haiti and the Dominican Republic. Outbreaks in Indonesia, Thailand, Laos, South Africa, Sudan, and Pakistan have occurred since 2011 (travel information about diphtheria is available at the Centers for Disease Control and Prevention [CDC] web site).

Did You Know...

  • Routine vaccination has made diphtheria rare in developed countries.

The bacteria that cause diphtheria are usually spread in droplets of moisture coughed into the air. Usually, the bacteria multiply on or near the surface of the mucous membranes of the mouth or throat, where they cause inflammation. This form of diphtheria is called respiratory diphtheria.

Some types of Corynebacterium diphtheriae release a potent toxin, which can damage the heart, kidneys, and nervous system.

A milder form of diphtheria affects only the skin and occurs mainly in adults. This form is more common among people with poor hygiene (for example, homeless people). It spreads through contact with infected skin sores.


The illness typically begins a few days (an average of 5 days) after exposure to the bacteria. Diphtheria symptoms then begin over a few days, with sore throat, painful swallowing, hoarseness, a general feeling of illness (malaise), and a low-grade fever (about 100.4 to 102° F, or 38 to 38.9° C). Children may also have a fast heart rate, nausea, vomiting, chills, and a headache. The lymph nodes in the neck may swell (called bull neck). The inflammation may make the throat swell, narrowing the airway and making breathing extremely difficult.

A pseudomembrane forms near the tonsils or other parts of the throat. This membrane is a tough, gray sheet of material made by the bacteria. It is composed of dead white blood cells, bacteria, and other substances. The pseudomembrane narrows the airway. The roof of the mouth may be paralyzed. When inhaling, the pseudomembrane may cause people to make a noisy gasping sound. Also, the pseudomembrane may extend into the windpipe or airway or suddenly become detached and block the airway completely. As a result, people may be unable to breathe.

The toxin produced by certain types of diphtheria bacteria sometimes affects certain nerves, particularly those to the muscles of the face, throat, arms, and legs, causing symptoms such as difficulty swallowing or moving the eyes, arms, or legs. The diaphragm (the most important muscle used for inhaling) may become paralyzed, sometimes causing respiratory failure. These symptoms take weeks to resolve. The bacterial toxin may also cause inflammation of the heart muscle (myocarditis), sometimes leading to abnormal heart rhythms, heart failure, and death.

Severe infection can also damage the kidneys or cause high blood pressure.

If diphtheria affects only the skin, it causes what look like scrapes (abrasions) and sores that vary in appearance. These sores appear on the arms and legs and resemble other skin disorders, such as eczema, psoriasis, and impetigo. A few people have open sores that do not heal. Sores may be painful and red and ooze.

Images of Diphtheria

Overall, about 3% of people with diphtheria die. The risk of dying is increased if

  • People delay going to the doctor.

  • Diphtheria affects the heart or kidneys.

  • Diphtheria develops in children under 15 years old or people over 40 years old.


  • Culture of a sample of infected material

  • If doctors suspect the heart is affected, electrocardiography

A doctor suspects diphtheria in a sick person who has a sore throat with a pseudomembrane, particularly if the roof of the mouth is paralyzed and if the person was not vaccinated. The diagnosis is confirmed by sending a sample of material from the person's throat to a laboratory where bacteria can be grown (cultured).

If doctors suspect that the heart is affected, electrocardiography (ECG) is done.

Doctors suspect skin diphtheria when people develop sores during an outbreak of respiratory diphtheria. To confirm the diagnosis, doctors take a sample from a sore and send it to a laboratory to be cultured.


Children are routinely immunized against diphtheria (see Table: Routine Vaccinations for Infants, Children, and Adolescents). The diphtheria vaccine is usually combined with vaccines for tetanus and pertussis (whooping cough). After the initial immunization schedule is completed, a booster shot for diphtheria (combined with tetanus) is given every 10 years.

If people are exposed to diphtheria and have not been completely vaccinated or if it has been more than 5 years since they have had a booster shot, they are given a booster shot.

After exposure to diphtheria

If people have close contact with a person who has diphtheria, they are tested for the infection and given antibiotics for 7 days. Samples from their throat and nose are sent to a laboratory to be cultured. If diphtheria bacteria are identified in the samples, these people must take antibiotics for an additional 7 days (for a total of 14 days).


  • Diphtheria antitoxin

  • Antibiotics

People who have symptoms of respiratory diphtheria are typically hospitalized in an intensive care unit (ICU) and given antibodies (antitoxin) by injection to neutralize the diphtheria toxin. Doctors also give antibiotics, such as penicillin or erythromycin, to kill the diphtheria bacteria. Antibiotics are given for 14 days. People must be kept in isolation (to prevent other people from being exposed to infected secretions) until two cultures, taken after the antibiotics are stopped, confirm that the bacteria have been killed.

For skin diphtheria, doctors thoroughly clean sores with soap and water, and people are given antibiotics for 10 days.

People who have had a severe infection recover slowly. They are advised not to resume activities too soon. If the heart was affected, even normal physical exertion may be harmful.

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