Merck Manual

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Larry M. Bush

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

Maria T. Vazquez-Pertejo

, MD, FACP, Wellington Regional Medical Center

Reviewed/Revised Mar 2023
Topic Resources

Diphtheria is an acute pharyngeal or cutaneous infection caused mainly by toxigenic strains of the gram-positive bacillus Corynebacterium diphtheriae and rarely by other, less common Corynebacterium species. Symptoms are either nonspecific skin infections or pseudomembranous pharyngitis followed by myocardial and neural tissue damage secondary to the exotoxin. An asymptomatic carrier state also exists. Diagnosis is clinical and confirmed by culture. Treatment is with antitoxin and penicillin or erythromycin. Childhood vaccination and booster doses should be routine.

Corynebacterium diphtheriae usually infect the nasopharynx (respiratory diphtheria) or skin (cutaneous diphtheria).

Diphtheria is now rare in the US and other high-income countries because childhood immunization is widespread. Susceptibility in high-income countries has also increased because booster immunization rates in adults are declining.

Diphtheria is endemic in many countries in Asia, the South Pacific, the Middle East, Eastern Europe, Venezuela, Haiti, and the Dominican Republic. Outbreaks in Indonesia, Thailand, Vietnam, 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 for travelers' health). Diphtheria may be present in returning travelers or migrants from countries where diphtheria is endemic.

Diphtheria toxin

Diphtheria strains infected by a beta-phage, which carries a toxin-encoding gene, produce a potent toxin. This toxin first causes inflammation and necrosis of local tissues and then can damage the heart, nerves, and sometimes the kidneys.

Nontoxigenic strains of C. diphtheriae can also cause nasopharyngeal infection and sometimes systemic disease (eg, endocarditis, septic arthritis).


Humans are the only known reservoir for C. diphtheriae. The organism is spread by

  • Respiratory droplets

  • Contact with nasopharyngeal secretions (including from asymptomatic carriers)

  • Contact with infected skin lesions

  • Fomites (rare)

A carrier state is common in endemic regions but not in high-income countries. Immunity derived from vaccination or active infection may not prevent patients from becoming carriers; however, most patients who are adequately treated do not become carriers. Patients with clinical illness or asymptomatic carriers may transmit the infection.

Poor personal and community hygiene contributes to the spread of cutaneous diphtheria.

Symptoms and Signs of Diphtheria

Symptoms of diphtheria vary depending on

  • Where the infection is

  • Whether the strain produces toxin

Most respiratory infections are caused by toxigenic strains. Cutaneous infections are caused by toxigenic and nontoxigenic strains. Toxin is poorly absorbed from the skin; thus, toxin complications are rare in cutaneous diphtheria.

Pharyngeal infection

After an incubation period, which averages 5 days, and a prodromal period of between 12 and 24 hours, patients develop mild sore throat, dysphagia, low-grade fever, and tachycardia. Nausea, emesis, chills, headache, and fever are more common among children.

If a toxigenic strain is involved, the characteristic membrane appears in the tonsillar area. It may initially appear as a white, glossy exudate but typically becomes dirty gray, tough, fibrinous, and adherent so that removal causes bleeding. Local edema may cause a visibly swollen neck (bull neck), hoarseness, stridor, and dyspnea. The membrane may extend to the larynx, trachea, and bronchi and may partially obstruct the airway or suddenly detach, causing complete obstruction.

If a large amount of toxin is absorbed, severe prostration, pallor, tachycardia, stupor, and coma may occur; toxemia may cause death within 6 to 10 days.

Mild disease with a serosanguineous or purulent discharge and irritation of the external nares and upper lip occur in patients who have only nasal diphtheria.

Manifestations of Diphtheria

Skin infection

Skin lesions usually occur on the extremities and are varied in appearance, often indistinguishable from chronic skin conditions (eg, eczema, psoriasis, impetigo). A few patients have nonhealing, punched-out ulcers, occasionally with a grayish membrane. Pain, tenderness, erythema, and exudate are typical. If exotoxin is produced, lesions may be numb. Concomitant nasopharyngeal infection occurs in 20 to 40% by direct or indirect inoculation with the organism, often from preexisting chronic skin lesions.


The main complications of diphtheria are cardiac and neurologic.

Myocarditis Myocarditis Myocarditis is inflammation of the myocardium with necrosis of cardiac myocytes. Myocarditis may be caused by many disorders (eg, infection, cardiotoxins, drugs, and systemic disorders such... read more Myocarditis is usually evident by the 10th to 14th day but can appear any time during the 1st to the 6th week, even while local respiratory symptoms are subsiding; risk of cardiac toxicity is related to degree of local infection. Insignificant ECG changes occur in 20 to 30% of patients, but atrioventricular dissociation, complete heart block, and ventricular arrhythmias may occur and are associated with a high mortality rate. Heart failure may develop.

Nervous system toxicity is uncommon (about 5%) and limited to patients with severe respiratory diphtheria. The toxin causes a demyelinating polyneuropathy that affects cranial and peripheral nerves. The toxic effects usually begin during the 1st week of illness with loss of ocular accommodation and bulbar palsy, causing dysphagia and nasal regurgitation. Peripheral neuropathy appears during the 3rd to 6th week. It is both motor and sensory, although motor symptoms predominate. The diaphragm may become paralyzed, sometimes causing respiratory failure. Resolution occurs over many weeks. Autonomic dysfunction in the form of loss of vasomotor tone (tachycardia, arrhythmias, and arterial hypotension) is also a complication of diphtheria.

In severe cases, acute renal failure may occur because the toxin damages the kidneys.

Overall mortality is 3%; it is higher in those with any of the following:

  • Delayed presentation

  • Acute renal failure

  • Myocarditis

  • Age < 15 years or > 40 years

Diagnosis of Diphtheria

  • Gram stain and culture

Pharyngeal diphtheria needs to be considered in patients with nonspecific findings of pharyngitis, cervical adenopathy, and low-grade fever if they also have systemic toxicity plus hoarseness, palatal paralysis, or stridor. The appearance of the characteristic membrane suggests the diagnosis.

Gram stain of a specimen from the membrane may reveal gram-positive bacilli with metachromatic (beaded) staining in typical Chinese-character configuration, with club-shaped swelling at one or both ends. Material for culture should be obtained from below the membrane, or a portion of the membrane itself should be submitted. The laboratory should be notified that C. diphtheriae is suspected, so that special culture media (Loeffler or Tindale) can be used. In vitro testing for toxin production (modified Elek test) is done to differentiate toxigenic from nontoxigenic strains. Polymerase chain reaction testing for the diphtheria toxin gene can be done.

Cutaneous diphtheria should be considered when a patient develops skin lesions during an outbreak of respiratory diphtheria. Swab or biopsy specimens should be cultured. Cutaneous diphtheria lesions may be coinfected with group A streptococci or Staphylococcus aureus.

ECG should be done to look for ST-T wave changes, QTc prolongation, and/or 1st-degree heart block related to myocarditis, which often becomes evident as the respiratory symptoms resolve.

Treatment of Diphtheria

  • Diphtheria antitoxin

  • Penicillin or erythromycin

Symptomatic patients with respiratory diphtheria should be hospitalized in an intensive care unit to monitor for respiratory and cardiac complications. Isolation with respiratory-droplet and contact precautions is required and must continue until 2 sequential cultures, taken starting 24 and 48 hours after antibiotics are stopped (after at least 14 days of treatment), are negative.

Diphtheria antitoxin

Diphtheria antitoxin must be given early without waiting for culture confirmation because the antitoxin neutralizes only toxin not yet bound to cells. The use of antitoxin for cutaneous disease, without evidence of respiratory disease, is of questionable value because toxic sequelae have rarely been reported in cutaneous diphtheria; however, some experts recommend it.

In the US, antitoxin must be obtained from the Centers for Disease Control and Prevention (CDC) through the CDC’s Emergency Operations Center at 770-488-7100 (see also the CDC’s information page regarding distribution of antitoxin).

The dose of antitoxin, ranging from 20,000 to 100,000 units IM or IV, is determined by the following:

  • Site and severity of symptoms

  • Duration of the disease

  • Complications

Allergic reactions include anaphylaxis occurring within 30 minutes of administration and delayed allergic reactions (serum sickness, a type III hypersensitivity reaction Type III Allergic (including atopic) and other hypersensitivity disorders are inappropriate or exaggerated immune reactions to foreign antigens. Inappropriate immune reactions include those that are... read more ). If anaphylaxis occurs, 0.3 to 1 mL epinephrine 1:1000 (0.01 mL/kg) should immediately be injected subcutaneously, IM, or slowly IV. IV administration of antitoxin is contraindicated in patients who are allergic to the antitoxin.


Antibiotics are required to eradicate the organism and prevent spread; they are not substitutes for antitoxin.

Patients may be given either of the following:

  • Erythromycin 10 mg/kg orally or by injection every 6 hours (maximum, 2 g a day) for 14 days

  • Procaine penicillin G IM (300,000 units every 12 hours for those weighing ≤ 10 kg and 600,000 units every 12 hours for those weighing > 10 kg) for 14 days

When patients are able to tolerate oral antibiotics, they should be switched to penicillin 250 mg orally 4 times a day or erythromycin 500 mg (10 mg/kg for children) orally every 6 hours for a total of 14 days of treatment.

Vancomycin or linezolid can be used if antibiotic resistance is detected. Organism elimination should be documented by 2 consecutive negative throat and/or nasopharyngeal cultures done 1 to 2 days after and again 2 weeks after completion of antibiotic treatment.

Other treatments

For cutaneous diphtheria, thorough cleansing of the lesion with soap and water and administration of systemic antibiotics for 10 days are recommended.

Recovery from severe diphtheria is slow, and patients must be advised against resuming activities too soon. Even normal physical exertion may harm patients recovering from myocarditis.

Vaccination is required after recovery for patients who had diphtheria because infection does not guarantee immunity.

Prevention of Diphtheria

Prevention consists of

  • Infection control measures (respiratory droplet isolation until 2 cultures at least 24 hours apart are negative)

  • Vaccination (primary and postexposure)

  • Antibiotics


See Diphtheria-Tetanus-Pertussis Vaccine Diphtheria-Tetanus-Pertussis Vaccine Vaccines that contain diphtheria toxoid, tetanus toxoid, and acellular pertussis help protect against diphtheria, tetanus, and pertussis, but they do not prevent all cases. For more information... read more for more information, including indications Indications Vaccines that contain diphtheria toxoid, tetanus toxoid, and acellular pertussis help protect against diphtheria, tetanus, and pertussis, but they do not prevent all cases. For more information... read more , contraindications and precautions Contraindications and Precautions Vaccines that contain diphtheria toxoid, tetanus toxoid, and acellular pertussis help protect against diphtheria, tetanus, and pertussis, but they do not prevent all cases. For more information... read more , dosing and administration Dose and Administration Vaccines that contain diphtheria toxoid, tetanus toxoid, and acellular pertussis help protect against diphtheria, tetanus, and pertussis, but they do not prevent all cases. For more information... read more , and adverse effects Adverse Effects Vaccines that contain diphtheria toxoid, tetanus toxoid, and acellular pertussis help protect against diphtheria, tetanus, and pertussis, but they do not prevent all cases. For more information... read more . See also the vaccine schedules for children and adults from the Centers for Disease Control and Prevention (CDC) and DTaP/Tdap/Td vaccine recommendations from the Advisory Committee on Immunization Practices (ACIP).

The vaccine for diphtheria contains diphtheria toxoid; it is available only in combination with other vaccines.

Everyone should be vaccinated at prescribed intervals using the following:

After exposure, diphtheria immunization should be updated in all contacts (including hospital personnel) who have not completed a primary series or who have gone > 5 years since their last booster dose. The vaccine should also be given if immunization status is unknown. An age-appropriate diphtheria toxoid-containing vaccine is used.

Postexposure antibiotics

All close contacts should be examined; surveillance for evidence of disease is maintained for 7 days.

Nasopharyngeal and throat cultures for C. diphtheriae should be done regardless of immunization status because the vaccine protects only against the effects of diphtheria toxin; it does not prevent infection with C. diphtheriae.

Asymptomatic contacts should be treated with erythromycin 500 mg (10 to 15 mg/kg for children) orally every 6 hours for 7 days or, if adherence is uncertain, a single dose of penicillin G benzathine (600,000 units IM for patients < 30 kg and 1.2 million units IM for those > 30 kg).

If cultures are positive, an additional 10-day course of erythromycin should be given; carriers should not be given antitoxin. After 3 days of treatment, carriers can safely resume work while continuing to take antibiotics. Cultures should be repeated; 24 hours after the completion of antimicrobial therapy, 2 consecutive culture sets of the nose and throat should be collected 24 hours apart. If results are positive, another course of antibiotics is given and cultures are done again.

Key Points

  • Usually, diphtheria is a cutaneous or nasopharyngeal infection, but a potent toxin can damage the heart, nerves, and sometimes the kidneys.

  • Diphtheria is rare in high-income countries because of widespread vaccination but is endemic in many low- and middle-income countries; rates are increasing slightly in high-income countries because rates of vaccination and revaccination are declining.

  • Pharyngeal infection causes a characteristic membrane in the tonsillar area; it may initially appear as a white, glossy exudate but typically becomes dirty gray, tough, fibrinous, and adherent.

  • Treat with diphtheria antitoxin and penicillin or erythromycin; document cure by culture.

  • Vaccinate patients after recovery, and vaccinate close contacts who have not completed a primary series or who have gone > 5 years since their last booster.

  • Do nasopharyngeal and throat cultures of close contacts regardless of their immunization status.

  • Give antibiotics to close contacts; duration of treatment depends on culture results.

More Information

The following English-language resources may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

Drugs Mentioned In This Article

Drug Name Select Trade
A/T/S, Akne-mycin, E.E.S., Emcin Clear , EMGEL, E-Mycin, ERYC, Erycette, Eryderm , Erygel, Erymax, EryPed, Ery-Tab, Erythra Derm , Erythrocin, Erythrocin Lactobionate, Erythrocin Stearate, Ilosone, Ilotycin, My-E, PCE, PCE Dispertab , Romycin, Staticin, T-Stat
Adrenaclick, Adrenalin, Auvi-Q, Epifrin, EpiPen, Epipen Jr , Primatene Mist, SYMJEPI, Twinject
FIRVANQ, Vancocin, Vancocin Powder, VANCOSOL
Zyvox, Zyvox Powder, Zyvox Solution
Bicillin L-A, Permapen
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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