Diphtheria is usually transmitted by contact with respiratory droplets from infected persons or asymptomatic carriers. It can also be transmitted by contaminated objects or foods (such as contaminated milk). The incubation period is 2 to 5 days.

The bacteria primarily infect the nose and throat, although they may initially infect the skin, producing skin lesions. Corynebacterium diphtheriae produces a toxin that causes tissue damage (necrosis) in the immediate area of the infection, usually the nose and throat.

It can also spread via the bloodstream to other organs, where it can cause significant damage. Although the toxin can damage any tissue, the heart and nervous system are most frequently and most severely affected.

Localized infection in the throat and tonsillar area produces a characteristic membrane that is gray to black, tough, and fibrous. This membrane can cause airway obstruction.

Diphtheria may be mild and unrecognized or it may become progressive. If toxin enters the bloodstream, the patient may develop inflammation of the heart muscle (myocarditis), which is the most common and most worrisome complication. Toxic effects on the nervous system may also cause temporary paralysis.

The disease is now rare in many parts of the world because of widespread immunization. Risk factors include crowding, poor hygiene, and lack of immunization. The incidence of diphtheria in the U.S. is fewer than 5 cases per year.

In 1993 and 1994, the states of the former USSR experienced a diphtheria epidemic, with more than 150,000 reported cases and 5,000 deaths. The epidemic was related to a drop in routine childhood DPT immunization to less than 60% of the population, failure to give booster doses to adults, and worsening economic conditions in the affected countries.

• Sore throat (may range from mild to severe with painful swallowing and hoarseness)
• Drooling (suggests airway obstruction is about to occur)
• Fever and chills
• Bloody, watery (serosanguineous) drainage from nose
• Croup-like (barking) cough
• Stridor, difficulty breathing, or rapid breathing
• Absent breathing (apnea) may occur
• Bluish coloration of the skin may be caused by lack of oxygen
• Skin lesions can be seen in cutaneous diphtheria (usually seen in the tropics.

Physical examination may reveal the characteristic gray membrane (pseudomembrane) in the throat, enlarged lymph glands, and swelling of the neck or larynx. If diphtheria is suspected, treatment should be started immediately, even before the results of bacterial tests are available.

Tests used may include:

• Gram stain of membrane or throat culture to identify Corynebacterium diphtheriae.
• In individuals with myocarditis, results of an electrocardiogram (ECG) are often abnormal.

Diphtheria antitoxin is given as an intramuscular or intravenous injection as soon as the diagnosis is suspected. The infection is then treated with antibiotics, such as penicillin or erythromycin.

People with diphtheria may require hospitalization for supportive treatment and during the administration of antitoxin. Insertion of an endotracheal tube and/or removal of the obstructing membrane may be required if airway obstruction is present. Intravenous fluids, oxygen, bed rest, and cardiac monitoring (due to the possibility of myocarditis) are usually indicated.

Immunization or booster shots for all contacts of the infected person, including health care personnel, should be given. Individuals found to be carriers of diphtheria are treated with antibiotics as well.

Protective immunity is not present longer than 10 years after the last vaccination, so it is important for adults to get a booster of tetanus-diptheria (Td) vaccine every 10 years.

• Myocarditis which can lead to heart failure.
• Neurologic palsies or peripheral neuritis causing uncoordinated movements and other symptoms (develops in 3 to 7 weeks). Severe nerve damage can cause paralysis.
• Kidney damage or nephritis may occur.