Diphtheria is a bacterial infection that causes a sore throat, fever, and sometimes serious or even fatal complications. Diphtheria is caused by the bacillus Corynebacterium diphtheriae. The disease is now rare in developed countries as a result of mass immunization. In the UK, the vaccine is given at 2, 3, and 4 months, 3–5 years, and 13–18 years of age.
The infection may begin in the throat or in the skin. In the throat, multiplication of bacteria causes the formation of a membrane that may cover the tonsils and spread up over the palate or down to the larynx (voicebox) and the trachea (windpipe), causing breathing difficulty and a husky voice. Other symptoms include enlarged lymph nodes in the neck, an increased heart rate, and mild fever. If infection is confined to the skin, the bacteria may produce a yellowish lesion covered by a hard membrane.
Life-threatening symptoms occur only in people who are not immune to the disease. They are caused by a toxin that is released by the bacteria and affects the heart and nervous system. Occasionally, a victim collapses and dies within a day of developing throat symptoms. More often, the victim is recovering from diphtheria when heart failure or paralysis of the throat or limbs develops. These complications can occur up to seven weeks after the onset of infection in the throat.
Diphtheria is treated with antibiotics; in addition, an antitoxin is given to neutralize the bacterial toxin. If severe breathing difficulties develop, a tracheostomy (the surgical introduction of a breathing tube into the windpipe) may also be necessary.
Diphtheria in detail - technical
Diphtheria is a potentially lethal infection caused by toxin-producing strains of Corynebacterium diphtheria, a Gram-positive bacillus. Humans are the only known reservoir, with spread via respiratory droplets or direct contact with skin lesions. Although now rare in developed countries, this vaccine-preventable disease remains an important problem in countries with poor or failing health systems, and is estimated to cause about 5000 deaths per year worldwide, most in children under 5 years of age.
Pathogenesis—diphtheria develops when toxigenic bacteria lodge in the upper airway or on the skin of a susceptible individual. An intense inflammatory reaction develops, leading to a characteristic greyish-coloured pseudomembrane that is adherent to underlying tissues. Systemic effects are caused by release of diphtheria toxin, carried by a lysogenic corynebacteriophage, a single molecule of factor A of which can kill a eukaryotic cell.
Clinical features—after an incubation period of 2 to 6 days the disease presents acutely in a number of ways, classified by the location of the pseudomembrane: (1) anterior nasal—usually relatively mild; (2) tonsillar (faucial)—the commonest form, with malaise, fever, sore throat, painful dysphagia and tender cervical lymphadenopathy; (3) tracheolaryngeal—with particular risk of airway obstruction; (4) malignant—with rapid onset, circulatory shock, cyanosis, gross cervical lymphadenopathy (‘bull neck’), and very poor prognosis; (5) cutaneous—usually mild but chronic; morphological features can be extremely variable. Later complications include (1) myocarditis—seen in 10% of cases; and (2) segmental demyelinative neuropathy—most often palatal paralysis, and more sinister paralyses of pharyngeal, laryngeal, respiratory and limb muscles.
Diagnosis—infection may be confirmed by bacterial culture, with detection of toxin production by one of several laboratory techniques, or of the toxin-producing gene by PCR.
Treatment and prognosis—aside from supportive care, this involves (1) antitoxin—20 000 to 100 000 units, depending on disease severity; preferably given within 48 h of the onset of symptoms; (2) antibiotics—benzylpenicillin (or penicillin V), or erythromycin in those allergic to penicillin; (3) maintaining the airway—life-saving procedures such as tracheostomy may be required. Recovery is usually complete if the patient survives.
Prevention—diphtheria is completely preventable by vaccination, but immunity is not life-long and may wane in adult life if booster doses are not given regularly. Similarly, infection does not necessarily confer complete protection and the disease may recur in previously infected individuals.
Diphtheria is an acute and potentially highly lethal infection of the upper respiratory tract caused by toxigenic strains of Corynebacterium diphtheriae and C. ulcerans. Today diphtheria has been virtually eliminated from most developed countries by mass immunization, yet it remains a threat in countries with poor vaccine coverage. During the 1990s there was a huge epidemic in parts of the former Soviet Union. Smaller outbreaks have been reported in several other countries.
Since ancient times diphtheria has been one of the most feared childhood diseases, characterized by devastating outbreaks. Diphtheria was recognized as an infectious disease by Brentonneau in 1819. The causative bacillus was described by Löffler in 1884 and a soluble toxin was identified by Roux and Yersin in 1889. In 1890, Fränkel developed an attenuated vaccine and von Behring produced an antitoxin, the first therapeutic antiserum that was first used clinically by Roux in 1894. Before the introduction of antitoxin, mortality in some epidemics had exceeded 50%. In 1913, von Behring produced a successful vaccine and the Schick (skin) test was used to detect immunity. In the United Kingdom there was an average of 50 000 cases and 4000 deaths each year from 1915 to 1942 and it was the leading cause of death among children aged 4 to 10 years.
During the Second World War, more than a million cases were reported, including 50 000 deaths. In the United States of America, W. Barry Woods Jr declared in 1961 that: “Were it possible merely to apply what is now known about diphtheria to every part of the world, this devastating malady could be wiped from the face of the earth”. However, even in that country, epidemic outbreaks continued in major cities, e.g. the 1970 San Antonio epidemic involving 201 cases with 3 deaths mainly in the unimmunized poor nonwhite population aged less than 15 years. In the United Kingdom, mass vaccination had reduced diphtheria to approximately 8–10 notified cases each year. In 2002 there were still an estimated 5000 deaths from diphtheria worldwide, of which 4000 were in children under 5 years of age.
C. diphtheriae are slender pleomorphic Gram-positive rods or clubs. There are four biotypes: gravis, intermedius, belfanti, and mitis, any of which can cause diphtheria if they produce exotoxin. Early manifestations of diphtheria, including pseudomembrane formation, result from an inflammatory reaction to the multiplying toxigenic C. diphtheriae. Fluid and leucocytes move from dilated blood vessels to surround necrotic epithelial cells. The fluid clots to enmesh dead cells, leucocytes, diphtheria bacilli, cellular debris, and occasionally small blood vessels. The resulting pseudomembrane is therefore adherent to underlying tissues and bleeds when pulled away.
C. diphtheriae does not usually pass beyond the pseudomembrane site; it is the toxin that causes the later complications of diphtheria. Diphtheria toxin is a 535-amino acid residue 62-kDa exotoxin consisting of three domains, A (enzymatic), B (binding), and T (translocation). Domain B binds on the cell surface to heparin-binding epidermal growth factor (EGF)-like growth factor precursor and CD9 complex, allowing the lethal factor A to pass through the endosome membrane into the cytosol where it catalyses the NAD+-dependent ADP-ribosylation of eukaryotic elongation factor 2 preventing protein synthesis leading to cell death, facilitated by apoptosis. Delivery of a single molecule of factor A to the cytosol of a eukaryotic cell will kill it. Employing this mechanism, recombinant diphtheria toxin with its B domain truncated and fused with the human interleukin (IL)-2 receptor is marketed as denileukin diftitox (DT388-IL2) for the treatment of cutaneous T-cell lymphoma, chronic lymphocytic leukaemia, and non-Hodgkin’s lymphoma.
The structural gene of the toxin (TOX) is carried by a lysogenic corynebacteriophage. However, TOX gene expression is regulated by the bacterial chromosome and requires low extracellular iron concentrations. Locally the toxin causes tissue necrosis and, when absorbed into the bloodstream, systemic complications. In addition to bacterial exotoxin, cell wall components such as the O- and K-antigens are important in disease pathogenesis.
Pathological changes may be seen in all human cells, but the most profound changes are seen in the myocardium, peripheral nerves, and kidneys. Common cardiac changes include fatty degeneration of cardiac muscle (myocarditis) and infiltration of the interstitium with leucocytes, which may involve the conduction fibres. Although the heart can recover completely from these effects, severe fibrosis and scarring may lead to death in late convalescence. Mural endocarditis may cause embolism leading to cerebral infarction and hemiplegia. Valvular endocarditis is extremely uncommon. Neuritic changes may be seen in the nerves to the heart during the late paralytic stage of the disease. Diphtheria toxin also causes demyelination and degeneration of both sensory and motor nerves. It affects the nerves to the eye, palate, pharynx, larynx, heart, and limb muscles. It is unclear whether the toxin crosses the blood–brain barrier to cause central lesions.
Humans are the only known reservoir for C. diphtheriae. In most cases transmission to susceptible individuals results in transient pharyngeal carriage rather than disease. Spread is via respiratory droplets or direct contact with skin lesions. Cutaneous diphtheria is more contagious than respiratory diphtheria and chronic skin infections are the main reservoir in environments of poverty and overcrowding. Patients may become carriers of the infection and continue to harbour the organism for weeks or months. The organism can survive for up to 5 weeks in dust or on fomites.
Today diphtheria remains an important health problem in countries with poor vaccine coverage. In these areas, children generally meet C. diphtheriae early, sometimes becoming a carrier, and young children may have severe or fatal attacks of diphtheria. C. diphtheriae tends to die out in highly immunized populations, and children may grow to adult life without encountering the bacillus. Recent serological studies in several countries indicate that up to 50% of adults are susceptible to diphtheria, and their immunity decreases significantly with increasing age. This potential risk is becoming increasingly important with the growth in international travel.
Immunity to systemic disease depends on the presence of IgG antitoxin antibodies. Type-specific protection against carriage and mild forms of local disease is induced by antibodies to the variable K antigens of the bacterial cell wall. Infection does not always confer protective immunity and outbreaks of mild disease have been reported even in highly vaccinated populations. In endemic countries protective immunity is boosted naturally through circulating strains of toxigenic C. diphtheriae.
Diphtheria is a devastating but preventable disease. Experience suggests that declining immunity in adults poses the risk of outbreaks, but is probably not sufficient in itself to sustain a large diphtheria epidemic unless there are large numbers of susceptible children and adolescents. In the newly independent states of the former Soviet Union (NIS), economic hardship, large urban migration, and low vaccination coverage due to failing health systems probably contributed to the massive outbreak of the 1990s. This started in Russia but spread to all the NIS, leading to more than 150 000 cases and 5000 deaths between 1990 and 1998 and more than 2700 cases subsequently. Widespread immunization campaigns have largely controlled the epidemic but the risk of diphtheria remains in all countries of the former Soviet Union (e.g. there were outbreaks in Western Siberia in 2003 and the southern Urals in 2004) and rare cases of diphtheria continue to be reported in tourists and travellers to the NIS.
Diphtheria has an incubation period of 2 to 6 days and presents acutely in a variety of forms, classified according to the location of the pseudomembrane:
This is usually unilateral and relatively mild unless it coexists with other forms. It is relatively common in infancy. There is a nasal discharge, initially watery, then purulent and blood-stained. The nostril may be sore or crusted and a thin pseudomembrane can sometimes be seen within the nostril itself.
This is the commonest form of diphtheria. Malaise, sore throat, and moderate fever develop gradually. At the onset of symptoms only a small, yellow-grey spot of pseudomembrane may be present on one or both tonsils and is easily mistaken for other types of tonsillitis; it is associated with marked fetor. The surrounding areas are dull and inflamed. Over the next few days the pseudomembrane enlarges and may extend to cover the uvula, soft palate, oropharynx, nasopharynx, or larynx. There is tender cervical lymphadenopathy, nausea, vomiting, and painful dysphagia. The pseudomembrane becomes greenish-black and eventually sloughs off.
Some 85% of tracheolaryngeal presentations are secondary to faucial diphtheria, but occasionally there may be no pharyngeal pseudomembrane. Initial symptoms include moderate fever, hoarseness, and a nonproductive cough. Over the next day or two, as the pseudomembrane and associated oedema spread, the patient becomes increasingly dyspnoeic with severe chest recession, cyanosis, and eventual asphyxiation unless the obstruction is relieved. Tracheostomy brings instant relief if the obstruction is confined to the larynx and upper trachea. In a minority of cases the pseudomembrane also involves the bronchi and bronchioles and tracheostomy has little effect.
The onset is rapid, with high fever, tachycardia, hypotension, and cyanosis. Pseudomembrane spreads from the tonsils to cover much of the nasopharynx. It has a thick edge and as this advances the earlier parts become necrotic and foul-smelling. There is gross cervical lymphadenopathy. Individual lymph nodes are difficult to feel because of surrounding oedema; this is the characteristic ‘bull neck’ of malignant diphtheria. The patient may bleed from the mouth, nose, or skin. Cardiac involvement with heart block occurs within a few days. Acute renal failure may ensue. Survival is unlikely.
In contrast to respiratory forms, cutaneous diphtheria is usually chronic but mild. The morphological features of individual lesions can be extremely variable as C. diphtheriae can colonize any pre-existing skin lesion (such as impetigo, scabies, surgical wounds, or insect bites) without altering their picture. However, the ulcerative form is the most frequent and typical. Initially vesicular or pustular, and filled with straw-coloured fluid, it soon breaks down to leave a punched-out ulcer several millimetres to a few centimetres across. Common sites are the lower legs, feet, and hands. During the first 1 to 2 weeks it is painful and may be covered with a dark pseudomembrane which separates, revealing a haemorrhagic base, sometimes with a serous or serosanguinous exudate. The surrounding tissue is oedematous and pink or purple in colour. Spontaneous healing to leave a depressed scar usually takes 2 to 3 months, and sometimes much longer. Systemic complications such as myocarditis are rare. Occasionally, the affected limb becomes paralysed.
A mild conjunctivitis may accompany faucial diphtheria. Occasionally, pseudomembrane forms in the lower conjunctiva and spreads over the cornea causing considerable damage. Dysphagia may indicate that pseudomembrane has spread from the tonsils to the oesophagus. Other parts of the gastrointestinal tract are not usually affected, but melaena with colicky abdominal pain is described. Diphtheria may spread by fingers from the throat to vulva or penis causing localized sores. C. diphtheriae occasionally invades the vagina and cervix, allowing the absorption of toxin. Endocarditis is rare, but at least one reported case recovered following antimicrobial treatment.
Diphtheria caused by other corynebacteria
C. ulcerans produces two toxins, one of which seems to be the same as diphtheria toxin. It may cause membranous tonsillitis but toxic manifestations are rare. C. ulcerans has been spread to humans in cows’ milk.
C. pseudodiphtheriticum is commonly present in the flora of the upper respiratory tract. It is nontoxigenic, but can cause exudative pharyngitis with a pseudomembrane identical to that produced by C. diphtheriae. More commonly it causes endocarditis in patients with anatomical abnormalities or infections of the lungs, trachea, or bronchi in immunosuppressed patients or those with pre-existing respiratory disease.
Patients surviving acute diphtheria may develop one or more later complications. These result from delayed effects of the toxin following haematogenous spread. The risk and severity of complications correlates directly with the extent of the pseudomembrane and the delay in administration of antitoxin.
Approximately 10% of patients with diphtheria will develop myocarditis, usually those with clinically severe infection. There is a much greater frequency of cardiac involvement in laryngeal and malignant diphtheria than in faucial diphtheria, and where antitoxin administration was delayed more than 48 h after onset of symptoms.
Cardiac toxicity usually appears after the first week of illness, but in malignant diphtheria can occur after just a few days. Patients complain of upper abdominal pain and may vomit. They become very lethargic and tired. Examination reveals a rapid, thready pulse with hypotension. At this stage profound shock may lead to death. In less severe cases, congestive cardiac failure may develop with a displaced apex beat, gallop rhythm, and murmurs audible over all areas of the heart. Profound bradycardia may result from heart block. There is hepatomegaly and oliguria.
Electrocardiography (ECG) is the best way to demonstrate cardiac involvement. The most common abnormalities are T-wave inversion with ST-segment changes in one or more chest leads and prolonged QTc and PR intervals. There may be right or left axis deviation, bundle branch block, or heart block. Very occasionally, atrial fibrillation or tachyarrhythmias are seen. Many more bursts of arrhythmias can be demonstrated if 24-h ECG monitoring is performed. Numerous ectopic beats have been recorded in patients who lacked other manifestations of cardiac involvement. Although most patients surviving myocarditis recover completely, the presence of left bundle branch block at discharge is associated with poor long-term outcome.
Diphtheria toxin causes a segmental demyelinative neuropathy. Neurological complications usually appear weeks after the onset of the disease, when the patient appears to be recovering, and may show a temporal progression. Palatal paralysis is relatively common and may be seen from the third week onwards. The patient develops a nasal voice and regurgitates fluids through the nose. This usually resolves within a week or so. From the third to the fifth week there may be blurred vision from paralysis of accommodation, or a transient squint from external rectus paralysis. About the sixth or seventh week more sinister paralyses may develop involving pharyngeal, laryngeal, respiratory, and limb muscles. The nerves to the heart may be affected causing tachycardia and dysrhythmias. In severe cases patients may become profoundly hypotonic over a few hours and can die from respiratory arrest. However, if intensive care facilities and skilled staff are available, complete recovery over the following weeks or months should ensue.
Clinical diagnosis is difficult where diphtheria is rare. The differential diagnosis includes infectious mononucleosis, streptococcal or viral tonsillitis, peritonsillar abscess, Vincent’s angina, oral thrush, anthrax, Lassa fever, and leukaemia and other blood dyscrasias. The bull neck of malignant diphtheria may be mistaken for mumps. In adults, secondary syphilis can sometimes cause a glairy (resembling egg white) exudate on the tonsils, and may be accompanied by rash and laryngitis.
Bacterial culture of C. diphtheriae is the mainstay of investigation. Material for culture should be obtained preferably from the edges of the mucosal lesions and inoculated onto appropriate selective media. Suspected colonies may be tested for toxin production by gel precipitation (Elek’s test), guinea pig inoculation, or enzyme immunoassay. Direct smears of infected areas of the throat are often used for diagnostic purposes, but are only of value in experienced hands. More reliably the diphtheria toxin gene may be detected directly in clinical specimens using polymerase chain reaction techniques.
Criteria for diagnosis
In areas where diphtheria is relatively common and during outbreaks, the disease should be suspected in any patient with exudate in the throat. Treatment must not be delayed until the disease is confirmed, except in cases of suspected cutaneous diphtheria without associated respiratory symptoms.
Other corynebacterial skin infections
C. diphtheriae and some other corynebacteria are associated with cutaneous ‘desert sores’. Erythrasma is caused by C. minutissimum and, in HIV-immunosuppressed patients, C. striatum can cause exuberant ulceration.
Antitoxin is the mainstay of treatment, but to be maximally effective it must be given before the toxin has reached tissues such as the heart and kidneys, preferably within 48 h of the onset of symptoms, implying that it must be given empirically before bacteriological confirmation. Dosage depends on the site of primary infection, the extent of pseudomembrane, and the delay between the onset of symptoms and antitoxin administration. Between 20 000 and 40 000 units are given for faucial diphtheria of less than 48 h duration or for cutaneous infection, 40 000 to 80 000 units for faucial diphtheria in excess of 48 h duration or for laryngeal infection, and 80 000 to 100 000 units for malignant diphtheria. For doses over 40 000 units a portion is given intramuscularly followed by the bulk of the dose intravenously after an interval of 30 min to 2 h. Anaphylaxis can occur following antitoxin administration, and adrenaline (epinephrine) should always be available.
Antibiotics are given to eradicate the organism and prevent further toxin production. Benzylpenicillin 150 000 to 250 000 units/kg per day (90–150 mg/kg per day) is given intravenously in four to six divided doses in children aged 1 month to 12 years. In adults the dosage is 12 million to 20 million units/day (7.2–12 g/day) in four to six divided doses. Oral penicillin V is substituted when the patient is able to swallow. Erythromycin may be used for penicillin-sensitive individuals, but it may not be as effective in eradicating carriage. Antibiotic therapy should continue for 10 to 14 days.
Facilities for urgent tracheostomy should always be available in case of respiratory obstruction. Indications include increasingly laboured breathing and agitation. This procedure will be lifesaving in many cases. Most tracheostomies can be closed after just a few days. Steroids may be used in conjunction with tracheostomy to reduce airway swelling, but there have been no controlled trials to support their use. Steroids are of no benefit in preventing myocarditis or neuritis.
Patients with signs or symptoms of cardiac involvement need to be managed in intensive care units. Oxygen should be given. Temporary cardiac pacing is useful in patients with heart block, but is of doubtful value in cases of malignant diphtheria. An isoprenaline infusion may buy valuable time while the patient is transferred to a centre with facilities for pacing. Digoxin has been used in congestive cardiac failure. It has been suggested that carnitine may prevent some cases of myocarditis.
There is no specific treatment for neuritis. The severest cases will need mechanical ventilation and intragastric or intravenous feeding. With skilled nursing care full recovery can be expected. Patients recovering from clinical disease should complete active immunization during convalescence.
Diphtheria toxoid is highly effective in conferring protection against clinical disease. Circulating antitoxin levels of less than 0.01 IU/ml are considered nonprotective, while levels of 0.01 IU/ml may confer some protection. Levels of 0.1 IU/ml or more are considered fully protective, and levels above 1.0 IU/ml are associated with long-term protective immunity. The potency of diphtheria vaccine is reduced in children aged 7 years and older so that reactogenicity is minimized.
The recommended schedule for vaccination against diphtheria varies between countries. In the United Kingdom three primary doses of adsorbed diphtheria–tetanus–pertussis–haemophilus influenzae type b vaccine (DTP-Hib) are given at 2, 3, and 4 months; a first booster dose with DTP at age 3 to 5 years, and a second booster dose with DT at school leaving. The primary course does not need to be repeated if boosters are delayed. People living in low-endemic or nonendemic countries should receive booster doses of DT approximately every 10 years. It is now recommended by the World Health Organization that DT rather than T (tetanus toxoid alone) should be used when tetanus prophylaxis is needed following injury.
Where diphtheria is endemic the primary course alone should be sufficient to prevent an epidemic of diphtheria, as natural mechanisms such as frequent skin infections caused by C. diphtheriae probably contribute to maintaining immunity. One or two DT or DTP booster doses may need to be added to the routine schedule in areas at increased risk of diphtheria. Adults in developing countries do not require routine immunization.
Aggressive action is needed in the event of a diphtheria outbreak. Groups at risk should be immunized, there should be prompt diagnosis and management of cases, and identification of close contacts should be made so that the spread of infection can be halted. A single dose of DTP should be used for children under 3 years of age, and DT for children aged over 3 years and adults. Additional doses of vaccine will be needed in nonimmunized (Schick test positive) people.
Susceptibility to diphtheria may be assessed using the Schick test: 0.1 ml of toxin is injected into the skin of one forearm (test site) and the same quantity of a heat-inactivated toxin injected into the other forearm (control site). A positive reaction occurs in individuals without toxin-neutralizing antibodies and consists of an area of redness appearing after 24 to 36 h at the test site only and persisting for 4 to 5 days. If no toxin-neutralizing antibodies are present there will be either no reaction at either site (negative test) or a pseudoreaction at either site due to antibodies to substances other than diphtheria toxin in the test materials. This test is no longer commonly performed due to limited availability of the test materials.