Pediatric Diphtheria 

Diphtheria is an acute toxin-mediated disease caused by Corynebacterium diphtheriae. Nontoxigenic strains also cause disease, which is mostly cutaneous and usually mild. Three biotypes (ie, mitis, gravis, intermedius), each capable of causing diphtheria, are differentiated by colonial morphology, hemolysis, and fermentation reactions.

The "strangling angel of children," as diphtheria was once called, can be traced to the fourth-to-fifth century BC and was one of the most common causes of death among children in the prevaccine era. Klebs was the first to identify the organism in 1884, and Loeffler was first to cultivate the bacterium a year later. Roux and Yersin purified the toxin in 1889, and the antitoxin was invented shortly afterwards. In the 1920s, the toxoid was developed.

Unlike other diphtheroids (eg, coryneform bacteria), which are ubiquitous in nature, C diphtheriae is an exclusive inhabitant of human mucous membranes and skin. Spread primarily occurs via contact with airborne respiratory droplets, direct contact with respiratory secretions of symptomatic individuals, or contact with exudate from infected skin lesions. Asymptomatic respiratory carriers are important in transmission.

In the prevaccine era, diphtheria was a dreaded highly endemic childhood disease found in temperate climates. Despite a gradual decline in deaths in most industrialized countries in the early 20th century (associated with improving living standards), diphtheria remained one of the leading causes of death in children until widespread vaccination was implemented. In England and Wales, as recently as 1937-1938, diphtheria was second only to pneumonia among all causes of death in children, with an annual death rate of 32 per 100,000 in children younger than 15 years.

Superimposed on the high rates of endemic disease was a rough incidence periodicity that demonstrated peaks every several years. Epidemic waves were characterized by extremely high incidence in Spain in the early 1600s, New England in the 1730s, and Western Europe from 1850-1890. Deaths were sporadic.

The factors governing the periodicity of diphtheria outbreaks are not understood. In the United States, Canada, and many countries in Western Europe, the widespread use of diphtheria toxoid for childhood vaccination, beginning in the 1930s and 1940s, led to a rapid reduction in diphtheria incidence. However, in the 1930s, a gradual rise in diphtheria incidence to 200 cases per 100,000 in the prewar period occurred in Germany and several other central European countries with partially implemented vaccination programs. The onset of World War II in 1939 and the occupation by German troops of many Western European countries led to the last diphtheria pandemic in western industrialized countries.

Diphtheria organisms usually remain in the superficial layers of skin lesions or respiratory mucosa, inducing local inflammatory reaction. The organism's major virulence lies in its ability to produce the potent 62-kd polypeptide exotoxin, which inhibits protein synthesis and causes local tissue necrosis.

Diphtheriae toxin, which is secreted by toxigenic strains of C diphtheriae, is a single polypeptide of Mr 58,342. Toxigenic strains of C diphtheriae carry the tox structural gene found in lysogenic corynebacteriophages beta-tox +, gamma-tox +, and omega-tox +.

Highly toxic strains have 2 or 3 tox + genes inserted into the genome. Expression of the gene is regulated by the bacterial host and is iron dependent. In the presence of low concentrations of iron, the gene regulator is inhibited, resulting in increased toxin production. Toxin is excreted from the bacterial cell and undergoes cleavage to form 2 chains, A and B, which are held together by an interchain disulfide bond between cysteine residues at positions 186 and 201. As toxin concentrations increase, the toxic effects extend beyond the local area because of distribution of the toxin by the circulation. Diphtheriae toxin does not have a specific target organ, but myocardium and peripheral nerves are most affected.

Within the first few days of respiratory tract infection, a dense necrotic coagulum of organisms, epithelial cells, fibrin, leukocytes, and erythrocytes forms, advances, and becomes a gray-brown adherent pseudomembrane. Removal is difficult and reveals a bleeding edematous submucosa. Paralysis of the palate and hypopharynx is an early local effect of the toxin. Toxin absorption can lead to necrosis of kidney tubules, thrombocytopenia, cardiomyopathy, and demyelination of nerves. Because cardiomyopathy and demyelination of nerves can occur 2-10 weeks after mucocutaneous infection, the pathophysiologic mechanism may be immunologically mediated in some patients.

In the classic description of diphtheria, the primary focus of infection is the tonsils or pharynx in more then 90% of patients; the nose and larynx are the next most common sites. After an average incubation period of 2-4 days, local signs and symptoms of inflammation develop. Fever is rarely higher than 39°C.

Frequency

United States

Diphtheria cases remain isolated, with the last outbreaks reported between 1972-1982. Diphtheria incidence continued to decline steadily throughout the vaccine era in the United States and Western Europe (after the immediate postwar period). Cases of clinical diphtheria became extremely uncommon after the 1970s. Residual indigenous cases have been concentrated among incompletely vaccinated or unvaccinated persons of low socioeconomic status.^[1]

International

Diphtheria is endemic in many parts of the world, including countries of the Caribbean and Latin America. During the last 10 years, large epidemics of diphtheria have occurred in the former Soviet Union, where diphtheria had been well controlled. The largest outbreak of diphtheria in the developed world occurred from 1990-1995 throughout the states of the former Soviet Union.^{[2, 3]} Since 1994, with the initiation of aggressive immunization efforts, the number of reported cases has decreased. Outbreaks also have been reported in Central Asia, Algeria, and Ecuador.^[4]

A feature of these epidemics concerns the age group; most cases have occurred in adolescents and adults, rather than in children. Protocols in all countries of the European Union call for at least 3 doses of diphtheria vaccine during the first 2 years of life. Vaccination in France, Greece, Ireland, Luxembourg, Portugal, and the United Kingdom begins at age 2 months; in Austria, Belgium, Finland, Germany, Italy, the Netherlands, Spain, and Sweden vaccination begins at age 3 months; and in Denmark, it begins at age 5 months. Consecutive injections are usually separated by 1-2 months, but 9 months elapse between the second and third doses in Denmark.

Booster doses are administered in most countries 1 year after the third injection, then approximately every 5 years. Childhood immunization stops at age 6 years in Belgium, Ireland, Italy, and Portugal; at age 10 years in the Netherlands and Sweden; at age 15 years in Greece and Luxembourg; at age 15-19 years in the United Kingdom; and at age 18-20 years in France. Adult immunity, with tetanus toxoid and a low dose of diphtheria vaccine (Td) every 10 years, is maintained systematically only in Austria, Finland, and Germany. The epidemic of diphtheria in the former Soviet Union led the World Health Organization (WHO) to recommend systematic immunization of travelers to these countries.

Mortality/Morbidity

Death due to mechanical airway obstruction or cardiac involvement with circulatory collapse occurs in at least 10% of patients with respiratory tract diphtheria. The mortality rate has not improved and was approximately 20% in the outbreak that occurred in the newly independent states of the Soviet Union during the early 1990s.

Prognosis depends on the virulence of the organism (with the gravis strain usually accounting for the most severe disease), the age and immunization status of the patient, the site of involvement, and the speed with which antitoxin is administered. For patients in whom disease is recognized on day 1 and therapy is promptly initiated, the mortality rate is approximately 1%. If appropriate treatment is withheld until day 4, the mortality rate rises to 20%.

Diphtheria was no longer considered to be a child killer until large epidemics in several Eastern European countries drew attention to this forgotten disease in the 1990s. Reports from developing countries suggest that different epidemiologic patterns of the disease occur in populations with different immunization histories. The outbreaks had high case fatality rates and a large proportion of patients with complications.

Race

No racial predilection is observed.

Sex

No difference has been described for acute infection; however, in surveys from around the world, lack of immunity was more pronounced in elderly women than in men.

Age

When diphtheria was endemic, it primarily affected children younger than 15 years; recently, the epidemiology has shifted to adults who lack natural exposure to toxigenic C diphtheriae in the vaccine era and those who have low rates of receiving booster injections. In the 27 sporadic cases of respiratory tract diphtheria reported in the United States in the 1980s, 70% occurred in persons older than 25 years.

Data from Europe are particularly noteworthy because the childhood immunization rate exceeds 95% in some countries (eg, Sweden), but approximately 20% of persons younger than 20 years and as many as 75% of persons older than 60 years lack the protective antibody. Other broad serosurveys have identified large subgroups of underimmunized individuals in the United States and other countries in which immunization is believed to be universal; these individuals would be at risk if the organism were introduced. In serosurveys in the United States and other developed countries with almost universal immunization during childhood, such as Sweden, Italy, and Denmark, 25% to more than 60% of adults lacked protective antitoxin levels, with particularly low levels found in elderly persons.