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Chapter: Microbiology and Immunology: Bacteriology: Corynebacterium

Epidemiology of Corynebacterium diphtheria

During the early 1990s, diphtheria was still endemic in many parts of the world including the Indian subcontinent, Indonesia, Philippines, Brazil, Nigeria, and republics of the former Soviet Union.

Epidemiology

 Geographical distribution

During the early 1990s, diphtheria was still endemic in many parts of the world including the Indian subcontinent, Indonesia, Philippines, Brazil, Nigeria, and republics of the former Soviet Union.

·           The largest outbreak of diphtheria in the developed world occurred from 1990 to 1996 throughout the states of the former Soviet Union. Most cases were reported among adolescents and adults, rather than children. More than 110,000 cases of diphtheria and 2900 fatalities from diph-theria were reported during the epidemic. Incidence declined in 1996, possibly due to immunization and early detection activities that were carried out following the outbreak.

·           Outbreaks have also been reported in Central Asia, Algeria, and Ecuador. In the United States, Europe, and Eastern Europe, recent outbreaks of diphtheria have occurred largely among alcohol and/or drug abusers.

·           Since 1994, with the advent of active immunization procedures, case fatality due to diphtheria has reduced significantly.

 Habitat

The upper respiratory tract of an infected host is the primary habitat of C. diphtheriae. The bacteria also inhabit the superfi-cial layers of the skin lesions.

 Reservoir, source, and transmission of infection

Humans are the only natural host of C. diphtheriae and thus are the only significant reservoirs of infection. Infective droplets or nasopharyngeal secretions are the common sources of infection.

·           Direct human contact facilitates transmission of the disease. Patients with active infection are more likely to transmit diphtheria.

·           C. diphtheriae is most commonly transmitted by closecontacts through droplets of nasopharyngeal secretions or infected skin lesions.

·           It is known that toxigenic strains may directly colonize the nasopharyngeal cavity. In addition, the tox gene can be spread indirectly by the release of toxigenic corynebacterio-phage and by lysogenic conversion of nontoxigenic autoch-thonous C. diphtheriae in situ. Asymptomatic respiratory carrier states are believed to be important in transmitting diphtheria and immunization appears to reduce the likeli-hood of carrier state. Dust and clothing also may contrib-ute to the transmission. The organism can survive up to 6 months in dust and fomites.

When diphtheria was endemic, the disease was most commonly seen in children younger than 15 years, but recently, the epide-miology has shifted to adults because this group of population lacked natural exposure to toxigenic C. diphtheriae in the vaccine era and also received less booster doses. In serosurvey in the  United States and other developed countries, such as Sweden, Italy, and Denmark, 25% to more than 60% of adults did not show protective antitoxin levels in their serum with particularly low levels found in elderly persons.

 Typing

C. diphtheriae strains are classified into various serotypes byagglutination reactions. Mitis strains have been classified into 40 types, intermedius into 4 types, and gravis into 13 types. Gravis type II strains are found worldwide, while types I and III are commonly found in Great Britain. Type IV is mainly found in Egypt, while type V in the United States.

      Typing of C. diphtheriae strains can also be done by biotyping, lysotype, and by using molecular biology techniques. The latter includes techniques, such as restriction endonuclease digestion patterns of C. diphtheriae chromosomal DNA and genetic probe for cloned corynebacterial insertion sequences.


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