Cutaneous diphtheria is caused by a cluster of genetically related bacteria called the Corynebacterium diphtheriae species complex (CdSC), which comprises Corynebacterium diphtheriae, the main pathogen responsible for diphtheria, along with additional species such as Corynebacterium ulcerans, which is now the more common pathogen in the United Kingdom. These strains can cause both cutaneous and respiratory diphtheria.

Corynebacterium diphtheriae is a pleomorphic, aerobic, gram-positive bacillus that causes symptoms through the production of a bacteriophage-dependent exotoxin. Human beings are the only known reservoir for C diphtheriae, which can only inhabit mucous membranes and skin and is therefore spread by respiratory droplets or contact with the exudate of a diphtheritic cutaneous lesion, including objects contaminated with discharge from the lesion.

There are 3 forms of cutaneous diphtheria:

1. Primary infection, which begins as a pustule or vesicle on previously normal skin
2. Superinfection of an eczematized lesion
3. Secondary infection of a wound

Cutaneous diphtheria typically follows an indolent course, usually remaining local, although the exotoxin released by C diphtheriae can rarely lead to complications, including myocarditis, renal impairment, or polyneuropathy, as seen more frequently with upper respiratory tract diphtheritic infections. However, cutaneous infection is associated with more bacterial shedding and higher rates of transmission of cutaneous diphtheria to close contacts. The skin is often an importation reservoir for C diphtheriae and is associated with sporadic cases of respiratory diphtheria.

Risk factors for cutaneous diphtheria include skin trauma, inadequate hygiene, injection drug use, and absence of diphtheria vaccination. Children, older adults, and the immunocompromised are most commonly affected. Endemic areas include Russia, Ukraine, and Latvia, ongoing clusters of cases have been reported in Vancouver and Seattle, and outbreaks have been reported in Europe in refugee populations, including East Africa.