Cutaneous diphtheria is a bacterial infection of the skin caused by Corynebacterium diphtheria and less commonly Corynebacterium ulcerans. The condition is endemic in the developing countries, but sporadic cases in Europe, Australia and North America, have been documented. The clinical presentation comprises chronic non-healing ulcers that are only rarely accompanied by signs of a systemic infection. The diagnosis rests on clinical criteria and microbiological identification of the bacterial pathogens.
The widespread vaccination against diphtheria toxin produced by Corynebacterium diphtheriae has led to a profound decrease in the number of infections caused by this gram-positive bacteria throughout the world. However, cutaneous diphtheria is still considered endemic in tropical countries and other developing parts of the world, including the Philippines, Thailand, Vietnam, Bangladesh, Nepal, Malaysia, Indonesia, Afghanistan, India, and China  . Additionally, a growing number of reports show both imported and domestic cases of cutaneous diphtheria in the United Kingdom, France, Austria, Norway, Canada, and Australia      . Although respiratory infections are the primary focus when it comes to Corynebacterium spp. (due to their life-threatening nature), cutaneous diphtheria is also important, not only due to its potential to cause a systemic infection (although they are rarely seen, particularly in vaccinated individuals), but because it is able to spread from person-to-person rapidly and lead to outbreaks   . Infection usually occurs after an insect bite or trauma of the skin that exposes the subcutaneous tissue to the bacterial pathogen  . The clinical presentation is characterized by the presence of shallow ulcers on the skin that may develop on virtually any site of the body  . The ulcers are chronic in nature and do not heal spontaneously, and coinfections of the ulcers by other bacteria (eg. Staphylococcus aureus and Streptococcus pyogenes) is common    .
When suspicion of cutaneous diphtheria arises, the diagnostic workup should start with a thorough history taking during which the patient should be asked about recent travel (or if the patient is a resident in any of the endemic countries mentioned previously) and previous vaccination for diphtheria. Some studies have shown that a poor socioeconomic status, even in developing cities and countries, is an important risk factor , implying that sociodemographic characteristics must also be covered in the interview. A detailed clinical examination, including a complete inspection of the skin, should follow. If a chronic non-healing ulcer has been detected, microbiological studies need to be employed. A swab of the tip of the ulcer is tested by gram-staining or by culturing the organism, which are usually the first methods used for detecting Corynebacterium spp., although the presence of various microbiological organisms that are a part of the skin flora (or responsible for co-infections) may compromise the results  . For this reason, selective cultivating media or more advanced methods, such as polymerase chain reaction (PCR), must be used to confirm cutaneous diphtheria. PCR amplification is the method of choice when it comes to identifying C. diphtheriae and C. ulcerans, but even more superior procedures, for example matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), provide more reliable and faster results   .