Diphtheria is a respiratory disease caused by Corynebacterium diphtheriae (C. diphtheriae), a gram-positive aerobic bacillus, which becomes toxic only if infected by a virus carrying the toxin gene. The disease manifests itself in a serious form when this happens. The bacterium comes in four biotypes: belfanti, gravis, intermedius and mitis, all capable of producing toxins that can cause serious pathologies.⁽¹⁾

Symptoms include the formation of a gray-green or black fibrous pseudomembrane in the back of the throat and on the tonsils that can impede breathing and swallowing. Other symptoms are swollen lymph nodes in the neck, sore throat, general weakness, fever and chills⁽²⁾. If toxins enter the bloodstream, further complications may occur, including neuritis, myocarditis, proteinuria, pneumonia, thrombocytopenia, and death.⁽³⁾

Diphtheria is contagious and is spread from person to person through respiratory secretions (coughing, sneezing). It can also be transmitted by touching a wound on a person with diphtheria or by touching objects contaminated with the bacteria. Vaccinated people are still able to shed the bacterium because vaccination does not eliminate the carriage of Corynebacterium diphtheriae in the back of the throat or on the skin.⁽⁴⁾

The incubation period varies from one to 10 days, usually between two and five days. The disease can affect any mucous membrane, with symptoms that vary depending on the location, affecting areas such as the skin (cutaneous diphtheria), the nostrils, the tonsils, the pharynx, the larynx, the eyes (ocular diphtheria), the external ear and the genitals⁽⁵⁾. Some strains of the bacterium do not produce toxins and cause only mild or moderate symptoms of throat inflammation⁽⁶⁾. Diphtheria can recur in individuals who have already had the disease.⁽⁷⁾

C. diphtheriae affects humans exclusively and carriers are often asymptomatic. The disease is very rare in Italy and other developed countries with high sanitation standards. Since 2015, 8 cases have been reported in Italy, including one in 2016 in Northern Italy caused by a toxigenic strain of C. diphtheriae responsible for cutaneous diphtheria. The other cases were attributed to non-toxin-producing strains⁽⁸⁾.