Cutaneous anthrax
After infection via an abrasion, cut, or possible insect bite, a small pimple or papule will develop within two to three days, although there are reports of incubation periods as short as 12 hours or as long as 19 days.35 Over the next 24 hours a ring of vesicles develops, followed by ulceration of the central papule, which dries to form the classic black eschar, which in turn enlarges to cover the drying vesicle. Pus will only be present if the lesion becomes secondarily infected with pyogenic bacteria, such as Staphylococcus aureus. The lesion, which is always painless, may be small or large and is always surrounded by oedema. Usually, by the fifth or sixth day a thick black eschar, firmly adherent to the underlying tissue, has developed. The bacilli remain localised to the lesion in uncomplicated cutaneous anthrax, although adenitis of the regional lymph nodes is not uncommon. Fever is rarely present. Ten days after the appearance of the original lesion the eschar begins to resolve slowly, over two to six weeks, irrespective of treatment,36 and resolution is usually completed with minimal scarring. In untreated anthrax, about 20% of patients may develop septicaemia and die, but with the use of appropriate antibiotics the mortality rate is < 1%. Cutaneous anthrax should always be considered when patients who have had contact with animals or animal products present with painless ulcers associated with vesicles and oedema.