Question
Malignant pustule
Answer
Malignant pustule is the older, classical term for cutaneous anthrax, the most common clinical form of anthrax (caused by Bacillus anthracis), acquired through inoculation of spores into broken/abraded skin (typically from contact with infected animals, hides, wool, or animal products) — despite its alarming name, it is usually the mildest and most treatable form of anthrax when appropriately managed.
Clinical evolution: begins as a small, painless (or minimally painful), pruritic papule at the site of inoculation, typically on an exposed area of skin; over 1–2 days, this evolves through a vesicular stage and then ulcerates, developing a characteristic black, depressed, necrotic eschar surrounded by marked, often extensive, non-pitting oedema — the “malignant” descriptor refers to this striking, out-of-proportion oedema and dark necrotic appearance, not to neoplastic malignancy. Regional lymphadenopathy and mild systemic symptoms may accompany the lesion.
Pathogenesis: local germination of spores at the inoculation site, with bacterial multiplication and toxin production (protective antigen, oedema factor, lethal factor) driving the marked local oedema and tissue necrosis; the lesion is characteristically painless, an important distinguishing feature from other bacterial skin infections.
Complications: if untreated, can progress to bacteraemia and systemic anthrax with high mortality, though the cutaneous form has a good prognosis (<1% mortality) with prompt antibiotic treatment.
Laboratory diagnosis: Gram stain and culture of vesicular fluid/eschar material (large, Gram-positive, spore-forming, box-car shaped bacilli, non-haemolytic on blood agar, characteristic non-motile “Medusa head” colonies); PCR where available.
Treatment: ciprofloxacin or doxycycline; surgical incision/manipulation of the lesion is avoided, as it risks precipitating systemic dissemination.