Cutaneous Anthrax

Organism

B. anthracis. Large Gram+, spore-forming, NON-MOTILE (distinctive — unlike most Bacillus spp). “Bamboo-stick”/jointed chains on Gram stain. Spores: survive DECADES in soil. Bioterrorism agent (2001 US postal attacks).

Virulence Factors (2 plasmids)

Capsule (pXO2): Poly-D-glutamic acid — antiphagocytic. UNUSUAL: polypeptide (not polysaccharide like most encapsulated bacteria).

Tripartite exotoxin (pXO1): 3 individually nontoxic proteins, combine in pairs:

PA (Protective Antigen) — common cell-binding component, needed for either toxin’s entry
PA+EF (Edema Factor, adenylate cyclase) = EDEMA TOXIN — disrupts water/ion homeostasis, massive local edema
PA+LF (Lethal Factor, protease) = LETHAL TOXIN — disrupts signaling, macrophage/cell death, septic shock (systemic disease)

Transmission/Clinical Forms

Zoonosis. Herbivores (cattle/sheep/goat) graze contaminated soil. Humans = incidental via animal/product contact.

Cutaneous (>95% natural cases): skin break + infected hide/wool/carcass. “Wool-sorter’s disease.” Occupational: farmers, wool-sorters, tannery/abattoir workers.
Inhalational: spores → alveolar macrophage → mediastinal LN → germinate → hemorrhagic mediastinitis. RARE natural, MOST LETHAL (~90% mortality untreated).
GI: undercooked contaminated meat → hemorrhagic enteritis. Rare, substantial mortality.
Injection: newer form, contaminated heroin, IDUs (Europe reports).

Clinical — Cutaneous Anthrax

Evolution over ~1 week: small PAINLESS papule (notable — NOT painful, unlike most bacterial skin infection; ±itching/tingling) at inoculation site (exposed areas: face/neck/arms) → vesiculates 1-2 days → ulcerates → dries into depressed BLACK NECROTIC ESCHAR (“anthrax”=Greek for coal) + striking non-pitting EDEMA disproportionate to lesion size. ± regional LAD, mild systemic symptoms (low fever, malaise).

Untreated mortality ~20% (progression to systemic sepsis). Treated mortality <1%. Generally self-limited even untreated in most cases — eschar separates, heals without significant scarring.

Laboratory Diagnosis

Direct microscopy: Gram stain (vesicular fluid/eschar) — large boxcar Gram+ bacilli chains. M’Fadyean stain (polychrome methylene blue): capsule as PINK HALO around blue bacilli = classic rapid presumptive test.

Culture: non-fastidious, grows readily. “Medusa head”/curled-edge colonies, NON-hemolytic. Confirmatory: String-of-pearls test (chains with penicillin), PCR (toxin+capsule genes), gamma-phage susceptibility.

Serology: retrospective/epidemiological, not useful for acute diagnosis.

Treatment

Ciprofloxacin or doxycycline = 1st line. Naturally-acquired cutaneous: 7-10 days. Bioterrorism/possible inhalational exposure: 60 days (spore reactivation concern). Systemic (inhalational/GI/complicated cutaneous): combination IV antibiotics + antitoxin (raxibacumab or anthrax immune globulin, targets PA, blocks cell entry) + aggressive supportive care.

Prevention

Animal vaccination (endemic regions, PRIMARY control, interrupts zoonotic cycle). Proper carcass disposal/incineration (NEVER bury intact — spores persist). PPE for at-risk occupations. Human vaccine (cell-free PA-based) — high-risk occupational/military only, not general population.

The Organism

Bacillus anthracis is a large, Gram-positive, spore-forming, non-motile (a genuinely distinctive feature, since most other Bacillus species are motile) rod, forming long chains that give a characteristic “bamboo-stick” or “jointed” appearance on Gram stain. Its spores are exceptionally hardy, surviving in contaminated soil for decades, which is exactly why anthrax remains endemic in certain grazing regions long after any acute animal outbreak has passed, and why the organism has real historical significance as a bioterrorism agent (weaponized spore preparations, notably the 2001 US postal attacks).

Virulence Factors

B. anthracis owes its virulence to two plasmid-encoded factors, both essential and both individually attenuating if lost:

Poly-D-glutamic acid capsule (plasmid pXO2) — antiphagocytic, chemically unusual among bacterial capsules (a polypeptide rather than the polysaccharide capsules typical of most encapsulated pathogens, as covered under Bacterial Morphology and Physiology).
Tripartite exotoxin (plasmid pXO1) — three individually non-toxic proteins that combine in pairs to produce two distinct toxic effects: protective antigen (PA) is the common cell-binding component required for either toxin to enter a host cell; PA + oedema factor (EF) (an adenylate cyclase) forms oedema toxin, disrupting cellular water/ion homeostasis and causing the massive local oedema anthrax is known for; PA + lethal factor (LF) (a protease) forms lethal toxin, which disrupts cell signalling and drives macrophage/cell death, contributing to septic shock in systemic disease.

Transmission and Clinical Forms

Anthrax is fundamentally a zoonosis — herbivores (cattle, sheep, goats) become infected grazing on spore-contaminated soil, and humans are infected incidentally through contact with infected animals or animal products. The clinical form directly tracks the route of exposure:

Cutaneous anthrax (>95% of naturally occurring cases): spores enter through a break in the skin during contact with infected animal hides, wool, or carcasses (occupational risk for farmers, wool-sorters, tannery/abattoir workers — “wool-sorter’s disease” is an older name reflecting this exposure).
Inhalational anthrax: spores inhaled (from contaminated wool/hides in an occupational setting, or historically from bioterrorism-weaponized preparations) reach alveolar macrophages, are transported to mediastinal lymph nodes, and germinate there, causing haemorrhagic mediastinitis — rare naturally, but the most lethal form, with mortality historically approaching 90% without prompt treatment.
Gastrointestinal anthrax: from ingesting undercooked contaminated meat, causing severe haemorrhagic enteritis — rare, but with substantial mortality.
Injection anthrax: a more recently recognized form among injection drug users exposed to contaminated heroin, particularly reported in parts of Europe.

Clinical Manifestations of Cutaneous Anthrax

The lesion evolves through a distinctive, recognizable sequence over about a week: a small, painless (a genuinely notable feature — the lesion is characteristically not painful, distinguishing it from most bacterial skin infections, though there can be surrounding itching/tingling) papule appears at the inoculation site, typically on an exposed area (face, neck, arms); this enlarges and vesiculates over 1–2 days, then ulcerates and dries into a depressed, black, necrotic eschar (“anthrax” derives from the Greek for coal, describing this black lesion) surrounded by striking, often extensive, non-pitting oedema disproportionate to the size of the lesion itself. Regional lymphadenopathy and mild systemic symptoms (low-grade fever, malaise) can accompany it. Untreated, cutaneous anthrax has a mortality of roughly 20% from progression to systemic disease/sepsis; with prompt antibiotic treatment, mortality falls below 1%, and the disease is generally self-limited even without treatment in most cases — the eschar eventually separates, healing without significant scarring in the majority.

Laboratory Diagnosis

Direct microscopy: Gram stain of vesicular fluid or eschar material shows large, boxcar-shaped Gram-positive bacilli in chains; a polychrome methylene blue (M’Fadyean) stain of the same material demonstrating the antiphagocytic capsule as a pink halo around blue-stained bacilli is a classic, rapid, presumptive confirmatory test, historically used for veterinary diagnosis in suspect animal carcasses and still useful in human specimens.

Culture: grows readily on ordinary media (non-fastidious), producing characteristic non-haemolytic, “Medusa head” or curled-edge colonies; confirmatory identification includes a positive string-of-pearls test (chains of spherical bacilli forming when grown with penicillin) and, definitively, PCR targeting the toxin and capsule genes, or specific phage lysis (gamma-phage susceptibility).

Serology: available for retrospective/epidemiological confirmation, less useful for acute diagnosis given the delay in antibody development.

Treatment

Ciprofloxacin or doxycycline are first-line for naturally acquired cutaneous anthrax, typically for 7–10 days for confirmed naturally acquired cutaneous disease (though a longer course, 60 days, is used if bioterrorism-associated inhalational exposure is a possibility, given the concern that inhaled spores could remain dormant and later reactivate). Systemic (inhalational, GI, or complicated cutaneous) disease needs combination IV antibiotic therapy plus, where available, antitoxin (raxibacumab or anthrax immune globulin, targeting protective antigen directly to block toxin entry into cells) alongside aggressive supportive care.

Prevention

Animal vaccination in endemic regions (the primary control measure, interrupting the zoonotic cycle at its source), proper disposal/incineration of infected animal carcasses (never buried intact, since spores would simply persist in the soil), protective equipment for at-risk occupational groups, and a human vaccine (cell-free protective-antigen-based) reserved for high-risk occupational or military exposure rather than general population use.