Anthrax
Summary
Anthrax is a potentially fatal infection caused by the spore-forming, Gram-positive bacterium Bacillus anthracis. It is primarily a disease of herbivores (Cattle, Sheep, Goats), but humans become infected through contact with infected animals or animal products. It presents in three main forms: Cutaneous (95%) – the classic painless black eschar; Inhalation (5%) – "Woolsorter's disease" with high mortality; and Gastrointestinal – rare, from contaminated meat. Anthrax is also a Category A bioterrorism agent due to spore stability and aerosol potential (2001 US Postal Attacks). B. anthracis produces lethal toxin and oedema toxin, which cause tissue necrosis and systemic shock. Treatment requires prompt antibiotic therapy (Ciprofloxacin or Doxycycline) plus antitoxin in severe cases. Post-exposure prophylaxis and vaccine are available for high-risk groups. Mortality for inhalation anthrax is 45-80% even with treatment. [1,2,3]
Clinical Pearls
"Malignant Pustule" = Painless Black Eschar: The cutaneous lesion is classically PAINLESS. A black eschar surrounded by oedema is pathognomonic.
"Widened Mediastinum": CXR finding in Inhalation Anthrax due to haemorrhagic mediastinitis/lymphadenitis. Classic exam image.
Bioterrorism Agent: Category A. Spores survive decades in soil. Weaponised powder form used in 2001 postal attacks.
Ciprofloxacin is First-Line: IV Ciprofloxacin for systemic disease. Doxycycline is an alternative.
Demographics
| Factor | Notes |
|---|---|
| Geographic Distribution | Endemic in agricultural regions of Africa, Asia, Middle East, Central/South America. Rare in UK/Europe (Occasional import). |
| Occupational Risk | Farmers, Veterinarians, Abattoir workers, Wool/Hide processors ("Woolsorters"). |
| Historical Outbreaks | 2001 US Postal Attacks (22 cases, 5 deaths). UK "Inject Anthrax" in heroin users (2009-2012). |
Transmission
| Route | Disease Form | Notes |
|---|---|---|
| Cutaneous Contact | Cutaneous Anthrax (95%) | Spores enter through skin abrasion/cut. |
| Inhalation | Inhalation Anthrax | Breathing spores (Wool/Hide processing, Bioterrorism). |
| Ingestion | GI Anthrax | Eating undercooked contaminated meat. Rare. |
| Injection | Injectional Anthrax | Contaminated heroin (UK outbreak). |
Organism
- Bacillus anthracis: Large, Gram-positive, Aerobic, Spore-forming rod.
- "Box-car" appearance: Square-ended bacilli in chains on Gram stain.
- Capsule: Poly-D-glutamic acid (Antiphagocytic). Unique polypeptide capsule.
- Spores: Extremely resistant (Survive decades in soil, Heat, Disinfection).
Virulence Factors (Toxins)
| Toxin | Components | Effect |
|---|---|---|
| Lethal Toxin (LeTx) | Protective Antigen (PA) + Lethal Factor (LF) | Macrophage cytolysis → Cytokine storm → Shock and Death. |
| Oedema Toxin (EdTx) | Protective Antigen (PA) + Oedema Factor (EF) | Increases cAMP → Massive oedema (Like adenylate cyclase). |
Pathogenesis
- Entry: Spores enter via skin, lungs, or gut.
- Germination: Spores germinate into vegetative bacilli at entry site.
- Local Replication: Toxin production → Local tissue necrosis (Eschar).
- Lymphatic Spread: Bacteria enter regional lymph nodes → Haemorrhagic lymphadenitis.
- Bacteraemia: Systemic spread → Septic shock, Haemorrhagic meningitis, Death.
| Condition | Key Features |
|---|---|
| Cutaneous Anthrax | Painless black eschar, Massive oedema, Farm/Animal contact. |
| Spider Bite (Brown Recluse) | Painful necrotic ulcer. No oedema. No systemic prodrome. |
| Necrotising Fasciitis | Rapidly spreading, VERY painful, Crepitus. |
| Ecthyma Gangrenosum (Pseudomonas) | Black eschar in immunocompromised. Septic patient. |
| Tularaemia | Ulceroglandular form. Painful lymphadenopathy. Tick/Animal exposure. |
| Plague (Bubonic) | Painful bubo. Rodent/Flea exposure. |
| Community-Acquired Pneumonia | Productive cough. CXR shows lobar consolidation (Not widened mediastinum). |
Cutaneous Anthrax (95% of Cases)
| Stage | Day | Features |
|---|---|---|
| Papule | Day 1-2 | Pruritic papule at inoculation site. |
| Vesicle | Day 2-3 | Vesicle → Ring of satellite vesicles. |
| Ulcer | Day 3-5 | Central necrosis. Ulcer with black eschar (Greek: Anthrax = Coal). |
| Eschar | Day 7-10 | Painless, Dry, Black, Depressed eschar. Massive non-pitting oedema around it. |
| Resolution | Week 2-3 | Eschar separates. Heals with scar. |
Key Point: The lesion is remarkably PAINLESS unless secondary infection occurs.
Inhalation Anthrax ("Woolsorter's Disease")
| Phase | Features |
|---|---|
| Prodrome (Day 1-5) | Flu-like illness: Fever, Myalgia, Malaise, Dry cough. |
| Fulminant Phase (Day 5-7) | Sudden deterioration: Stridor, High fever, Dyspnoea, Cyanosis, Shock. Death within 24-36 hours of fulminant onset. |
Key CXR Finding: Widened Mediastinum (Haemorrhagic mediastinal lymphadenitis). May have pleural effusions (Haemorrhagic).
Gastrointestinal Anthrax
| Form | Features |
|---|---|
| Oropharyngeal | Oral/Oesophageal ulcers, Neck swelling, Lymphadenopathy, Dysphagia. |
| Intestinal | Nausea, Vomiting, Bloody diarrhoea, Acute abdomen, Ascites. |
Injectional Anthrax
Anthrax Meningitis
Diagnosis
| Test | Specimen | Notes |
|---|---|---|
| Gram Stain | Lesion fluid, Blood | Large Gram-positive rods, "Box-car" chains, Capsule (India ink stain). |
| Blood Culture | Blood | Positive in systemic disease. Non-haemolytic colonies. |
| PCR | Lesion, Blood, CSF | Rapid confirmation. |
| Serology | Blood | 4-fold rise in anti-PA antibodies. Retrospective. |
Imaging
| Imaging | Findings |
|---|---|
| CXR (Inhalation) | Widened Mediastinum, Hilar lymphadenopathy, Pleural effusions. NO parenchymal infiltrates initially. |
| CT Chest | Haemorrhagic mediastinal lymph nodes, Pleural effusions. |
Management Algorithm
SUSPECTED ANTHRAX
(Occupational/Travel exposure, Characteristic lesion/CXR)
↓
IMMEDIATE ACTIONS
- Notify Public Health immediately (Notifiable disease)
- Suspect bioterrorism if multiple unexplained cases
- Isolate patient (Standard precautions)
- Do NOT incise/drain cutaneous lesion
↓
ANTIBIOTIC THERAPY
┌──────────────────────────────────────────────────────────┐
│ CUTANEOUS ANTHRAX (Uncomplicated): │
│ - **Ciprofloxacin 500mg BD PO** for 60 days │
│ OR **Doxycycline 100mg BD PO** for 60 days │
│ (Long course due to spore germination time) │
│ │
│ SYSTEMIC/INHALATION/GI ANTHRAX: │
│ - **IV Ciprofloxacin 400mg BD** (or IV Doxycycline) │
│ - PLUS second agent (e.g., Clindamycin for toxin │
│ inhibition, Meropenem, Linezolid) │
│ - Duration: IV until stable → Complete 60 days PO │
│ │
│ ANTITOXIN (Severe Disease): │
│ - **Anthrax Immune Globulin (AIG)** or │
│ - **Raxibacumab** (Anti-PA monoclonal antibody) │
│ - Neutralises toxin. Reduces mortality. │
└──────────────────────────────────────────────────────────┘
↓
SUPPORTIVE CARE
- ICU admission for inhalation anthrax
- Ventilatory support
- Vasopressors for shock
- Drainage of pleural effusions
Post-Exposure Prophylaxis (PEP)
| Indication | Regimen |
|---|---|
| Known Spore Exposure (Bioterrorism) | Ciprofloxacin 500mg BD PO for 60 days + Anthrax Vaccine (3 doses). |
Public Health Response
- Notifiable Disease: Immediate notification to PHE/CDC.
- Decontamination: Sporicidal agents for surfaces (Bleach, Formaldehyde).
- Contact Tracing: Identify exposure source.
| Complication | Notes |
|---|---|
| Septic Shock | Common in systemic disease. High mortality. |
| Haemorrhagic Meningitis | Blood-stained CSF. Very poor prognosis. |
| Respiratory Failure | Inhalation anthrax. Rapid deterioration. |
| Multi-Organ Failure | Toxin-mediated. |
| Secondary Infection | Cutaneous lesion if disturbed. |
| Form | Mortality (Treated) | Notes |
|---|---|---|
| Cutaneous | less than 1% | Excellent if treated. May be fatal if untreated and progresses to sepsis. |
| Inhalation | 45-80% | Very high even with treatment. Better with early antitoxin. |
| GI | 25-60% | Depends on early recognition. |
| Meningitis | >90% | Almost universally fatal. |
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| Anthrax Guidance | CDC (2014) | Treatment regimens, PEP. |
| PHE Green Book | PHE | Vaccine recommendations for high-risk groups. |
| Bioterrorism Response | WHO | Public health response protocols. |
Prevention
- Anthrax Vaccine Adsorbed (AVA): Available for high-risk occupational groups (Military, Lab workers). 5-dose series.
- Animal Vaccination: Primary prevention in endemic areas.
- Safe Handling: Wool, Hides, Meat in endemic regions.
What is Anthrax?
Anthrax is a serious bacterial infection caused by Bacillus anthracis, which lives as spores in soil. It mainly affects farm animals like sheep and cattle. Humans can catch it from contact with infected animals or animal products.
How do people catch it?
- Skin contact: Most common. Spores enter a cut or graze.
- Breathing in spores: Very rare, but very serious. Historically in wool/hide factories.
- Eating contaminated meat: Rare.
Is it contagious?
No, anthrax does NOT spread person-to-person. You cannot catch it from someone who is infected.
What does it look like?
The skin form causes a painless black scab surrounded by swelling. If you have this after contact with animals or animal products, seek medical attention immediately.
Is it treatable?
Yes, with antibiotics (Ciprofloxacin). The skin form is almost always curable if treated. The lung form is much more serious and needs hospital treatment.
Primary Sources
- Hendricks KA, et al. Centers for Disease Control and Prevention Expert Panel Meetings on Prevention and Treatment of Anthrax in Adults. Emerg Infect Dis. 2014;20(2):e130687. PMID: 24447897.
- Sweeney DA, et al. Anthrax infection. Am J Respir Crit Care Med. 2011;184(12):1333-1341. PMID: 21852539.
- Price EP, et al. Bacillus anthracis. Clin Microbiol Rev. 2021;34(3):e00049-20. PMID: 33762410.
Common Exam Questions
- Classic Lesion: "Describe the appearance of cutaneous anthrax."
- Answer: Painless black eschar surrounded by massive non-pitting oedema.
- CXR Finding: "What is the characteristic CXR finding in inhalation anthrax?"
- Answer: Widened mediastinum (Haemorrhagic mediastinal lymphadenitis).
- First-Line Antibiotic: "What is the treatment?"
- Answer: Ciprofloxacin (IV for systemic, PO for cutaneous).
- Bioterrorism: "Why is anthrax a Category A agent?"
- Answer: Spores are stable, easily aerosolised, high mortality.
Viva Points
- "Box-car" Bacilli: Large Gram-positive rods with square ends in chains on Gram stain.
- 2001 Postal Attacks: Bioterrorism example – spore powder in letters. 22 cases, 5 deaths.
- Heroin Anthrax (UK 2009-2012): Injectional anthrax in IV drug users from contaminated Afghan heroin.
- 60-Day Course: Long antibiotic course required because spores can germinate weeks after initial exposure.
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