Inhalation anthrax is the most lethal form of anthrax, caused by inhalation of Bacillus anthracis spores. It is a CDC Category A bioterrorism agent with historically 45–90% case fatality rates even with treatment. [1-2] The disease follows a characteristic biphasic course: an initial nonspecific prodrome mimicking influenza, followed by abrupt fulminant deterioration with shock, respiratory failure, and death within hours. [3-4]
1. History
- Key HPI questions: Occupational exposure (wool, hides, animal products), travel to endemic areas, suspicious mail/packages, known bioterrorism event, military service
- Symptom characterization: Biphasic illness — initial "flu-like" prodrome (fever, malaise, myalgia, nonproductive cough) lasting hours to days, sometimes followed by a brief period of apparent improvement, then abrupt fulminant phase [3-4]
- Timing: Incubation period 1–7 days (median 4 days in 2001 attacks), but can extend up to 6–9 weeks post-exposure [1][6]
- Associated symptoms: Drenching sweats, dyspnea, chest pain, nausea/vomiting, headache, abdominal pain [2-3]
- Critical negative: Rhinorrhea is characteristically absent — a key distinguishing feature from influenza and URI [1]
2. Alarm Features
- Abrupt onset of high fever, severe dyspnea, diaphoresis, cyanosis, and shock (marks transition to fulminant phase) [3-4]
- Stridor — from extrinsic tracheal compression by massive mediastinal lymphadenopathy [4]
- Meningismus, altered mental status, obtundation — hemorrhagic meningitis occurs in up to 50% of cases and is nearly universally fatal [1][3]
- Widened mediastinum on CXR in a previously healthy patient with overwhelming febrile illness is essentially pathognomonic [3]
- All 4 patients in the 2001 attacks who presented with fulminant illness before antibiotics were started died [2]
3. Medications
- Treatment (systemic anthrax per CDC 2023 guidelines): [7]
- Two bactericidal agents from different classes PLUS a protein synthesis inhibitor (PSI) or RNA synthesis inhibitor
- Preferred bactericidal: ciprofloxacin (or levofloxacin/moxifloxacin) + meropenem (if meningitis not ruled out)
- Preferred PSI: linezolid or clindamycin (inhibit toxin production) [7]
- Doxycycline/minocycline are preferred PSIs from a PK/PD standpoint [7]
- Antitoxin therapy (adjunctive): Obiltoxaximab or raxibacumab (monoclonals preferred over polyclonal AIGIV) [7]
- Postexposure prophylaxis: Ciprofloxacin or doxycycline for 60 days + AVA vaccine (3-dose series at 0, 2, 4 weeks) [2][7]
- Contraindicated: Oral antibiotics alone are inadequate for systemic disease — IV combination therapy is mandatory [8]
- Caution: Rifampin should never be used as monotherapy (rapid resistance development) [7]
4. Diet
- No specific dietary triggers or recommendations
- Aggressive IV fluid resuscitation is critical during the fulminant phase as part of sepsis management [1]
- NPO status likely required given ICU-level care and potential need for intubation
5. Review of Systems
- Constitutional: Fever, chills, malaise, drenching sweats, fatigue
- Respiratory: Dyspnea, nonproductive cough, chest tightness, pleuritic pain (no rhinorrhea)
- GI: Nausea, vomiting, abdominal pain (can be prominent)
- Neurologic: Headache, confusion, neck stiffness, altered mental status (meningitis screen)
- Cardiovascular: Palpitations, chest pain, syncope
6. Collateral History and Family History
- Exposure context is paramount: Coworkers or household contacts with similar symptoms (cluster of cases suggests intentional release) [9]
- Occupational history: Animal hide/wool workers, veterinarians, laboratory workers
- Recent travel to endemic regions (Central/South Asia, sub-Saharan Africa)
- Receipt of suspicious mail or packages
- Family history is not relevant to susceptibility, but household contacts may need PEP if shared exposure occurred
- Notify public health authorities and law enforcement immediately if bioterrorism is suspected [9]
7. Risk Factors
- Bioterrorism exposure (aerosolized spore release) — primary concern [7]
- Occupational exposure to animal products (wool sorters, hide workers — "woolsorter's disease")
- Laboratory workers handling B. anthracis
- Military personnel in endemic regions
- Comorbidities associated with more severe disease: diabetes, obesity, hypertension, COPD [7]
- No persons younger than 24 years were among the 66 deaths in the Sverdlovsk outbreak, suggesting possible age-related susceptibility variation [5]
8. Differential Diagnosis
- Community-acquired pneumonia — most similar presentation; distinguished by mediastinal widening, absence of rhinorrhea, elevated hematocrit, and normal WBC [10-11]
- Influenza/ILI — rhinorrhea and myalgias more prominent; anthrax patients more likely to have tachycardia, high hematocrit, low albumin/sodium [10]
- COVID-19 — overlapping constitutional symptoms [9]
- Acute bacterial mediastinitis (esophageal perforation) — similar mediastinal widening
- Fibrous mediastinitis (Histoplasma capsulatum) — chronic, not acute [5]
- Ruptured/dissecting aortic aneurysm — widened mediastinum without infectious prodrome [5]
- Superior vena cava syndrome — mediastinal widening from malignancy
- Hemorrhagic meningitis from other causes (if CNS involvement present) [5]
- Pearl: A clinical algorithm using mediastinal widening + altered mental status + elevated hematocrit is 100% sensitive and 98.3% specific for distinguishing inhalation anthrax from CAP [12]
9. Past Medical History
- Prior anthrax vaccination status (military personnel)
- Immunocompromised state (affects PEP duration — requires full 60-day antimicrobial course even after treatment) [7]
- Chronic lung disease (COPD increases severity) [7]
- Prior episodes of anthrax (extremely rare; survivors develop natural immunity) [7]
- Allergy history to fluoroquinolones, tetracyclines, or beta-lactams (affects antibiotic selection)
10. Physical Exam
- Vital signs: Fever (present in all 2001 cases), tachycardia (majority at presentation), hypotension (fulminant phase), tachypnea, hypoxemia [2]
- HEENT: No rhinorrhea; possible subcutaneous edema of chest/neck in advanced disease [4]
- Lungs: May be surprisingly clear early; decreased breath sounds with effusions; stridor if mediastinal compression [4]
- Cardiovascular: Tachycardia, signs of shock; pericardial effusion possible [1]
- Neurologic: Meningismus, nuchal rigidity, altered mental status, obtundation (hemorrhagic meningitis in up to 50%) [1][3]
- Skin: Cyanosis in fulminant phase; check for concurrent cutaneous anthrax lesions (painless black eschar)
11. Lab Studies
- Blood cultures (MOST USEFUL test) — obtain BEFORE antibiotics; growth in 6–24 hours; sterilized after even 1–2 doses of antibiotics [2-3]
- Gram stain of peripheral blood — bacilli may be visible in advanced disease (massive bacteremia) [3]
- CBC: Hemoconcentration (elevated hematocrit), normal or mildly elevated WBC (notably NOT markedly leukocytic — distinguishes from typical bacterial pneumonia) [1][10]
- CMP: Elevated transaminases, low sodium, low albumin [2][10]
- Coagulation studies: Coagulopathy/DIC in ~33% of inhalation cases [7]
- ABG/VBG: Hypoxemia (present in majority at presentation) [2]
- Lumbar puncture: Required to rule out hemorrhagic meningitis (hemorrhagic CSF) [1]
- PCR testing: Definitive — performed at Laboratory Response Network (LRN) reference labs [1]
- Alert the microbiology lab — B. anthracis may be dismissed as B. cereus contaminant if not specifically suspected [2-3]
12. Imaging
- First-line: PA and lateral CXR [8]
- Widened mediastinum (70–100% of cases) — essentially pathognomonic in context [2][11]
- Pleural effusions (72–80% of inhalation cases, usually bilateral) [2][7]
- Infiltrates (70%) — not true pneumonia but atelectasis, effusions, early ARDS [2]
- CT chest without contrast (if CXR equivocal or for further characterization): [13]
- Hyperdense (hemorrhagic) mediastinal lymphadenopathy — characteristic finding on unenhanced CT [1]
- Bilateral pleural effusions, mediastinal widening, edema of mediastinal fat [2]
- Echocardiogram: Evaluate for pericardial effusion [1]
- Pearl: All 10 initial patients in the 2001 attacks had abnormal CXR; a normal CXR makes inhalation anthrax less likely [2][8]
13. Special Tests
- Clinical screening algorithm (Kyriacou et al.): Mediastinal widening or pleural effusion on CXR + altered mental status + elevated hematocrit → 100% sensitive, 98% specific for inhalation anthrax vs. CAP [12]
- Scoring system (Kuehnert et al.): Tachycardia + high hematocrit + low albumin/sodium + absence of rhinorrhea/myalgias → score ≥4 captured all 11 anthrax cases and excluded 96% of ILI [10]
- Pleural fluid analysis: Gram stain/culture (83% positive pre-antibiotics); spectrographic analysis can detect anthrax lethal factor in pleural fluid [7][14]
- Immunohistochemical staining, gamma phage assays — performed at LRN reference labs [15]
- Drainage of pleural effusions: Both diagnostic AND therapeutic — drainage associated with OR 38.3 for survival [14]
14. ECG
- Arrhythmias observed in ~7% of adults with inhalation anthrax [7]
- ECG indicated to evaluate for pericardial effusion (low voltage, electrical alternans) and arrhythmias
- Tachycardia is the most common finding
- Monitor for signs of hemodynamic compromise from toxin-mediated cardiovascular collapse
15. Assessment
Inhalation anthrax is a medical emergency and potential public health crisis. Key clinical pearls:
- Biphasic illness: The initial prodrome is nearly indistinguishable from influenza — a high index of suspicion is required, especially in the context of known or suspected exposure [1][9]
- Not a true pneumonia: The pathology is hemorrhagic mediastinitis and toxin-laden effusions, not bronchopneumonia [4]
- Mortality: Historically 89–90%; reduced to ~45% in the 2001 attacks with modern critical care, early antibiotics, and pleural drainage. Adults who died had a median hospital stay of only 1 day [1-2][7]
- Complications: Sepsis (~70%), secondary meningitis (~33%), coagulopathy (~33%), respiratory failure requiring ventilation (~23%) [7]
- Meningitis confers near-100% mortality — all patients with meningitis in the 2001 attacks died [1]
16. Treatment Plan
Initial stabilization
Antimicrobial therapy (CDC 2023): [1][7]
- If meningitis not ruled out: Fluoroquinolone (ciprofloxacin 400 mg IV q8h) + CNS-penetrating beta-lactam (meropenem 2g IV q8h) + PSI (linezolid 600 mg IV q12h)
- If meningitis ruled out: Fluoroquinolone + PSI (linezolid or clindamycin)
- Duration: ≥2 weeks IV, then transition to oral; total 60 days if aerosol exposure suspected [7]
Antitoxin: [7]
- Obiltoxaximab or raxibacumab as adjunctive therapy for all noncutaneous systemic anthrax
- Monoclonal antitoxins preferred over polyclonal (AIGIV) [7]
Procedural interventions
- Drainage of pleural effusions — strongly associated with survival (OR 38.3); bilateral chest tubes often needed [14]
- Pericardiocentesis if pericardial effusion with tamponade physiology
- Glucocorticoids may be considered for anthrax meningitis per bacterial meningitis protocols [1]
Postexposure prophylaxis for exposed contacts: [2][7]
- Ciprofloxacin 500 mg PO BID or doxycycline 100 mg PO BID × 60 days
- AVA vaccine (0, 2, 4 weeks) as adjunct
17. Disposition
- All suspected inhalation anthrax → ICU admission [1]
- No role for outpatient management of symptomatic disease
- Observation: Asymptomatic exposed individuals on PEP may be managed outpatient with close follow-up [2]
- Specialist consultation: Infectious disease (mandatory), pulmonology/critical care, neurosurgery (if meningitis), cardiothoracic surgery (effusion drainage)
- Immediate notification: Hospital infection control, state/local public health department, CDC Emergency Operations Center (770-488-7100), FBI if bioterrorism suspected [9]
- Inhalation anthrax is not transmitted person-to-person — standard precautions are sufficient [3]
18. Follow Up / Return Precautions
- Survivors: Median hospital stay 14–16 days for systemic anthrax without meningitis [7]
- PEP patients: Must complete full 60-day antimicrobial course + 3-dose AVA series; compliance was <50% in the 2001 attacks — close follow-up and counseling are essential [1]
- Return precautions for PEP patients: Report immediately any flu-like symptoms or febrile illness during or after the prophylaxis period — evaluate for possible inhalation anthrax [2]
- Monitoring: GI tolerance of prolonged antibiotics; ~2% of postal workers in 2001 sought care for suspected allergic reactions to PEP medications [2]
- Long-term: Immunocompromised patients require extended PEP even after treatment of active disease [7]
- Expected recovery: Survivors of systemic anthrax without meningitis who receive early combination therapy have improving outcomes with modern critical care (~55% survival in the 2001 attacks) [1][16]
References
1. Clinical Management of Potential Bioterrorism-Related Conditions. — Adalja AA, Toner E, Inglesby TV. The New England Journal of Medicine. 2015.
2. Anthrax as a Biological Weapon, 2002: Updated Recommendations for Management. — Inglesby TV, O'Toole T, Henderson DA, et al. The Journal of the American Medical Association. 2002.
3. Anthrax as a Biological Weapon: Medical and Public Health Management. — Inglesby TV, Henderson DA, Bartlett JG, et al. The Journal of the American Medical Association. 1999.
4. Recognition and Management of Anthrax — An Update. — Swartz MN. The New England Journal of Medicine. 2001.
5. Anthrax. — Dixon TC, Meselson M, Guillemin J, Hanna PC. The New England Journal of Medicine. 1999.
6. Clinical Features of Patients Hospitalized for All Routes of Anthrax, 1880-2018: A Systematic Review. — Hendricks K, Person MK, Bradley JS, et al. Clinical Infectious Diseases : An Official Publication of the Infectious Diseases Society of America. 2022.
7. CDC Guidelines for the Prevention and Treatment of Anthrax, 2023. — William A. Bower MD, Yon Yu PharmD, Marissa K. Person MSPH, et al United States Centers for Disease Control and Prevention. 2025.
8. Differentiating Inhalational Anthrax From Other Influenzalike Illnesses in the Setting of a National or Regional Anthrax Outbreak. — Cinti SK, Saravolatz L, Nafziger D, Sunstrum J, Blackburn G. Archives of Internal Medicine. 2004.
9. Bioterrorism. — Rathjen NA, Shahbodaghi SD. American Family Physician. 2021.
10. Clinical Features That Discriminate Inhalational Anthrax From Other Acute Respiratory Illnesses. — Kuehnert MJ, Doyle TJ, Hill HA, et al. Clinical Infectious Diseases : An Official Publication of the Infectious Diseases Society of America. 2003.
11. Clinical Predictors of Bioterrorism-Related Inhalational Anthrax. — Kyriacou DN, Stein AC, Yarnold PR, et al. Lancet. 2004.
12. Discriminating Inhalational Anthrax From Community-Acquired Pneumonia Using Chest Radiograph Findings and a Clinical Algorithm. — Kyriacou DN, Yarnold PR, Stein AC, et al. Chest. 2007.
13. Conference Report on Public Health and Clinical Guidelines for Anthrax. — Stern EJ, Uhde KB, Shadomy SV, Messonnier N. Emerging Infectious Diseases. 2008.
14. Clinical Characteristics and Survival of Hospitalized Anthrax Patients With Pleural, Pericardial, or Peritoneal Fluid Collections. — Holty JC, Person MK, Binney S, et al. Clinical Infectious Diseases : An Official Publication of the Infectious Diseases Society of America. 2026.
15. ACCF/AHA/CDC Conference Report on Emerging Infectious Diseases and Biological Terrorism Threats. Task Force III: Prevention and Control of Cardiovascular Complications of Emerging Infectious Diseases and Potential Biological Terrorism Agents and Diseases. — Cooper LT, Mensah GA, Baddour LM, et al. Journal of the American College of Cardiology. 2007.
16. Systematic Review of Hospital Treatment Outcomes for Naturally Acquired and Bioterrorism-Related Anthrax, 1880-2018. — Person MK, Cook R, Bradley JS, et al. Clinical Infectious Diseases : An Official Publication of the Infectious Diseases Society of America. 2022.