Anthrax natural history, complications and prognosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]


The natural history of anthrax depends on the mode of exposure to the disease (cutaneous, ingestion, inhalation, or injection). In cutaneous anthrax, a small painless skin sore develops into a blister and later into a skin ulcer, with a central black area. The resolution of the lesion takes several weeks, depending on its size, location and severity. The anthrax lesions might lead to scarring and contractures. Inhalation anthrax is characterized by a mild initial phase of nonspecific symptoms, that is followed by sudden development of dyspnea, cyanosis, disorientation with coma, and death.[1] In oropharyngeal anthrax, the lesion is generally localized in the oral cavity. This type may progress rapidly, and edema around the lymph nodes may result in extensive swelling of the neck and anterior chest wall.[1] The gastrointestinal anthrax lesions may occur anywhere within the gastrointestinal tract, potentially bleeding, and lead to fatal hemorrhage. Some cases are complicated by massive ascites,shock and death.[1] The prognosis of anthrax depends on the form of the disease, how early it is diagnosed, the strain of bacteria, the patient's age and his health condition. Pulmonary anthrax has the highest mortality rate.[2]

Natural History

Cutaneous Anthrax

The incubation period of anthrax ranges from as little as 9 hours to 3 weeks, mostly 2 to 6 or 7 days. The natural history of cutaneous anthrax is shown below.

Day 0

  • There is entry of the infecting B. anthracis (usually as spores) through a skin lesion (cut, abrasion, etc.) or (possibly as vegetative forms or vegetative forms and spores) by means of a fly-bite.

Days 2-3

Days 3-4

  • A ring of vesicles develops around the papule. Vesicular fluid may be exuded. Unless the patient was treated, capsulated B. anthracis can be identified in appropriately stained smears of this fluid, and the bacterium can be isolated by culture.
  • Marked edema starts to develop.
  • Unless there is secondary infection, there is no pus and pathognomonically the lesion itself is not painful, although painful lymphadenitis may occur in the regional lymph nodes and a feeling of pressure may result from the edema.
  • The lesion is usually 1-3 cm in diameter and remains round and regular. Occasionally a lesion may be larger and irregularly shaped.[1]

Days 5-7

Day 10

  • The eschar begins to resolve; resolution takes several weeks and is not hastened by treatment.
  • Clinicians unaware of this suffer from concern that the treatment has been ineffective.
  • A small proportion of untreated cases (20%) develop sepsis or meningitis with hyperacute symptoms.[3]

Time to Resolution

  • Time to resolution will depend on the size, location and local severity of the lesion.
  • The initial crust is separated several weeks after the onset, with subsequent healing by granulation. Sometimes the separation of the crust is delayed, and the lesion may become secondarily infected. In this situation, the crust should be excised surgically.
  • Lesions characterized by “malignant edema” can take months to heal.
  • Very large lesions may require skin grafts, and lesions in locations such as the eyelid may require surgical intervention due to scarring.[1]

Shown below are images of the development and resolution of uncomplicated cutaneous anthrax lesion.

Inhalation Anthrax

Initial Phase

  • Symptoms prior to the onset of the final hyperacute phase are nonspecific, and suspicion of anthrax depends on the knowledge of the patient’s history. The mild initial phase of nonspecific symptoms is followed by the sudden development of dyspnea, cyanosis, disorientation with coma, and death.

Acute Phase

Ingestion Anthrax

Oropharyngeal anthrax

  • The oral lesion is generally 2-3 cm in diameter and covered with a grey pseudomembrane surrounded by extensive edema.

Gastrointestinal Anthrax

  • The symptoms of gastrointestinal anthrax may be divided in 2 clinical forms:[3]
  • Abdominal
  • Oropharyngeal
  • The time between onset of symptoms to death has frequently varied from 2 to 5 days
  • There is evidence that not all untreated cases end in toxemia, sepsis and death and that, after the initial symptoms, recovery may occur.


Cutaneous and Injection Anthrax

Gastrointestinal Anthrax

Inhalation Anthrax


  • Any form of anthrax is treatable, if the diagnosis is made early enough and with the appropriate supportive therapy.
  • Following recovery, resolution of small- to medium-size cutaneous lesions is generally complete with minimal scarring. With larger lesions, or lesions on mobile areas, scarring and contractures may require surgical correction to return normal functioning and large cutaneous defects may require skin grafting.


  • In the non-cutaneneous forms, a correct early diagnosis is harder to reach, so these are associated with particularly high mortality. The pulmonary anthrax is the one with highest mortality rate.[2]
  • The mortality rate for each form of anthrax is:
    • Pulmonary anthrax: 45%[2]
    • Gastrointestinal anthrax: 40%[4]
    • Injection anthrax: 28%[5]
    • Cutaneous anthrax: < 2%[6]


  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 "Anthrax in Humans and Animals" (PDF).
  2. 2.0 2.1 2.2 Barakat LA, Quentzel HL, Jernigan JA, Kirschke DL, Griffith K, Spear SM; et al. (2002). "Fatal inhalational anthrax in a 94-year-old Connecticut woman". JAMA. 287 (7): 863–8. PMID 11851578.
  3. 3.0 3.1 3.2 Spencer RC (2003). "Bacillus anthracis". J Clin Pathol. 56 (3): 182–7. PMC 1769905. PMID 12610093.
  4. Beatty ME, Ashford DA, Griffin PM, Tauxe RV, Sobel J (2003). "Gastrointestinal anthrax: review of the literature". Arch Intern Med. 163 (20): 2527–31. doi:10.1001/archinte.163.20.2527. PMID 14609791.
  5. "An Outbreak of Anthrax Among Drug Users in Scotland, December 2009 to December 2010" (PDF).
  6. "Centers for Disease Control and Prevention Expert Panel Meetings on Prevention and Treatment of Anthrax in Adults".