Anthrax differential diagnosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Differentiating Anthrax from Other Diseases

Cutaneous Anthrax

• A history of exposure to contaminated animal materials, occupational exposure, and living in an endemic area is crucial when considering a diagnosis of anthrax. A painless, pruritic papule, surrounding vesicles and edema, usually on an exposed region of the body should raise a concern of cutaneous anthrax, which is confirmed by the demonstration of Gram-positive encapsulated bacilli from the lesion and/or positive culture for Bacillus anthracis from the lesion and/or positive specialized tests.

• The differential diagnosis of the anthrax eschar includes a wide range of infectious and non-infectious conditions including:

• Boil (early lesion)
• Arachnid bites
• Ulcer (especially tropical)
• Erysipelas
• Glanders
• Plague
• Syphilitic chancre
• Ulceroglandular tularemia
• Clostridial infection
• Rickettsial diseases
• Rhizomucor infections
• Orf
• Vaccinia
• Cowpox
• Rat-bite fever
• Leishmaniasis
• Ecthyma gangrenosum
• Herpes

• Generally these other diseases and conditions lack the characteristic edema of anthrax. The absence of pus, the lack of pain, and the patient’s occupation may provide further diagnostic clues. The outbreak of Rift Valley fever, initially thought to be anthrax in livestock, also affected numerous humans.

• In the differential diagnosis of the severe forms, orbital cellulitis, dacryocystitis and deep tissue infection of the neck should be considered in the case of severe anthrax lesions involving the face, neck and anterior chest wall. Necrotizing soft tissue infections, particularly group A streptococcal infections and gas gangrene, and severe cellulitis due to staphylococci, should also be considered in the differential diagnosis of severe forms of cutaneous anthrax. Gas and abscess formation are not observed in patients with cutaneous anthrax. Abscess formation is only seen when the lesion is infected with other bacteria such as streptococci or staphylococci.

Ingestional Anthrax (Oropharyngeal and Gastrointestinal Anthrax)

• In the differential diagnosis of oropharyngeal anthrax, diphtheria and complicated tonsillitis, streptococcal pharyngitis, Vincent's angina, Ludwig's angina, parapharyngeal abscess, and deep-tissue infection of the neck should be considered.

• The differential diagnosis in gastrointestinal anthrax includes food poisoning (in the early stages of intestinal anthrax), acute abdomen due to other reasons, and hemorrhagic gastroenteritis caused by other microorganisms, particularly necrotizing enteritis caused by Clostridium perfringens and dysentery (amebic or bacterial).

Inhalational Anthrax (Pulmonary, Mediastinal, and Respiratory Anthrax)

• Alternative diagnoses to be considered are Mycoplasma pneumoniae, Legionnaires' disease, psittacosis, tularemia, Q fever, viral pneumonia, histoplasmosis, coccidiomycosis, and malignancy.

Anthrax Meningitis

• Differential diagnosis should take into account acute meningitis of other bacterial etiologies and other cerebral afflictions, such as cerebral malaria or subarachnoid hemorrhage. The definitive diagnosis is obtained by visualization of the capsulated bacilli in the cerebrospinal fluid and/or by culture.

Anthrax Sepsis

• In the differential diagnosis, sepsis due to other bacteria should be considered. Definitive diagnosis is made by the isolation of Bacillus anthracis from the primary lesion and from blood cultures or by detection of the toxin or DNA (deoxyribonucleic acid) of B. anthracis in these specimens.