Anthrax Differential Diagnoses

Updated: Apr 05, 2021
  • Author: David J Cennimo, MD, FAAP, FACP, FIDSA, AAHIVS; Chief Editor: Michael Stuart Bronze, MD  more...
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Diagnostic Considerations

Early diagnosis is difficult, and a high index of suspicion is required. The differential diagnosis varies among cutaneous, inhalational, and intestinal anthrax. A triage checklist has been developed in case of a mass exposure event. [7]

Cutaneous anthrax

Physicians must differentiate cutaneous anthrax from bubonic plague or lymphocutaneous tularemia. Patients with plague have painful adenopathy, usually in the groin or axilla. No ulcer is present, and ulcer edema and eschar characteristic of anthrax are absent. Patients with bubonic plague appear more toxemic than patients with uncomplicated cutaneous anthrax.

Patients with anthrax have an appropriate history of contact with animal products. In contrast, patients with bubonic plague may have a history of contact with infected cats that may have been in contact with sylvatic rodents in plague-endemic areas. Patients with bubonic plague may also provide a history of contact with armadillos or infected prairie dogs in the western United States.

Inhalational anthrax

Do not confuse inhalational anthrax with the zoonotic atypical pneumonias. Pulmonary tularemia usually presents as a community-acquired pneumonia with bilateral hilar adenopathy and bloody pleural effusion. The primary clinical manifestation of inhalational anthrax is hemorrhagic mediastinitis with bloody pleural effusions. No pulmonary infiltrate is present, and a widened mediastinum is observed on early chest CT scans. Mediastinitis very closely resembles inhalational anthrax on chest radiographs, but their clinical presentations are different.

The initial phase of inhalational anthrax may resemble bacterial mediastinitis, but it is associated with hemoptysis, severe substernal chest pain, and shock, which is very different from bacterial mediastinitis. Patients with bacterial mediastinitis have a history of previous esophageal tear or recent thoracic surgery. Patients with inhalational anthrax have a history of exposure to sources of anthrax spores.

Intestinal anthrax

Intestinal anthrax is a difficult diagnosis that must be distinguished from dysentery. Dysentery may manifest as bloody diarrhea, as does intestinal anthrax, and may be accompanied by abdominal pain (eg, in cases of Shigella or amebic dysentery). A history of ingesting meat possibly contaminated with anthrax is helpful in suspected cases of intestinal anthrax.

In tropical areas where bacillary and amebic dysentery are common, clinically differentiating intestinal anthrax from these endemic causes of dysentery is very difficult unless a cluster of dysentery cases or an outbreak is known to exist. Stool examination provides rapid confirmation of bacillary or amebic dysentery. Stools negative for amebic cysts or trophs and for Shigella suggest the possibility of intestinal anthrax in patients residing near areas where anthrax is endemic (ie, in pastures where herbivores graze) or after ingestion of spores from hand/food contact.

Ulceroglandular tularemia is characterized by purple ulcerative lesions that are painful, not pruritic, and not surrounded by a gelatinous edematous halo. Patients with anthrax, tularemia, or plague may report headache and have fever associated with shaking chills.

The chancre of primary syphilis may also be confused with cutaneous anthrax. The chancre of primary syphilis is painless, as is the lesion of cutaneous anthrax, but the syphilitic chancre is not pruritic and is not surrounded by an edematous halo. Generalized rather than local adenopathy accompanies syphilis, which is the opposite of what is expected with cutaneous anthrax.

Exudates from the ulcers of both ulceroglandular tularemia and cutaneous anthrax reveal organisms when properly stained. The ulcer of syphilis does not reveal organisms, but Treponema pallidum may be visualized using dark-field examination.

Other problems to be considered include the following:

  • Ecthyma (Pseudomonas aeruginosa and staphylococcal infections)

  • Glanders (Pseudomonas pseudomallei)

  • Histoplasmosis

  • Leprosy

  • Orf (Rickettsia akari)

  • Psittacosis

  • Rat-bite fever (Streptococcus moniliformis, Spirillum minus)

  • Rickettsia

  • Tularemia

  • Typhoid

Differential Diagnoses