Gastrointestinal (GI) anthrax is one of 4 major syndromes caused by Bacillus anthracis, an encapsulated, gram-positive, spore-forming bacillus. The other "forms" are inhalational, cutaneous, and injection anthrax.

Bacillus anthracis has been classified by the Centers for Disease Control and Prevention (CDC) as a Category A bioterrorism agent. If anthrax were to be intentionally dispersed, the most likely syndromes will be inhalational and cutaneous. Cutaneous anthrax occurs in endemic areas (eg, Turkey) due to exposure to infected animals, and it is the most common natural form of anthrax disease (95%).

Both intestinal and oropharyngeal variants of GI anthrax can occur after ingestion of contaminated meat, usually raw or undercooked. The mortality rate for untreated intestinal anthrax approaches 100%; for oropharyngeal anthrax, the rate varies between 25% and 60% without treatment. Whether acquired naturally or as an agent of bioterrorism, the incubation period can be from 3-7 days. GI symptoms include fever, abdominal pain, anorexia, nausea, vomiting, and accumulation of serous fluid in the abdominal cavity. The patient may vomit blood, and severe prostration accompanies bloody diarrhea. It usually progresses to an acute abdomen and septic shock, often with frank intraabdominal pus that demonstrates gram-positive bacilli on Gram stain. Many case reports describe large to massive ascites. Oropharyngeal symptoms include sore throat, ulcerated lesions on the pharynx, difficulty swallowing, noticeably swollen neck, and regional lymphadenopathy. If untreated, the disease may progress to hemorrhagic meningitis, septicemia, and death.

There are no documented cases of person-to-person transmission of intestinal or oropharyngeal anthrax.

In 2009, a woman in the Northeastern United States presented with GI anthrax 10 days after participating in a drum circle (it is presumed that animal skin for one of the drums came from an infected animal and 2 potential drums were implicated). She developed flu-like symptoms 1 day after exposure, had worsening symptoms over the subsequent week, and sought medical attention nearly 10 days after exposure. She had profound leukocytosis (WBC >40 K), hemoconcentration (hematocrit >60), and mild hyponatremia. An abdominal / pelvis CT revealed massive ascites with focal nonspecific small bowel pathology and abnormal retroperitoneal lymphadenopathy. She underwent an exploratory laparotomy with bowel resection, but the diagnosis was not made until her initial blood cultures yielded gram-positive rods that were identified as B. anthracis.