Shigellosis, or bacillary dysentery, is an infectious disease caused by the Gram-negative bacteria Shigella, a member of the Enterobacteriaceae family.

Shigella is transmitted through fecal-to-oral contact or by ingestion of contaminated food or water. Contamination occurs when an infected person prepares food without washing their hands or when food or water is exposed to other contaminated foods, sewage, or infected flies. A very small inoculum of 50-100 bacilli can cause infection.

One to three days after exposure, the patient will present with fever, crampy abdominal pain, and voluminous watery diarrhea. Dehydration may occur. This may proceed to dysentery with bloody diarrhea, tenesmus, rectal ulcerations, and constitutional symptoms. Without treatment, the illness can last up to 30 days, although the average is 7 days.

In the United States, an estimated 500 000 cases occur annually. Two-thirds of the cases of shigellosis in the United States are caused by Shigella sonnei and the remainder by Shigella flexneri. Infection with S flexneri may result in reactive arthritis (Reiter syndrome), a chronic autoimmune condition producing arthralgia, eye problems, and dysuria.

International travelers are at increased risk for infection with multidrug-resistant Enterobacteriaceae, and the US Centers for Disease Control and Prevention (CDC) reports that, due to repeated importation by travelers, ciprofloxacin-resistant S sonnei is now circulating domestically in the United States. There have been reports of strains with reduced susceptibility to azithromycin in New York City. In 2022, 5% of Shigella infections reported to the CDC were classified as extensively drug resistant (XDR) to all commonly recommended empiric and alternative antibiotics (azithromycin, ciprofloxacin, ceftriaxone, trimethoprim-sulfamethoxazole [TMP-SMX], and ampicillin); an increase from 0% in 2015.

There is no vaccine against shigellosis. The CDC has designated it a possible bioterrorism agent (food safety threat).

People at higher risk for contracting shigellosis are residents of dormitories, nursing homes, reservations, and military camps, as well as children in day care centers. Other specific populations at risk include Orthodox Jews, international travelers, HIV-infected patients, and men who have sex with men.