For health professionals: Shigellosis (Shigella)

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What health professionals need to know

Shigellosis is an acute infectious diarrheal disease caused by a group of bacteria called Shigella. Shigella bacteria are extremely acid-tolerant. After ingestion, the bacteria progress from the stomach to the small intestine, where they multiply. Large numbers of bacteria then advance to the colon and enter the colonic epithelium.

Humans are the only natural reservoir for Shigella, however, prolonged outbreaks have occurred in primate colonies.

Shigellosis is a common cause of diarrhea in Canada. In recent years, approximately 880 cases of shigellosis have been reported annually in Canada.

Causative Agent

There are 4 species of Shigella: S. sonnei, S. flexneri, S. dysenteriae, and S. boydii.

S. dysenteriae is considered the most virulent. It can produce a potent cytotoxin known as Shigatoxin.


Shigella bacteria spread through the direct or indirect fecal-oral route. The illness is highly infectious and can also be spread from person to person. Transmission typically occurs by:

Flies that come into contact with contaminated feces or water have also been linked to the spread of infection.

Clinical manifestations

The incubation period for shigellosis is 1 to 7 days.

Shigellosis is an acute infection with onset of symptoms. In particular, it causes watery diarrhea usually occurring within 24 to 48 hours of ingestion of the etiologic agent.

Infection may be mild or asymptomatic. Illness can range from mild watery diarrhea to severe inflammatory bacillary dysentery or shigellosis. Symptoms include:

Occasionally, it can lead to complications, such as:

Infections are usually self-limiting. They can become life-threatening in immunocompromised patients. The illness usually lasts for 4 to 7 days. People are infectious while they are sick, and infectivity could last for up to 4 weeks after illness. Some people may not experience symptoms after they have been infected with Shigella. However, their feces may still be contagious for up to a few weeks.


Shigellosis can be clinically diagnosed in most patients based on fresh blood in the stool. Patients presenting with watery diarrhea and fever should be suspected of having shigellosis.

Bloody, mucoid stools are highly indicative of shigellosis, however, the differential diagnosis should include:

Blood is common in the stools of patients with amebiasis, but it is usually dark brown rather than bright red, as in Shigella infections.

Laboratories can confirm diagnosis by the isolation of Shigella sp. from an appropriate clinical specimen (e.g. sterile site, deep tissue wounds, stool, vomit or urine).


Most patients recover without complications within 5 to 7 days without specific treatment.

With proper oral rehydration or electrolyte replacement, shigellosis is generally a self-limiting disease. Fluid replacement is essential in dehydrated patients and can lead to recovery within days. This is particularly important for seniors, children and those with weakened immune systems. In severe cases, patients may need to be given fluids intravenously.

Antibiotics are prescribed based on the severity of disease, the age of the patient and the likelihood of further transmission of the infection.


Shigellosis is a nationally notifiable disease. Health professionals are to report cases to their provincial or territorial public health authorities.

Health professionals in Canada play a critical role in identifying and reporting cases of shigellosis. See the surveillance section for more information on surveillance in Canada.

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