In the not-very politically correct Middle Ages, dysentery was known as ‘the bloody flux’. Both dysentery and flux are general terms that describe severe diarrhea, often with blood present in the stool. The condition has various causative agents; Shigella is one of them.

The microorganism Shigella was first identified in 1897 during the course of an epidemic of dysentery. It was named after its discoverer, Japanese scientist Kiyoshi Shiga. Diarrheal illness known to be caused by Shigella is most specifically referred to as Shigella infection or shigellosis.

Classification of Shigella

Shigella belongs firstly to the group of organisms known as prokaryotes, that is, organisms lacking a true nucleus within the cell. 

Prokaryotes come in four basic shapes, and are classified accordingly as bacilli (rod shaped), cocci (round), spirochetes (spiral), or vibrio (comma-shaped). Shigella is a bacillus.

Some prokaryotes fall under the category of ‘bacteria’, and Shigella is one of them. Bacteria are further classified in two broad groups as Gram positive or Gram negative, in accordance with how they absorb crystal violet dye. In general, the Gram positive group is associated with infectious disease, but some Gram negative bacteria are also culprits. Shigella is one of them.

Shigella is a type of enterobacteria, meaning it resides in the gut. Shigella is found only in the digestive tracts of primates. Not surprisingly, it is a relative of Escherichia coli.

There are four serogroups of Shigella: 

A (S. dysenteriae)

B  (S. flexneri)

C  (S. boydii)

D  (S. sonnei)

These serogroups each contain multiple serotypes of the bacteria. The serogroup most identified with Shigellosis in the US is Group D; Group A is associated with epidemics in developing countries.

Shigella is potent stuff! The ‘infectivity dose’ of Shigella is very low, meaning it takes relatively few of the organisms to cause disease in a host. The incubation period, or time between introduction of infection and manifestation of symptoms, is twelve hours to seven days, depending on the number of organisms present. Two to four day incubation is most common.

Shigella bacteria enter the digestive tract in the large intestine through a process described as biological injection. Once they get to work, they destroy the epithelium of the intestinal mucosa, or the lining of the interior surface of the lower portions of the gut. Visit Medscape Reference for a very full, not to mention tasteful, explanation of the genetics, chemistry, and mechanics of this process.

The illness

Shigellosis is diagnosed based on clinical presentation and diagnostic testing. Some blood test results may be suggestive of bacterial infection; stool tests can give more specific diagnostic information but may not be absolutely conclusive. 

If you have ever experienced shigellosis or anything like it, the following paragraph may induce a flashback.

The illness presents with severe abdominal cramps and watery, high-volume diarrhea. Stools may also contain mucus or blood.  Diarrhea can be severe enough to cause incontinence in some cases. This may be followed by tenesmus, a powerful urge to defecate even though the colon is already empty.

Vomiting and fever may accompany the diarrhea, as may abdominal tenderness and headache. Seizures can occur in small children with shigellosis.

Symptoms of possible dehydration subsequent to fever and loss of body fluids include dry mouth, low blood pressure, and general malaise.

If infection is severe or if the host suffers poor health or compromised immunity, Shigellosis can lead to septic shock or coma. In individuals with a specific predisposition, Arthritis, Urethritis, and Conjunctivitis Syndrome can occur subsequent to an attack of shigellosis.

How you get it

Epidmiologists call the most common mode of transmission of this illness the fecal-oral route; your mother called it ‘not washing your hands after you use the bathroom’. Food handled by contaminated hands can carry the disease, as can vegetables that have been harvested from a field contaminated with sewage. Swimming in contaminated water may also expose one to the Shigella pathogen. The common housefly is a potential vector, or carrier, as well.


You remember Dr. Shiga, the scientist who identified the Shigella organism? In the early 20th century he collaborated with another very famous scientist, Dr. Paul Ehrlich. Ehrlich originated the concept of ‘the magic bullet’, a specific and surefire cure for a particular something that ails you. Sadly, the average shigellosis sufferer does not get a bullet, magic or otherwise. He or she just wishes for one.

Mild to moderate shigellosis in otherwise healthy individuals is usually not treated with antibiotics; the illness is considered self-limiting, generally running its course (no pun intended) in forty-eight to seventy-two hours. Ampicillin, trimethoprim-sulfamethoxazole (Bactrim), or Ciprofloxacin (Cipro) may be prescribed for compromised individuals, or if the illness is severe.  Antidiarrheal medications such as Lomotil are not recommended in most cases as they can prolong the duration of symptoms.

Dehydrated individuals may require hydration therapy by IV.

Who gets it?

Approximately 18,000 cases of Shigellosis are reported in the United States annually, but the actual number of cases, due to underreporting, is probably much higher. It is more prevalent during the summer months.

Children under five years of age in group daycare, people residing in institutions such as nursing homes, and those who care for them are at increased risk of developing shigellosis due to toileting/hygiene issues in the population, and a congregate setting. 

People living in crowded, unsanitary conditions, such as shelters, and people traveling in countries where sanitation is not optimal are also at increased risk.

Individuals with HIV and homosexual men are at higher than average risk.

How do you prevent it?

On an individual level, scrupulous hand hygiene around bathroom use is a key measure, as is diaper hygiene when caring for very young children. 

Proper handling, cooking, and refrigeration of food are also important preventive measures. Raw fruits and vegetables should be washed thoroughly and, if appropriate, peeled.

Children and staff with shigellosis should not be present in day care situations until symptoms have resolved and stool cultures are negative.

Recuperating individuals should not swim in public pools for two weeks after symptoms resolve.

On a larger scale, public education about hand washing is vital, as is reporting of occurrence of the disease to local health authorities, as required. Improvements in worker hygiene and sanitary conditions for those who harvest produce is another way to reduce the likelihood of shigellosis occurring. On a worldwide scale, proper sanitation and a safe water supply need to be insured for all.


Encyclopedia Britannica, “Shigella”, at

Medscape Reference, “Shigella Infection”, at