Shigellosis, also known as bacillary dysentery, is the infectious disease caused by the Shigella bacteria. Most who are infected with Shigella develop diarrhea, fever, and stomach cramps starting a day or two after exposure. The diarrhea is often bloody. Shigellosis usually resolves in 5 to 7 days. In some persons, especially young children and the elderly, the diarrhea can be so severe that the patient needs to be hospitalized. A severe infection with high fever may also be associated with seizures in children less than 2 years old. Some persons who are infected may have no symptoms at all, but may be carriers, and still pass the Shigella bacteria to others.
There are several different kinds of Shigella bacteria: Shigella sonnei, also known as “Group D”, accounts for much of the infection in the United States and is the strain most often linked to infection from food (shigellosis accounts for less than 10% of the reported outbreaks of foodborne illness in the United States.) A second type, Shigella flexneri, or “group B”, accounts for almost all of the rest of the infections in the U.S., and recent studies have determined a high percentage of infection by this strain is transmitted sexually. Other types of Shigella, such as Shigella dysenteriae type 1, are most often found in water contaminated with human feces. Such strains are rare in the U.S., although they continue to be important causes of death and disease in the developing world.
Shigellosis is more common in summer than winter. Children, especially toddlers aged 2 to 4, are the most likely to get shigellosis. Many cases are related to the spread of illness in child-care settings, and many more are the result of the spread of the illness in families with small children.
Increasingly, multidrug-resistant shigellosis cases are appearing in the US, often linked to travelers from outside the US.[1] These include Shigella sonnei that is resistant to ciprofloxacin, a main line antibiotic used to treat infections in adults. Shigella is already resistant to ampicillin. Since 2015, the CDC has tracked a sharp rise in extensively drug-resistant (XDR) Shigella—strains resistant to all five commonly recommended antibiotics (azithromycin, ciprofloxacin, ceftriaxone, trimethoprim-sulfamethoxazole, and ampicillin), for which no FDA-approved oral treatment currently exists. The proportion of reported U.S. infections caused by XDR strains rose from 0% in 2011–2015 to roughly 8.5% in 2023, prompting a national CDC Health Advisory in 2023. Whereas shigellosis has historically most affected young children, XDR infections have occurred mainly in adult men—many of them men who have sex with men or people living with HIV—and largely without recent international travel, indicating domestic transmission (CDC, 2023).
Where does Shigella come from?
The organism is frequently found in water polluted with human feces. Salads (potato, tuna, shrimp, macaroni, and chicken), raw vegetables, dairy products, and poultry are foods commonly listed as sources of Shigella. Contamination of these foods is usually through the fecal-oral route. Therefore, water polluted with human feces and unsanitary handling by food handlers is the most common causes of contamination.
An estimated 450,000 cases of shigellosis occur annually in the U.S. and Shigella infections cause an estimated 600,000 deaths per year worldwide.[2] The number of infections attributable to contaminated food is unknown, but as it takes very little Shigella to cause infection, it’s probably substantial. Most of these deaths occur in developing countries with poor hygiene and unsafe water supplies.
How is Shigella detected?
Determining that Shigella is the cause of the illness depends on laboratory tests that identify the bacteria in the stools of an infected person. These tests are sometimes not routinely performed unless the laboratory is instructed specifically to look for the organism. The laboratory can also do special tests to tell which type of Shigella the person has and which antibiotics, if any, would be best to treat it. Organisms like Shigella are difficult to isolate in foods because standard methods are insensitive to such toxins. Recently, a method to identify foodborne pathogens, like Shigella, by specifying segments of their DNA has been developed by FDA and is currently under field test. However, the isolation procedures are still inadequate.
What are the typical symptoms of Shigella?
Symptoms include abdominal pain, cramps, diarrhea, fever, vomiting, blood, pus, or mucus in stools, and painful rectal spasms. These symptoms normally manifest one to two days after ingestion. The disease is caused when virulent Shigella organisms attach to, and penetrate, the wall of the intestinal tract. After invasion, they multiply, and spread to other smooth surface cells within the body, resulting in tissue destruction. Some strains produce enterotoxin and Shiga toxin, very much like the verotoxin of E. coli O157:H7.
Once someone has had shigellosis, they are not likely to get infected with that specific type again for at least several years. However, they can still get infected with other types of Shigella. Infants, the elderly, and those with weakened immune systems are susceptible to the severest symptoms of disease, but all humans are susceptible to some degree. Shigellosis is a very common illness suffered by those with acquired immune deficiency syndrome (AIDS) and other immuno-compromised individuals.
What are the long-term and serious complications of Shigella?
Although most people with shigellosis recover completely within five to seven days, the infection can produce serious complications and lasting health problems. The most common post-infectious complication is reactive arthritis, historically referred to as Reiter’s syndrome. About 2 percent of people infected with certain types of Shigella—most often Shigella flexneri—later develop reactive arthritis, which can cause joint pain, eye irritation (conjunctivitis or uveitis), and painful urination. This reaction occurs mainly in people who are genetically predisposed, particularly those carrying the HLA-B27 gene, and typically appears one to several weeks after the infection. Reactive arthritis usually lasts three to five months, but in some cases it persists for years and progresses to chronic, difficult-to-treat arthritis.[3]
The most severe complication is hemolytic uremic syndrome (HUS), a form of kidney failure marked by the destruction of red blood cells (hemolytic anemia), a low platelet count (thrombocytopenia), and acute renal failure. HUS is associated with Shiga toxin–producing Shigella—most often Shigella dysenteriae type 1—and is the leading cause of death in S. dysenteriae outbreaks. Much like the same complication seen with E. coli O157:H7, HUS can result in permanent kidney damage, neurological injury, and death.[4]
Other complications, though less common, can be life-threatening, especially in young children, older adults, and people with weakened immune systems. These include bloodstream infections (bacteremia and sepsis), which occur when Shigella or other intestinal bacteria enter the bloodstream through the damaged lining of the intestine; generalized seizures in young children; and, more rarely, toxic megacolon, intestinal perforation, rectal prolapse, and encephalopathy. In some patients—particularly children in the developing world—repeated or severe infection interferes with the absorption of nutrients and contributes to malnutrition and impaired growth. Post-infectious irritable bowel syndrome and prolonged disturbances in bowel habits have also been reported following Shigella infection.[5]
How can Shigella infection be prevented?
Because it takes only a very small number of organisms—fewer than 100 bacteria—to cause illness, Shigellaspreads easily from person to person and through contaminated food and water. There is currently no vaccine to prevent shigellosis, so prevention depends on interrupting the fecal-oral route of transmission. The single most important measure is careful and frequent handwashing with soap and water, especially after using the bathroom, after changing a diaper or cleaning up after someone who is sick, before preparing or eating food, and before any sexual activity. Handwashing by young children should be supervised, particularly in child-care settings and in homes with children who are not yet toilet-trained.[6]
Additional precautions further reduce the risk of infection. Soiled diapers should be disposed of promptly in a covered, lined container, and diaper-changing areas should be cleaned and disinfected after each use. People should avoid swallowing water from ponds, lakes, and swimming pools, and anyone with diarrhea should not swim, share baths, or prepare food for others. A safe, chlorinated water supply and an effective sewage-disposal system are the most important protections in areas with substandard sanitation. When traveling to developing countries, travelers should drink only boiled or treated water and eat only foods that are cooked and served hot or fruits they peel themselves—the same precautions that help prevent other forms of traveler’s diarrhea.[7]
Because foodborne outbreaks are frequently caused by an infected food worker who contaminates food through poor hand hygiene, people who are ill should not prepare or serve food for others. The CDC advises food workers with a Shigella infection to stay home from the time they first feel sick until at least two days after their diarrhea has ended, and infected individuals should not return to food handling until stool testing confirms they are no longer shedding the bacteria. Shigellosis is a reportable disease in every state, and cases should be reported to state or local health authorities to help identify and control outbreaks.[8]
[1] See CDC “Multidrug-resistant Shigellosis Spreading in the United States.” April 2, 2015. Web April 23, 2015. http://www.cdc.gov/media/releases/2015/p0402-multidrug-resistant-shigellosis.html.
[2] Id.: “Shigella causes an estimated 500,000 cases of diarrhea in the United States every year. It spreads easily and rapidly from person to person and through contaminated food and recreational water. It can cause watery or bloody diarrhea, abdominal pain, fever, and malaise.”
[3] See CDC, “Signs and Symptoms of Shigella Infection,” at https://www.cdc.gov/shigella/signs-symptoms/index.html.
[5] See CDC, “Clinical Overview of Shigellosis,” at https://www.cdc.gov/shigella/hcp/clinical-overview/index.html.
[6] See CDC, “Preventing Shigella Infection,” at https://www.cdc.gov/shigella/prevention/index.html.
[8] See CDC, “Preventing Shigella Infection Among Food Service Workers and Managers,” at https://www.cdc.gov/shigella/prevention/preventing-shigella-infection-among-food-service-workers-and-managers.html.
