Shigella

Shigella is a group of bacteria that cause shigellosis — an intestinal infection whose most severe form is bacillary dysentery, meaning diarrhea with blood and mucus, cramping, and painful straining. What makes Shigella stand out among the germs that upset our guts is not how sick it usually makes people, but how astonishingly little of it is needed to do so. Swallowing as few as ten to a hundred organisms can start an infection — a dose so tiny that Shigella passes easily from one person's hands to another, sweeping through daycares, households, and crowded living conditions. This page explains what Shigella is, the four species that make up the genus, how it spreads, the illness it causes, why the Shiga toxin of one species is so dangerous, and how shigellosis is diagnosed, treated, and prevented. It also covers the growing problem of drug-resistant strains that has put Shigella on the radar of public-health agencies worldwide. The tone throughout is plain and honest: for most healthy people this is a rough week that clears on its own, but for young children in places without clean water it remains a leading cause of dangerous diarrhea.

Overview
The Bacterium: Four Species
How It Spreads
The Illness: Shigellosis
Shiga Toxin and the HUS Danger
Who Is Most at Risk
Diagnosis
Treatment
Drug-Resistant Shigella
Prevention
The Honest Bottom Line
Research Papers
Connections
Featured Videos

Overview

Shigella is one of the most contagious causes of diarrhea in the world. It is spread from feces to mouth, and because it takes so few organisms to cause disease, it travels person-to-person more efficiently than most other intestinal germs. The illness it causes, shigellosis, ranges from a few days of watery diarrhea to full-blown dysentery with blood, mucus, fever, and severe cramping.

The burden is uneven across the globe. In the United States, the CDC estimates roughly 450,000 infections each year, most of them self-limiting. But in low- and middle-income countries, Shigella is a heavyweight killer of children. The landmark Global Enteric Multicenter Study (GEMS) identified Shigella as one of the top causes of moderate-to-severe diarrhea in young children across Africa and South Asia, and later analyses of the Global Burden of Disease study attributed more than 200,000 deaths a year worldwide to shigellosis, a large share of them in children under five. Clean water, sanitation, and handwashing are what separate a nuisance from a life-threatening disease.

Two themes run through everything below: the tiny infectious dose that makes Shigella so easy to catch, and the rising tide of antibiotic resistance that is eroding our ability to treat the people who most need help.

The Bacterium: Four Species

Shigella is a Gram-negative, rod-shaped bacterium. Unlike many of its gut-dwelling relatives, it is non-motile — it has no flagella to swim with — and it does not form spores. It is a facultative anaerobe, meaning it can grow with or without oxygen, and it belongs to the large family Enterobacteriaceae. The genus is divided into four species, historically labeled as serogroups A through D:

Shigella dysenteriae (Group A). The most severe. Type 1 of this species produces Shiga toxin and has historically caused enormous, deadly epidemics of dysentery in Central America, sub-Saharan Africa, and South Asia. It is comparatively uncommon today but the most feared when it appears.
Shigella flexneri (Group B). The predominant cause of shigellosis in low- and middle-income countries, and the species most associated with the endemic childhood dysentery that drives the global death toll.
Shigella boydii (Group C). The least common of the four, found mainly on the Indian subcontinent.
Shigella sonnei (Group D). The dominant species in industrialized, high-income countries, including the United States and much of Europe. It usually causes a milder illness — but it is also the species now driving much of the alarming antibiotic-resistance problem.

One fact surprises many people: at the level of DNA, Shigella is essentially a type of Escherichia coli. The two are so genetically similar that if biology were reorganized from scratch today, Shigella would be folded into the E. coli species. The separate genus name survives for historical and clinical reasons — it has been in medical use since the bacterium's discovery by the Japanese microbiologist Kiyoshi Shiga in 1897, whose name lives on in both the genus and in Shiga toxin. What turns this otherwise E. coli-like organism into a dysentery-causer is a large virulence plasmid (a ring of extra DNA) that gives it the machinery to invade the cells lining the colon. A closely related group of true E. coli called enteroinvasive E. coli (EIEC) carries a similar plasmid and causes a nearly identical illness.

How It Spreads

Shigella spreads by the fecal-oral route: microscopic amounts of stool from an infected person reach another person's mouth. What makes this bacterium so successful is its extraordinarily low infectious dose. Careful human studies established decades ago that swallowing as few as 10 to 100 organisms is enough to cause disease in a healthy adult — orders of magnitude less than the thousands or millions typically required by Salmonella or the hundreds required by Campylobacter. Because so little is needed, and because that amount is invisible on an unwashed hand, Shigella spreads directly from person to person far more readily than most foodborne germs.

The main routes are:

Person-to-person contact. This is the dominant mode. Contaminated hands transfer the bacteria after using the toilet or changing a diaper, then pass them to other people or to shared objects. Toddlers in diapers are an especially efficient link in the chain.
Daycares, institutions, and crowded settings. The low dose makes Shigella notorious for outbreaks in childcare centers, nursing homes, jails, cruise ships, refugee camps, and any place where many people share space and sanitation is strained.
Contaminated food. Food handled by an infected person who did not wash their hands — particularly cold, ready-to-eat foods like salads and sandwiches that are not cooked afterward — can carry an infectious dose.
Contaminated water. Drinking or swimming in water polluted with sewage spreads Shigella. Recreational-water outbreaks in pools, splash pads, and lakes are well documented, because swallowing even a mouthful can be enough.
Flies. Houseflies can carry Shigella mechanically from feces to food, an underappreciated route in settings with poor sanitation.
Sexual transmission. Oral-anal sexual contact transmits Shigella, and there have been sustained outbreaks among men who have sex with men (MSM). This route is important because it has been closely tied to the spread of highly drug-resistant strains.

People who are infected can shed the bacteria in their stool for weeks after symptoms resolve, which is why returning to food handling, childcare, or healthcare work too soon can restart an outbreak.

The Illness: Shigellosis

Symptoms usually begin one to four days after swallowing the bacteria, occasionally up to a week. The illness classically unfolds in two phases. It often starts with watery diarrhea, fever, and a general feeling of being unwell. Then, in more pronounced cases, it progresses to true dysentery: frequent, small-volume stools containing blood and mucus, accompanied by intense abdominal cramping and tenesmus — the painful, urgent, and frustrating sensation of needing to pass stool even when little or nothing comes out. This reflects the bacterium invading and inflaming the lining of the colon and rectum.

The typical features are:

Diarrhea — initially watery, then often bloody and mucoid as dysentery develops.
Fever — frequently high, especially in children, in whom it can occasionally trigger febrile seizures.
Abdominal cramps and tenesmus — among the most distressing symptoms, driven by inflammation of the lower bowel.
Nausea and sometimes vomiting, malaise, and loss of appetite.

For most otherwise-healthy people, shigellosis is self-limiting and resolves in about five to seven days. The chief danger in ordinary cases is dehydration from fluid loss, which is riskiest for infants, the elderly, and anyone unable to keep up with drinking. A minority of patients develop complications: reactive arthritis (joint inflammation appearing after the infection, more common in people with a genetic marker called HLA-B27), rectal prolapse in young children straining against dysentery, rarely a dangerously dilated colon (toxic megacolon), bloodstream infection in vulnerable patients, and — most seriously — the hemolytic uremic syndrome described in the next section.

Shiga Toxin and the HUS Danger

The single most dangerous feature of the genus belongs to just one member: Shigella dysenteriae type 1, which produces Shiga toxin. This is the original, prototypical toxin of its family — the very same toxin (in a nearly identical "Shiga-like" form) made by the well-known Shiga-toxin-producing E. coli such as E. coli O157:H7. The everyday species S. sonnei and S. flexneri do not make meaningful amounts of Shiga toxin, which is why the great majority of shigellosis cases in wealthy countries do not carry this specific risk.

Shiga toxin works by shutting down protein production inside cells — it chemically damages the ribosome, the cell's protein factory — and it preferentially injures the delicate cells that line small blood vessels, especially in the intestine and the kidneys. When it reaches the kidneys, it can trigger hemolytic uremic syndrome (HUS), a life-threatening trio of problems:

Hemolytic anemia — red blood cells are torn apart as they pass through damaged vessels.
Thrombocytopenia — a dangerous drop in platelets, the cells that help blood clot.
Acute kidney injury — the kidneys begin to fail, sometimes requiring dialysis.

HUS is most feared in young children and is a major reason the historical epidemics of S. dysenteriae type 1 dysentery carried such high death rates. It is worth being clear about one point that often causes confusion: for E. coli O157:H7 infections, antibiotics are generally avoided because they may increase the risk of HUS. That specific caution does not transfer to Shigella — for confirmed or severe shigellosis, appropriate antibiotics are recommended, because they shorten the illness, reduce complications, and cut down the contagious shedding. Any sign of HUS — paleness, reduced urination, unusual bruising, or swelling after a bout of bloody diarrhea — is a medical emergency.

Who Is Most at Risk

Anyone can catch Shigella, but some groups face a much higher chance of infection or of severe disease:

Young children, especially ages one to four. They have the highest rates of infection, and toddlers in diapers and daycare are at the center of most community outbreaks. Children in low-income countries bear the heaviest burden of severe and fatal disease.
International travelers. Shigella is a common cause of travelers' diarrhea, particularly for people visiting regions with less-developed water and sanitation systems.
People in crowded or low-sanitation settings. Refugee camps, correctional facilities, cruise ships, and shelters concentrate risk because the tiny infectious dose spreads so easily where hygiene is strained.
Men who have sex with men. Sexual transmission has produced ongoing outbreaks, several of them involving multidrug-resistant strains.
People with weakened immune systems. Those with advanced HIV or other immune compromise can suffer more severe, prolonged, or bloodstream infections.
Malnourished children. Poor nutrition both increases susceptibility and worsens outcomes, part of a vicious cycle in which repeated dysentery further impairs a child's growth and nutrition.

Diagnosis

Many mild cases of shigellosis are never specifically tested, because the illness resolves on its own and a laboratory diagnosis would not change what a healthy adult needs to do. Testing becomes important when diarrhea is bloody or severe, when the patient is very young, elderly, or immunocompromised, during outbreaks, and — increasingly — whenever a doctor needs to know which antibiotics will still work. The main approaches are:

Stool culture. Growing the bacterium from a stool sample remains the traditional reference test. Its great advantage today is that it yields a live organism for antibiotic susceptibility testing — which, in an era of spreading resistance, is often the most valuable piece of information the lab can provide.
Multiplex PCR stool panels. Modern molecular panels detect Shigella's genetic signature (along with many other pathogens) within hours, and are far more sensitive than culture. Their limitation is that they do not, by themselves, tell doctors which antibiotics will work — so a positive PCR is often reflexed to culture to obtain susceptibilities. These panels also cannot always separate Shigella from its close cousin enteroinvasive E. coli.
Supporting clues. Blood and white cells in the stool point toward an invasive, inflammatory diarrhea consistent with Shigella, helping distinguish it from watery, toxin-driven diarrheas.

Treatment

The foundation of treatment for shigellosis — as for nearly every diarrheal illness — is fluid and electrolyte replacement. Most people can rehydrate by mouth with oral rehydration solution (ORS), water balanced with the right amount of salt and sugar; severe dehydration may require intravenous fluids in a hospital. In children, the World Health Organization also recommends zinc supplementation, which shortens and reduces the severity of diarrheal episodes.

Whether to use antibiotics deserves a careful, honest explanation. Many healthy adults recover without them. But Shigella is treated more readily with antibiotics than most other causes of gastroenteritis, for three reasons: antibiotics shorten the illness, they reduce complications, and — important for public health — they cut the period of contagious shedding, helping stop this highly transmissible germ from spreading to others. For that reason, treatment is commonly recommended for confirmed cases, for severe or bloody disease, for young children, and for vulnerable patients.

The catch is that the antibiotics historically relied upon — ciprofloxacin, azithromycin, and ceftriaxone — are all losing ground to resistance (see the next section). Because of this, choosing an antibiotic increasingly depends on local resistance patterns and, ideally, susceptibility testing from a stool culture rather than a one-size-fits-all prescription.

One rule is firm: avoid anti-motility (antidiarrheal) drugs such as loperamide (Imodium) and diphenoxylate in dysentery. By slowing the bowel, these medicines trap the bacteria and their toxins inside, which can prolong the illness and worsen complications, including the risk of toxic megacolon. Fluids, rest, and — when indicated — the right antibiotic are the correct tools; drugs that simply "stop the diarrhea" are not.

Drug-Resistant Shigella

The most worrying modern development is that Shigella is becoming steadily harder to treat. The U.S. Centers for Disease Control and Prevention classifies drug-resistant Shigella as a "serious threat" in its national antibiotic-resistance assessment, and the World Health Organization has placed fluoroquinolone-resistant Shigella on its global priority list of pathogens for which new antibiotics are urgently needed. Multidrug resistance — strains resistant to several first-line drugs at once — is now widespread.

The frontier of concern is extensively drug-resistant (XDR) Shigella, defined as strains resistant to essentially all the antibiotics normally used against it: azithromycin, ciprofloxacin, ceftriaxone, ampicillin, and trimethoprim-sulfamethoxazole. In early 2023 the CDC issued a national health advisory warning that XDR Shigella was rising sharply in the United States, with cases concentrated among adults — particularly men who have sex with men, people experiencing homelessness, international travelers, and people living with HIV. When a strain is XDR, clinicians may be left with few or no reliably effective oral options.

Resistance spreads because the genes that confer it travel on mobile pieces of DNA (plasmids) that Shigella can swap with other bacteria, and because tightly connected transmission networks — whether a daycare, a travel corridor, or a sexual network — let a resistant strain propagate quickly once it appears. Being honest about the stakes: for most healthy people, shigellosis still clears on its own without antibiotics at all. The danger of resistance is what it means for the people who genuinely need treatment — young children, the immunocompromised, and those with severe disease — for whom a dwindling list of working drugs is a real and growing problem.

Prevention

Because the infectious dose is so small, the humble act of handwashing with soap and water is the single most powerful defense against Shigella — far more so than for germs that require a large dose. Thorough handwashing after using the toilet, after changing diapers, and before preparing or eating food breaks the fecal-oral chain at its most important point. (Alcohol-based sanitizers help but do not replace soap and water for this organism.) Practical prevention includes:

Wash hands well and often, especially around toileting, diapering, and food.
Keep sick people away from food handling, childcare, and healthcare until they are cleared, since shedding continues after symptoms fade.
Do not swim while ill, and avoid recreational water for a week or two after recovery, to protect pools and splash pads from contamination.
Use safe water and food when traveling — drink treated or boiled water, and follow the "boil it, cook it, peel it, or forget it" rule in areas with uncertain sanitation.
Reduce sexual transmission through hygiene and barrier methods, particularly during known community outbreaks.
Invest in sanitation. At the population level, clean water and proper sewage disposal are what turn Shigella from a major childhood killer into an occasional nuisance.

There is currently no licensed vaccine against Shigella, but developing one is a global public-health priority, and several candidate vaccines are moving through clinical trials — work aimed above all at protecting young children in the regions where shigellosis remains most deadly.

The Honest Bottom Line

For most healthy people in high-income countries, shigellosis is a miserable but self-limiting week: fluids, rest, and scrupulous hand hygiene are the mainstays, and antibiotics are reserved for those who truly need them. The real toll falls elsewhere — on young children in places without reliable clean water and sanitation, where Shigella remains a leading cause of dysentery and diarrheal death. Two things deserve everyone's respect: the tiny infectious dose that makes this germ spread so easily (which is exactly why handwashing works so well), and the rising drug resistance that is quietly narrowing our treatment options. Wash your hands, rehydrate, skip the anti-diarrheal pills if there is blood in the stool, and seek care for high fever, dehydration, bloody dysentery, or any warning sign of kidney trouble after the diarrhea.

Research Papers

Kotloff KL, Riddle MS, Platts-Mills JA, Pavlinac P, Zaidi AKM. Shigellosis. Lancet. 2018;391(10122):801–812. PMID 29254859 — Authoritative modern review of Shigella epidemiology, clinical features, treatment, and the vaccine pipeline.
Kotloff KL, Nataro JP, Blackwelder WC, et al. Burden and aetiology of diarrhoeal disease in infants and young children in developing countries (the Global Enteric Multicenter Study, GEMS): a prospective, case-control study. Lancet. 2013;382(9888):209–222. PMID 23680352 — Landmark multi-country study identifying Shigella as a leading cause of moderate-to-severe childhood diarrhea in Africa and South Asia.
DuPont HL, Levine MM, Hornick RB, Formal SB. Inoculum size in shigellosis and implications for expected mode of transmission. J Infect Dis. 1989;159(6):1126–1128. PMID 2656880 — The classic human-challenge study establishing that as few as 10–100 organisms can cause disease, explaining the efficient person-to-person spread.
Khalil IA, Troeger C, Blacker BF, et al. Morbidity and mortality due to shigella and enterotoxigenic Escherichia coli diarrhoea: the Global Burden of Disease Study 1990–2016. Lancet Infect Dis. 2018;18(11):1229–1240. PMID 30266330 — Global estimate attributing more than 200,000 deaths a year to Shigella, concentrated in young children.
Baker S, The HC. Recent insights into Shigella: a major contributor to the global diarrhoeal disease burden. Curr Opin Infect Dis. 2018;31(5):449–454. PMID 30048255 — Reviews the shifting epidemiology of S. sonnei and the drivers of emerging multidrug resistance.
Melton-Celsa AR. Shiga toxin (Stx) classification, structure, and function. Microbiol Spectr. 2014;2(4). PMID 25530917 — Detailed account of Shiga toxin biology, shared by S. dysenteriae type 1 and Shiga-toxin-producing E. coli, and its role in HUS.
Livio S, Strockbine NA, Panchalingam S, et al. Shigella isolates from the Global Enteric Multicenter Study inform vaccine development. Clin Infect Dis. 2014;59(7):933–941. PMID 24958238 — Serotype distribution across sites showing which strains a broadly protective vaccine would need to cover.
Bowen A, Grass J, Bicknese A, Campbell D, Hurd J, Kirkcaldy RD. Elevated risk for antimicrobial drug–resistant Shigella infection among men who have sex with men, United States, 2011–2015. Emerg Infect Dis. 2016;22(9):1613–1616. PMID 27533624 — U.S. surveillance data linking sexual transmission among MSM with drug-resistant strains.
Ranjbar R, Farahani A. Shigella: antibiotic-resistance mechanisms and new horizons for treatment. Infect Drug Resist. 2019;12:3137–3167. PMID 31632102 — Comprehensive review of how Shigella acquires resistance and the treatment options that remain.
Puzari M, Sharma M, Chetia P. Emergence of antibiotic resistant Shigella species: a matter of concern. J Infect Public Health. 2018;11(4):451–454. PMID 29066021 — Summarizes the global rise of multidrug-resistant Shigella and its public-health implications.
MacLennan CA, Grow S, Ma LF, Steele AD. The Shigella vaccines pipeline. Vaccines (Basel). 2022;10(9):1376. PMID 36146457 — Overview of vaccine candidates in development to protect children in endemic regions.
Tacconelli E, Carrara E, Savoldi A, et al. Discovery, research, and development of new antibiotics: the WHO priority list of antibiotic-resistant bacteria and tuberculosis. Lancet Infect Dis. 2018;18(3):318–327. PMID 29276051 — The WHO analysis that places fluoroquinolone-resistant Shigella among priority pathogens needing new antibiotics.

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