Shigellosis: Symptoms and Treatment

Shigellosis is an acute, highly contagious infectious disease caused by gram-negative bacteria of the genus Shigella, which includes four major species: S. dysenteriae, S. flexneri, S. boydii, and S. sonnei. It primarily affects the intestines, leading to inflammation of the colon (colitis), and is characterized by diarrhea that may range from mild to severe, often accompanied by fever, abdominal cramps, nausea, and the presence of mucus or blood in the stool.

Transmission occurs via the fecal–oral route, typically through ingestion of contaminated food or water, contact with contaminated surfaces, or direct person-to-person spread, particularly in crowded environments such as refugee camps, daycare centers, or institutions. The infectious dose of Shigella is notably low—fewer than 100 bacterial cells can cause illness—making it one of the most communicable bacterial diarrheal diseases.

The disease is most common in areas with poor sanitation, limited access to clean water, and inadequate hygiene practices, contributing significantly to morbidity and mortality in children under five years of age, especially in low- and middle-income countries. Symptoms generally appear one to three days after exposure and can last up to a week.

Diagnosis is confirmed through stool culture, though rapid antigen detection and molecular tests (e.g., PCR) are increasingly used for quicker identification. Treatment focuses on supportive care, particularly oral or intravenous rehydration to prevent dehydration. In moderate to severe cases, antibiotics such as ciprofloxacin or azithromycin may be prescribed; however, rising antimicrobial resistance among Shigella strains is a growing global concern.

Preventive measures include improving access to clean water and sanitation, promoting handwashing with soap, ensuring safe food handling practices, and public health education. Currently, there is no licensed vaccine for shigellosis, though several candidates are in development.

Bacterial Cause of Shigellosis

Shigellosis is caused by gram-negative, facultatively anaerobic bacteria of the genus Shigella, which belongs to the family Enterobacteriaceae. There are four recognized species: Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei. These species differ in their global prevalence, virulence, and association with disease severity.

• S. sonnei is the predominant species in high-income and industrialized nations, accounting for over 75% of cases in the United States and Western Europe.
• S. flexneri is more common in low- and middle-income countries and is responsible for the majority of shigellosis cases globally, especially in Africa and Southeast Asia.
• S. dysenteriae, particularly S. dysenteriae type 1, is known for causing large-scale outbreaks and is associated with more severe forms of the disease, including hemolytic uremic syndrome (HUS), due to its production of Shiga toxin.
• S. boydii is primarily found in the Indian subcontinent and is the least common of the four species.

Shigella species have a remarkably low infectious dose—fewer than 100 organisms are sufficient to cause disease. Once ingested, the bacteria target and invade the epithelial cells of the colonic mucosa, triggering intense inflammation, cell death, and ulceration. This invasion is facilitated by a Type III secretion system, encoded on a large virulence plasmid, which allows the bacteria to manipulate host cell functions and evade immune defenses.

The pathogenicity of Shigella is further enhanced by its ability to produce enterotoxins and cytotoxins:

• Shiga toxin, produced by S. dysenteriae type 1, inhibits protein synthesis in host cells, contributing to severe bloody diarrhea and potential systemic complications such as kidney damage.
• ShET-1 and ShET-2 (Shigella enterotoxins) are implicated in watery diarrhea during the early stages of infection.

A notable case study from a 2018 outbreak in South Asia linked multidrug-resistant S. sonnei to over 500 confirmed infections, illustrating the growing challenge of antimicrobial resistance. Genomic surveillance revealed the rapid spread of fluoroquinolone and azithromycin resistance genes, raising concern over limited treatment options.

Epidemiology and Prevalence

Shigellosis remains a significant global health challenge. It is estimated that Shigella causes between 80 and 165 million cases annually, resulting in approximately 74,000 to 700,000 deaths worldwide. The vast majority of these cases and deaths occur in low- and middle-income countries (LMICs). In 2010 alone, there were an estimated 188 million global cases, with approximately 62 million occurring among children under the age of five.

According to the 1999 Global Burden of Disease (GBD) study, around 165 million episodes of shigellosis occurred annually, including 163 million in LMICs and approximately 1.5 million in high-income countries. The disease was responsible for an estimated 1.1 million deaths each year.

Children under five years of age are disproportionately affected by shigellosis. An estimated 69% of all cases and 61% of associated deaths occur in this age group. In LMICs, the incidence rate among children under five ranges from 1 to 75 cases per 100 child-years, depending on region and sanitation levels. Between 1990 and 2016, regional estimates attributed approximately 63,700 deaths among children under five to shigellosis.

Shigellosis frequently causes outbreaks in densely populated or resource-limited settings. For instance, a 2012 study in Zambia found that Shigella was present in 34.7% of moderate-to-severe diarrheal cases among children under five. However, clinical dysentery was detected in only 8.5% of those cases.

In Somalia in 2019, a hospital-based study involving 180 children with diarrhea under the age of five reported a shigellosis prevalence of 20.6%. The most common species isolated was Shigella flexneri (70%), and all isolates exhibited multidrug resistance. Similarly, research conducted in 2012 in Mekelle, Ethiopia, revealed a prevalence of 13.3% among children under five, with the highest rates (22.6%) observed in those aged 12 to 23 months.

Asymptomatic carriage of Shigella is relatively common, with rates ranging from 4.9% to 17.8% in endemic regions. Such carriage is associated with long-term health consequences, including environmental enteric dysfunction and impaired growth in children. Recurrent infections in early childhood have been linked to stunting, cognitive development delays, and the onset of functional bowel disorders.

Although shigellosis is more prevalent in LMICs, high-income countries also report cases, often linked to travel or foodborne transmission. In Europe in 2022, the notification rate was approximately 1.5 per 100,000 population, rising to 3.0 per 100,000 among children under five. Infections peaked during the summer and autumn months, with S. sonnei accounting for approximately 96% of cases.

In the United States, laboratory-confirmed cases between 1990 and 1994 averaged around 17,500 annually, or approximately 6.5 per 100,000 population. However, actual incidence may be 20 to 100 times higher due to underreporting. Travel-associated cases are common; for instance, travelers returning from countries such as India or East Africa face incidence rates as high as 318 cases per 100,000 travelers.

Globally, Shigella flexneri is the most prevalent species, accounting for roughly 60% of cases in developing countries. It is followed by S. sonnei (about 15%), S. dysenteriae, and S. boydii, each contributing approximately 6% of cases.Nature In contrast, industrialized countries report a predominance of S. sonnei, which accounts for around 77% of cases, while S. flexneri contributes 16% and the other species are much rarer.

In low-income countries, infections often present without dysentery, making diagnosis more difficult and less reliant on the presence of bloody stool.

Shigellosis exhibits seasonal trends, with incidence typically peaking during warmer months. Institutional outbreaks are particularly common in high-density settings such as daycare centers, refugee camps, prisons, and other crowded environments. In developed countries, outbreaks are frequently linked to contaminated food or water and are often associated with international travel.

Transmission and Risk Factors

Shigellosis is transmitted primarily via the fecal–oral route, with infection occurring through ingestion of food or water contaminated with feces, or through direct person-to-person contact. A defining characteristic of Shigella is its extremely low infectious dose. As few as 10 to 100 bacterial cells can cause illness. This makes the pathogen highly contagious, particularly in environments where hygiene and sanitation are inadequate.

Contaminated food and water are common vehicles of transmission, especially in developing countries where wastewater treatment and food safety practices are lacking. Improperly washed produce, street-vended food, and shared water sources often contribute to outbreaks. A 2014 outbreak in Haiti, for example, was traced to a contaminated water system affecting over 800 individuals in a rural community.

Direct transmission occurs readily in crowded settings, such as:

• Daycare centers and schools, where young children may not practice adequate hand hygiene.
• Refugee camps and emergency shelters, where infrastructure is overwhelmed or absent.
• Long-term care facilities and prisons, where close contact and shared facilities facilitate spread.

Among travelers to endemic regions, shigellosis is one of the most commonly acquired bacterial diarrheal illnesses. The CDC reports that up to 15% of all traveler’s diarrhea is caused by Shigella, particularly in destinations across Asia, Africa, and Latin America.

Sexual transmission, particularly among men who have sex with men (MSM), has become an increasingly recognized route in developed countries. The bacteria can spread through oral-anal contact or via contaminated fingers or surfaces during sexual activity. In 2019, an outbreak in the United Kingdom linked to MSM communities revealed multidrug-resistant S. flexneri strains, sparking concern about difficult-to-treat cases spreading within high-risk sexual networks.

Other identified risk factors include:

• Poor hand hygiene, especially after toileting or diaper changing.
• Lack of access to clean water for washing hands and food.
• Malnutrition, which increases vulnerability to severe infection in children.
• HIV infection, which can predispose individuals to more severe or prolonged illness.

Transmission can be exacerbated by asymptomatic carriers, who shed bacteria for up to four weeks after symptoms resolve, contributing silently to the spread in communities.

Symptoms of Shigellosis

Shigellosis presents with a range of gastrointestinal and systemic symptoms, varying in severity depending on the Shigella species involved, host factors, and the individual’s immune response. The clinical spectrum can range from mild, self-limiting diarrhea to life-threatening dysentery and systemic complications.

Common Clinical Presentation

The hallmark symptom of shigellosis is diarrhea, which may begin as watery but frequently progresses to dysentery, characterized by blood, mucus, and pus in the stool due to inflammation and ulceration of the colonic mucosa. The diarrhea is often accompanied by:

• Moderate to severe abdominal cramps
• Fever, typically ≥38.5°C (101.3°F), lasting several days
• Tenesmus, or a painful and urgent feeling of needing to defecate, even when the bowel is empty
• Malaise and fatigue
• Nausea and vomiting, reported in 30–40% of hospitalized cases

The incubation period is usually 1 to 3 days, after which symptoms develop suddenly and can last 5 to 7 days. In some cases, especially among young children, the elderly, and immunocompromised individuals, symptoms may persist beyond a week or relapse after apparent recovery.

Complications Associated With Infection

Although most shigellosis cases resolve without intervention, complications can occur, particularly in vulnerable populations:

• Dehydration: The most common complication due to fluid and electrolyte loss from frequent diarrhea. In severe cases, hospitalization and intravenous fluid therapy may be required.
• Hemolytic Uremic Syndrome (HUS): Most often linked to Shigella dysenteriae type 1, HUS is a life-threatening condition characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. It occurs in approximately 5–10% of infections caused by Shiga toxin-producing strains and can result in long-term renal impairment or death, particularly in children.
• Seizures: Especially observed in febrile infants and toddlers. A retrospective analysis from a pediatric hospital in Egypt noted seizures in up to 8% of children admitted with shigellosis, often linked to high fever or neurotoxic effects of the infection.
• Reactive arthritis (Reiter’s syndrome): A post-infectious autoimmune response that can manifest days to weeks after the initial illness. It includes joint inflammation, conjunctivitis, and urethritis. It is more common in individuals with the HLA-B27 genotype and can become chronic.
• Bacteremia: Although rare, it can occur, particularly in malnourished children, HIV-infected individuals, and patients with underlying immunodeficiencies. When it occurs, it may lead to septic shock, necessitating immediate antibiotic therapy.

Diagnosis of Shigellosis

Diagnosis involves evaluating symptoms, conducting microbiological tests, and ruling out other causes of dysentery. Accurate identification is crucial for appropriate treatment and infection control.

Clinical Assessment

Initial diagnosis is guided by a detailed patient history and physical examination. Clinicians look for hallmark symptoms such as:

• Acute onset of diarrhea, which may be watery, mucoid, or bloody
• Fever and abdominal cramps
• Tenesmus
• Signs of dehydration, especially in children and older adults

Patients may report recent travel to endemic regions, exposure to infected individuals, or consumption of high-risk foods or untreated water.

Laboratory Testing

1. Stool Culture (Gold Standard) 

Stool culture remains the definitive diagnostic method, allowing for:

• Isolation of Shigella species
• Serogroup identification (e.g., S. flexneri, S. sonnei)
• Antibiotic susceptibility testing

However, culture sensitivity can be limited if the patient has already started antibiotics or if stool sample collection is delayed.

2. Molecular and Rapid Diagnostic Tests 

• Polymerase Chain Reaction (PCR): Detects Shigella DNA in stool with >95% sensitivity and faster turnaround time than cultures. Multiplex PCR panels can also test for other pathogens simultaneously.
• Rapid Antigen Detection Tests (RADTs): Provide quicker results but have lower specificity and are less commonly used in clinical settings.

In many low-resource settings, these advanced diagnostics are unavailable. Thus, reliance on clinical suspicion and basic microscopy remains common.

3. Microscopy and Supporting Tests 

Microscopic examination of stool may reveal:

• Fecal leukocytes and erythrocytes — indicating an invasive bacterial infection
• Occult blood in non-visible cases of dysentery

In severe cases or when systemic symptoms are present, complete blood counts (CBC) and electrolyte panels are useful to evaluate dehydration and inflammatory markers.

4. Antimicrobial Resistance Testing 

Due to rising resistance, especially to ampicillin, trimethoprim-sulfamethoxazole, and fluoroquinolones, susceptibility testing is critical. A multicenter surveillance study across Southeast Asia (2015–2022) reported that over 40% of Shigella isolates were multidrug-resistant.

Differential Diagnosis

Because shigellosis presents similarly to other enteric infections and inflammatory disorders, distinguishing it is vital:

Condition	Key Features
Salmonella or Campylobacter	Diarrhea (may be bloody), fever, abdominal pain; often linked to undercooked meat
EHEC (E. coli O157:H7)	Bloody diarrhea, little or no fever; risk of HUS
Amoebic Dysentery (E. histolytica)	Gradual onset, less fever, history of exposure to contaminated water
Inflammatory Bowel Disease (IBD)	Recurrent symptoms, weight loss, no clear infectious exposure

Integration of clinical presentation, lab confirmation, and epidemiological context ensures accurate diagnosis. In resource-limited or outbreak scenarios, treatment may begin based on symptoms and contact history while awaiting lab results.

Treatment and Management

Effective treatment of shigellosis involves targeted antimicrobial therapy, fluid and electrolyte replacement, and infection control measures. Management must be tailored to the severity of illness, local antibiotic resistance trends, and individual patient risk factors.

Antibiotic Therapy

Antibiotic treatment can significantly shorten illness duration, reduce bacterial shedding, and minimize complications, particularly in moderate to severe infections or in vulnerable populations (e.g., children under 5, elderly, immunocompromised individuals).

• Recommended Antibiotics Ciprofloxacin (500 mg twice daily for 3 days) – formerly the first-line agent, now limited by growing resistance

• Azithromycin (500 mg once daily for 3 days) – increasingly preferred, especially in pediatric and pregnant patients

• Ceftriaxone (1–2 g IV daily for 3–5 days) – reserved for hospitalized or severely ill patients

Antibiotic Resistance 

Global surveillance has revealed alarming trends in multidrug-resistant (MDR) Shigella:

• A 2022 WHO report found that >40% of isolates in parts of Asia and Africa were resistant to ampicillin and trimethoprim-sulfamethoxazole.

• Fluoroquinolone resistance, particularly among S. sonnei, has increased sharply in South Asia and is emerging in North America and Europe.

• Extended-spectrum beta-lactamase (ESBL)-producing Shigella strains are being reported with increasing frequency.

In light of these patterns, antibiotic selection should be guided by local resistance data and individual susceptibility testing whenever possible. Empirical therapy may require revision once lab results become available.

Supportive Care

Regardless of antibiotic use, supportive care remains a cornerstone of management, especially in resource-limited settings where diagnostics may be delayed.

Fluid and Electrolyte Replacement

• Oral Rehydration Salts (ORS) are recommended for mild to moderate dehydration.
• For severe dehydration, IV fluid therapy using Ringer’s lactate or normal saline is essential.

The Integrated Management of Childhood Illness (IMCI) guidelines recommend early and aggressive fluid replacement in children, particularly under age 5, where dehydration-related mortality remains high. WHO estimates that dehydration accounts for up to 60% of deaths from diarrheal diseases in children worldwide.

Nutritional Support

Continued feeding during illness is advised. Breastfeeding should not be interrupted. Bland, easy-to-digest foods like rice, bananas, and porridge are suitable during recovery.

Avoidance of Anti-Motility Agents

Drugs such as loperamide and diphenoxylate are contraindicated in shigellosis, as they can delay clearance of the bacteria and increase the risk of toxic megacolon, especially in children.

Monitoring and Infection Control:

Patients should be monitored for signs of worsening illness, such as:

• Persistent high fever
• Blood in stool that worsens
• Signs of systemic toxicity (e.g., lethargy, hypotension)
• Reduced urine output or altered mental status (suggestive of HUS or sepsis)

Discharge criteria may include:

• Resolution of fever
• Normalization of bowel movements
• Adequate oral intake and hydration

Preventing Transmission:

• Isolation precautions: Especially in healthcare or institutional settings
• Hand hygiene enforcement
• Public health notification: In many jurisdictions, confirmed Shigella cases must be reported to health authorities.

Prevention of Shigellosis

Preventing shigellosis relies on controlling sources of infection and interrupting transmission pathways. Core strategies include personal hygiene, safe food and water practices, and coordinated public health measures. These efforts are especially critical in regions with inadequate sanitation infrastructure.

Personal Hygiene Practices

Handwashing is the most effective way to reduce Shigella spread. Hands should be washed with soap and water for at least 20 seconds, especially after using the toilet, changing diapers, and before handling food.

Avoiding direct contact with fecal matter is crucial. Caregivers must disinfect surfaces and objects contaminated by stool. Nail hygiene also matters; keeping nails short limits bacterial harboring. Disinfecting surfaces and toys in households and daycares reduces indirect transmission. The U.S. Environmental Protection Agency (EPA) recommends using disinfectants with proven efficacy against Shigella species, such as sodium hypochlorite-based cleaners.

Alcohol-based hand sanitizers (ABHS) containing at least 60% ethanol can be useful when soap and water are not available. However, Shigella is partially resistant to alcohol-based sanitizers, especially in the presence of fecal matter. Therefore, handwashing with soap remains the gold standard.

Teaching children proper hygiene habits is essential to limit outbreaks in schools and daycare centers.

Food and Water Safety Measures

Proper food handling is essential for shigellosis prevention. Undercooked meat and raw produce have been implicated in multiple outbreaks. 

Safe cooking practices include:

• Cooking meat to safe internal temperatures (e.g., 74°C/165°F for poultry).
• Washing fruits and vegetables thoroughly under running water.
• Avoiding cross-contamination between raw and cooked foods.

Water safety is another major concern. Shigella is highly infectious—as few as 10–100 organisms can cause disease. Therefore, drinking water in high-risk areas should be boiled for at least one minute or chlorinated (0.5 mg/L free residual chlorine) before consumption.

In low-income regions, waterborne transmission is common. A 2002 shigellosis outbreak in Sierra Leone during the rainy season affected over 12,000 people, attributed to flooded latrines contaminating nearby water sources.

Food handlers play a key role. In many countries, asymptomatic carriers have initiated outbreaks. Periodic screening of food service workers, particularly in outbreak-prone regions, helps reduce risk. The World Health Organization (WHO) recommends excluding symptomatic food handlers from work until 48 hours after symptoms resolve and stool tests confirm no pathogen shedding.

Regular training and certification for food vendors is critical. In Mexico, implementation of the “Hygienic Handling of Food” program led to a 30% drop in foodborne shigellosis cases over five years

Public Health Strategies

• Surveillance Systems: The CDC’s FoodNet and WHO’s Global Foodborne Infections Network help detect and respond to outbreaks. In 2014, CDC surveillance detected an emergence of multidrug-resistant Shigella sonnei among international travelers, prompting a targeted response.

• Education Campaigns: Mass media and school programs can significantly impact behaviors. In Vietnam, a hygiene promotion campaign reduced shigellosis incidence by up to 50% in intervention communities.

• Sanitation Improvements: Access to improved sanitation (e.g., flush toilets or ventilated pit latrines) reduces transmission. WHO estimates that 88% of diarrheal diseases worldwide, including shigellosis, are attributable to unsafe water, inadequate sanitation, and poor hygiene.

• Vaccination: Currently, there is no licensed vaccine for shigellosis, though several candidates are in Phase II and III clinical trials. Live-attenuated and conjugate vaccines are being tested in endemic regions. WRAIR’s investigational vaccine has shown 63% efficacy in preliminary studies.

Shigellosis in Special Populations

Certain groups experience higher risks and complications from shigellosis. These populations require tailored considerations for diagnosis, treatment, and prevention.

Children and Infants

Children under five years of age bear the greatest global burden of shigellosis. According to the Global Enteric Multicenter Study (GEMS), Shigella was the second most common cause of moderate-to-severe diarrhea in infants and toddlers in low-resource countries. In this age group, symptoms often escalate to include high fever (>39°C), frequent bloody diarrhea (dysentery), vomiting, and rapid dehydration.

A study in Kenya (Kotloff et al., 2013) found that Shigella accounted for up to 11% of moderate-to-severe diarrheal cases in children under five. Mortality rates are higher in this group, especially when co-infected with other enteric pathogens or in malnourished children.

Transmission in childcare settings is common due to the challenges of enforcing hygiene among young children.

Treatment considerations include:

• Pediatric dosing adjustments for antibiotics such as azithromycin or ceftriaxone.
• Careful monitoring for neurological complications, including febrile seizures, which occur in up to 6% of pediatric cases.
• Watchfulness for toxic megacolon, though rare, especially in cases of delayed care or underlying health conditions.

Rehydration therapy—using ORS (Oral Rehydration Salts) or IV fluids—is essential. In severe cases with profuse diarrhea, electrolyte monitoring is necessary to prevent hypokalemia and acidosis.

Immunocompromised Individuals

Individuals with weakened immune systems, including those with HIV/AIDS, cancer patients on chemotherapy, organ transplant recipients, or those with autoimmune diseases receiving immunosuppressants, are at substantially higher risk for invasive and recurrent shigellosis.

Among HIV-positive populations, Shigella flexneri is the predominant species and is often multidrug-resistant (MDR). Studies from South Africa and India report up to 30–40% resistance to first-line drugs like ciprofloxacin and ampicillin. In these patients, shigellosis is frequently complicated by:

• Persistent diarrhea lasting >14 days
• Weight loss and malabsorption
• Bacteremia (occurs in approximately 3–7% of severe HIV-positive cases)
• Recurrent episodes, especially with CD4 counts <200 cells/mm³

Clinical management in these patients includes:

• Use of second-line antibiotics, often based on susceptibility testing (e.g., carbapenems or tigecycline in extreme MDR cases).
• Longer treatment durations, often 7–10 days instead of the standard 3–5 days.
• Prophylactic therapy may be considered in high-risk facilities during outbreaks.

Preventive strategies involve:

• Strict hand hygiene protocols
• Safe sexual practices, as sexual transmission of Shigella is increasingly recognized among MSM communities.
• Patient education regarding early symptom recognition and seeking prompt care.

Antibiotic Resistance in Shigellosis

Antibiotic resistance in Shigella species is a growing global concern. Resistance limits treatment options and complicates management of shigellosis.

Common antibiotics previously effective, such as ampicillin and trimethoprim-sulfamethoxazole, now show high resistance rates in many regions. Fluoroquinolone resistance has also been increasingly reported, especially in Asia and parts of Africa.

The emergence of resistance in Shigella species involves multiple mechanisms:

• Efflux Pumps: Such as AcrAB-TolC, which actively expel antibiotics from bacterial cells, reducing intracellular drug concentrations.

• Target Site Mutations: Mutations in gyrA and parC genes confer resistance to fluoroquinolones by altering DNA gyrase and topoisomerase IV binding sites.

• Beta-Lactamase Production: Including extended-spectrum beta-lactamases (ESBLs), which hydrolyze beta-lactam antibiotics such as cephalosporins and ampicillin.

• Macrolide Resistance Genes (mphA, ermB): These confer resistance to azithromycin, increasingly detected in Asia and among travelers returning from endemic regions.

The table below summarizes typical resistance patterns for key antibiotics:

Antibiotic	Resistance Status
Ampicillin	High resistance
Trimethoprim-sulfamethoxazole	High resistance
Fluoroquinolones	Increasing resistance
Cephalosporins	Variable resistance
Azithromycin	Emerging resistance
Carbapenems	Rare, reserved for MDR cases

Clinical and Public Health Implications 

• Treatment Failures: Empirical use of ineffective antibiotics can lead to prolonged illness, increased transmission, and complications.

• Hospitalization and Costs: Resistant infections are more likely to require hospitalization, IV therapy, and prolonged recovery. In a U.S. CDC estimate, MDR Shigella infections cost 20% more per patient compared to drug-susceptible cases.

• Vulnerable Groups at Higher Risk: Children, immunocompromised patients, and international travelers are disproportionately affected by resistant strains.