AUTHOR AND EDITOR INFORMATION

Section 1 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

INTRODUCTION

Section 2 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Background

Shigella organisms are a group of gram-negative, facultative intracellular pathogens. They were recognized as the etiologic agents of bacillary dysentery or shigellosis in the 1890s. Shigella was adopted as a genus in the 1950s and grouped into 4 species: Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei, also known as groups A, B, C, and D, respectively. S dysenteriae serotype 1 causes deadly epidemics, S boydii is restricted to the Indian subcontinent, and S flexneri and S sonnei are prevalent in developing and developed countries, respectively. S flexneri, an enteroinvasive gram-negative bacterium, is responsible for the worldwide endemic form of bacillary dysentery.

Pathophysiology

Shigella infection is a major public health problem in developing countries where sanitation is poor. Humans are the only natural reservoir. No natural food products harbor endogenous Shigella species, but a wide variety of foods may be contaminated.

Shigellosis is spread by means of fecal-oral transmission. Other modes of transmission include ingestion of contaminated food or water, contact with a contaminated inanimate object, and sexual contact. Vectors like the housefly can spread the disease by physically transporting infected feces.

The infectivity dose (ID) is extremely low. As few as 10 S dysenteriae bacilli can cause clinical disease, whereas 100-200 bacilli are needed for S sonnei or S flexneri infection.

The incubation period varies from 12 hours to 7 days, but typically it is 2-4 days, and it is inversely proportional to the load of ingested bacteria.

Bacterial shedding usually ceases within 4 weeks of the onset of illness. A chronic carrier state beyond 1 year is rare.

Virulence

Virulence in Shigella species involves both chromosomal- and plasmid-coded genes.

The characteristic virulence trait is encoded on a large plasmid, which is responsible for synthesis of polypeptides causing cytotoxicity. Shigellae those lose the virulence plasmid are no longer pathogenic. Escherichia coli (E coli O157:H7) that harbor this plasmid clinically behave as Shigella bacteria.

Plasmid-coded genes are described below.

Siderophores control the acquisition of iron from host cells from its protein-bound state. The ability to acquire nutrients is an essential attribute of any successful organism. In the extraintestinal phase of infection by gram-negative bacteria, iron becomes one of the major factors limiting further growth. This limitation exists because most of the iron in human body is sequestered in hemoproteins (ie, hemoglobin, myoglobin) or iron-chelating proteins involved in iron transport (transferrin and lactoferrin). Many bacteria can secrete iron chelating compounds, or siderophores, which chelate iron from the intestinal fluids and which bacteria then take up to obtain iron for its metabolic needs. These siderophores are under the control of plasmids and are tightly regulated by genes such that, under low iron conditions, expression of the siderophore system is high.

Regulatory genes control expression of virulence genes. Shiga toxin (Stx) is not essential for virulence of S dysenteriae type 1, but it contributes to the severity of dysentery. Both plasmid-encoded virulence traits and chromosome-encoded factors are essential for full virulence of shigellae.

Regarding chromosomally encoded enterotoxin, many pathogenic features of Shigella infection are due to the production of potent cytotoxins known as Stx, a potent protein synthesis–inhibiting exotoxin. Shigella strains produce distinct enterotoxins. These are a family of cytotoxins that contain 2 major immunologically non–cross-reactive groups called Stx1 and Stx2. The homology sequences between Stx1 and Stx2 are 55% and 57% in subunits A and B, respectively. These toxins are lethal to animals; enterotoxic to ligated rabbit intestinal segments; and cytotoxic for vero, HeLa, and some selected endothelial cells (human renal vascular endothelial cells) manifesting as diarrhea, dysentery, and hemolytic-uremic syndrome (HUS). Stx1 is synthesized in significant amount by S dysenteriae serotype 1 and S flexneri 2a and E coli (Shigella toxin–producing E coli, or ShET).

Stx1 and Stx2 both are encoded by a bacteriophage inserted into the chromosome. Stx 1 increases inflammatory cytokine production by human macrophages, which in turn leads to a burst of interleukin (IL)-8. This could be relevant in recruiting neutrophils to the lamina propria of the intestine in hemorrhagic colitis, and it accounts for elevated levels of IL-8 in serum of patients with diarrhea-associated HUS.

Stxs have 2 subunits. Subunit A is a 32-kD polypeptide that, when digested by trypsin, generates A1 with a 28-kD fragment and another small fragment, A2, which is 4 kD. A1 fraction acts like N-glycosidase; it removes single adenine residue from 28S rRNA of ribosome and inhibits protein synthesis. The A2 fraction is a pentamer polypeptide of 7.7-kD protein and is required to bind the A1 fraction to the B subunit. The main function of the B subunit is the binding of toxins to the cell surface receptor, typically globotriaosylceramide (Gb3), on the brush border of intestinal epithelial cells.

In summary, events that occur on exposure to Shigella toxin are as follows:

• The B subunit of holotoxin binds to the Gb3 receptor on the cell surface of brush-border cells of the intestines.
• The receptor-holotoxin complex is endocytosed.
• The complex moves to Golgi apparatus and then to the endoplasmic reticulum.

The A1 subunit is released and it targets 28S RNA of the ribosome, inhibiting protein synthesis. Stxs may play a role in the progression of mucosal lesions after colonic cells are invaded, or they may induce vascular damage in the colonic mucosa. Stx adheres to small-intestine receptors and blocks the absorption of electrolytes, glucose, and amino acids from intestinal lumen. The B subunit of Stx binds the host's cell glycolipid in the large intestine and in other cells, such as renal glomerular and tubular epithelia. The A1 domain internalized by means of receptor-mediated endocytosis and causes irreversible inactivation of the 60S ribosomal subunit, inhibiting protein synthesis and causing cell death, microvascular damage to the intestine, apoptosis in renal tubular epithelial cells, and hemorrhage (as blood and mucus in the stool).

Chromosomal genes control lipopolysaccharide (LPS) antigens in cell walls. LPS plays an important role in resistance to nonspecific host defense encountered during tissue invasion. These genes help in invasion, multiplication, and resistance to phagocytosis by tissue macrophages. LPS enhances the cytotoxicity of Stx on human vascular endothelial cells. Shigella chromosomes share most of their genes with E coli K12 strain MG1655, and the diversity of putative virulence genes acquired by means of bacteriophage-mediated lateral gene transfer is extensive. As a result of convergent evolution involving the gain and loss of functions, Shigella species have became highly specific human pathogens with variable epidemiologic and pathologic features.

A 3-kb plasmid that harbors information for the production of bacteriocin by S flexneri strains has been described. The production of this bacteriocin may be related to dysenteric diarrhea these bacterial strains produce.

Intestinal adherence factor

Intestinal adherence factor favors colonization in vivo and in animal models. This is 97-kD outer-membrane protein (OMP) encoded by each gene on chromosomes. This codes for intimin protein, and an anti-intimin response is observed in children with HUS.

Pathology

The host response to primary infection is characterized by the induction of an acute inflammation, which is accompanied by polymorphonuclear cell (PMN) infiltration, resulting in massive destruction of the colonic mucosa.

Gross pathology consists of mucosal edema, erythema, friability, superficial ulceration, and focal mucosal hemorrhage involving the rectosigmoid junction primarily.

Microscopic pathology consists of epithelial cell necrosis, goblet cell depletion, PMN infiltrates and mononuclear infiltrates in lamina propria, and crypt abscess formation. (Invasion of M cells, the specialized cells that cover the lymphoid follicles of the mucosa, overlying Peyer patches may be the earliest event.)

Frequency

United States

The reported incidence of Shigella infections was 2078 cases in 2005. Most cases are reported during summer months. S sonnei accounts for approximately 78%, and S flexneri and S boydii account for most of the remainder of all Shigella isolates in recent surveys from the Centers for Disease Control and Prevention (CDC). S flexneri causes 18% of Shigella infections in the United States. S dysenteriae is rare in the United States. See the CDC Web site.

The overall incidence of Shigella infection is 4.67 cases per 100,000 population, and the rate of HUS in pediatric patients <15 years is 0.49 per 100,000. The estimated annual incidence of Shigella infection decreased by 43% (confidence interval, 18-60%) from 1996-1998 to 2005. More than 95% of Shigella infections may be asymptomatic. Hence, the actual incidence may be 20 times higher than reported.

International

Worldwide, the incidence of shigellosis is estimated to be 164.7 million per year, of which 163.2 million were in developing countries, where 1.1 million deaths occurred. About 60% of all episodes and 61% of all deaths attributable to shigellosis involved children younger than 5 years. The incidence in developing countries may be 20 times greater than that in developed countries. Although the relative importance of various serotypes is not known, an estimated 30% of these infections are caused by S dysenteriae.

Case-fatality rates for S dysenteriae infections may approach 30%. Patients with malnutrition are at increased risk of having complicated course. Shigella infection in malnourished children often causes a vicious cycle of further impaired nutrition, recurrent infection, and further growth retardation.

Mortality/Morbidity

Although shigellosis-related mortality is rare in developed countries, S dysenteriae infection is associated with substantial morbidity and mortality rates in the developing world.

• The overall mortality rate in developed countries is less than 1%.
• In the Far East and Middle East, the mortality rates for infections of S dysenteriae may be as high as 20-25%.

Race

No racial predilection exists.

Sex

No sexual predilection exists.

Age

According to recent CDC reports, Shigella infection accounted for 28% of all the enteric bacterial infections. Children younger than 5 years had 7% of total reported cases, a rate indicating a disproportionate disease burden in this population.

CLINICAL

Section 3 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

History

• Populations that are at high-risk for shigellosis include the following:
  • Children in daycare centers
  • Orthodox Jews
  • Native Caucasians
  • Persons in custodial institutions
  • International travelers
  • Homosexual men
  • Persons in homes with inadequate water supply
  • People with HIV infection
• Symptoms include the following:
• • Sudden onset of severe abdominal cramping, high-grade fever, emesis, anorexia, and large-volume watery diarrhea. Seizures may be an early manifestation.
  • Abdominal pain, tenesmus, urgency, fecal incontinence, and small-volume mucoid diarrhea with frank blood may subsequently occur.
• Signs include the following:
• • Elevated temperatures (up to 104°F) and a generally toxic appearance
  • Tachycardia and tachypnea secondary to fever and dehydration (Depending on the degree of dehydration, dry mucous membranes, hypotension, prolonged capillary refill, and poor skin turgor may be present.)
  • Abdominal tenderness (usually central, though it may be generalized)
• Extraintestinal manifestations are as follows:
• • CNS symptoms include severe headache, lethargy, meningismus, delirium, and seizures lasting less than 15 minutes, especially with S dysenteriae. Severe toxic encephalopathy is rare, but lethal complications occur when initial symptoms are followed by sensory obtundation, seizures, coma, and death in 6-48 hours. The pathogenesis of neurologic manifestations during shigellosis is unclear. However, data now clearly demonstrate that Stx is not responsible.
  • Regarding HUS, microangiopathic hemolytic anemia, thrombocytopenia, and renal failure have been reported with S dysenteriae because of vasculopathy mediated by Stx. The principal organ affected in Stx1-mediated HUS is the kidney. This is presumed to be the consequence of the high renal blood flow and abundant baseline expression and high inducibility of the Stx glycolipid receptor Gb_e in the glomerular microcirculation. Manifestations of the disease arise due to 2 primary pathogenetic mechanisms: direct Stx-mediated injury to vascular endothelial cells leading to tissue ischemia and dysfunction and a systemic inflammatory response triggered by Stx-mediated release of a wide range of cytokines and chemokines, including IL-6, IL-8, and tumor necrosis factor-alpha.
  • Septicemia is rare except in malnourished children with S dysenteriae infection.
  • Septicemia sometimes is caused by other gram-negative organisms and related to loss of mucosal integrity by Shigella infection.
  • Shigella sepsis may be complicated with disseminated intravascular coagulation (DIC), bronchopneumonia, and multiple organ failure in lethal cases.
  • Reiter syndrome (arthritis, urethritis, conjunctivitis) is commonly observed in adults carrying human leukocyte antigen (HLA)-B27 histocompatibility antigen.
  • Hepatitis, if present, is usually mild.
  • Myocarditis is identified with cardiogenic shock, arrhythmias, and heart block.
• Shigellosis in the first 6 months of life is rare probably due to presence of antibodies to both virulence plasmid-coded antigens and lipopolysaccharides in the breast milk. Shigellosis in the neonatal period results from mother-to-infant fecal-oral transmission during labor and delivery, usually from asymptomatic mothers.
• • Symptoms usually begin on the third day of life.
  • Septicemia and chronic diarrhea are common.
  • Fever may be absent.
  • Diarrhea usually is not bloody.
  • Intestinal perforation and mortality are more common in this group than in older children.
• Shigellosis in patients with HIV infection is often a protracted, chronic, relapsing disease (even when treated with antibiotics). Bacteremia may be a complication.

Physical

• Physical examination during acute illness reveals a febrile ill-appearing child. Fever with a temperature as high as 39-40°C may be noted.
• The patient's hydration status should be assessed carefully. Especially note dryness of the mouth, a lack of tears, decreased urine output, and loss of skin turgor.
• Abdominal examination may reveal generalized mild-to-moderate tenderness with no guarding or rigidity.
• In a child who presents with febrile seizures, careful neurologic examination is mandatory to exclude meningitis.

Causes

Shigella infection is spread by means of fecal-oral transmission.

DIFFERENTIALS

Section 4 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Other Problems to be Considered

Clostridium difficile infection
Entameba histolytica infection

WORKUP

Section 5 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Lab Studies

• Hematology
• • The WBC count is often (5-15 X 10⁹/L [5000-15,000/mL]), with a high percentage of bands. Leukopenia or leukemoid reactions are occasionally detected.
  • In HUS, anemia and thrombocytopenia occur.
  • Bacteremia is rare, even in severe disease.
• Stool examination
• • Routine microscopy may reveal sheets of PMNs. Platelet counts are reduced.
  • In approximately 70% of patients with shigellosis, fecal blood or leukocytes (confirming colitis) are detectable in the stool. Fecal blood and leukocytes are present in 50% of patients.
• Stool culture
• • A sample for stool culture should be obtained in all suspected cases of shigellosis.
  • The yield from stool cultures is greatest early in the course of disease. Guidelines for obtaining specimens to improve the yield are as follows:
    • Process specimens immediately after collection.
    • If processing is delayed, use a transport medium (eg, buffered glycerol saline).
    • Collect more than 1 stool or rectal (not anal) swab and inoculate them promptly on at least 2 different culture media.
    • Specimens should be plated lightly onto MacConkey, xylose-lysine-deoxycholate, Hektoen enteric, or Salmonella-Shigella, or eosin-methylene blue agars.
  • If processing is delayed, a rectal-swab sample can be placed in Cary-Blair transport medium or buffered glycerol saline.
  • After overnight incubation, colorless, nonlactose-fermenting colonies may be tested by means of latex agglutination to establish a preliminary identification of Shigella infection.
  • Antimicrobial susceptibility tests of all confirmed isolates should be performed by using the agar diffusion technique. The agar and broth-dilution methods are also widely used. The new Epsilometer strip method (E test) is used to accurately determine the minimum inhibitory concentration (MIC).
  • Despite meticulous care in obtaining and processing specimens from patients infected with Shigella species, approximately 20% may fail to yield Shigella organisms.
• An enzyme immunoassay for Stx is used to detect S dysenteriae type 1 in the stool.
• With rapid techniques, gene probes or polymerase chain reaction (PCR) primers are directed toward virulence genes (invasion plasmid locus, ipl).

TREATMENT

Section 6 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Medical Care

The clinician should rapidly assess the patient's fluid and electrolyte status and institute parenteral or oral hydration along with antipyretics as needed. Prompt recognition and treatment of seizures and raised intracranial pressure are essential. Nutritional supplementation of vitamin A 200,000 IU can hasten clinical resolution in malnourished children.

Surgical Care

Surgical care may be required for complications (eg, intestinal perforation).

Consultations

• Consult a neurologist if seizures and altered sensorium predominate.
• Consult a nephrologist if HUS is suspected (eg, for patients with anemia, thrombocytopenia, oliguria, and renal failure).

Diet

The diet may need to be restricted according to the severity of the disease.

Activity

No restrictions are necessary.

MEDICATION

Section 7 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

A variety of antimicrobial agents are effective for the treatment of shigellosis, though options are becoming limited because of globally emerging drug resistance. Resistance of Shigella species to sulfonamides, tetracyclines, ampicillin, and trimethoprim-sulfamethoxazole (TMP-SMX) exists worldwide, and these agents are not recommended as empirical therapy. Most clinical infections with S sonnei are self-limited (48-72 h) and may not require antimicrobial therapy. Treatment is recommended for patients with severe disease and those with underlying immunosuppressive conditions.

At present, no US Food and Drug Administration (FDA)–approved vaccines are available.

Antibiotic treatment decreases the duration of illness, person-to-person spread, and cases in household contacts. Treatment in malnourished children (eg, in developing countries) is likely to reduce the risk of worsening malnutrition morbidity after shigellosis. In persons infected with S dysenteriae type 1, early administration of effective antibiotics is decreases Stx concentrations in the stool and lowers the risk of HUS. However, the risk of HUS caused by E coli, O157-H7 may be increased with the early administration of antibiotics. Prophylactic antibiotics are not recommended for contacts.

Antidiarrheal medications (diphenoxylate hydrochloride with atropine [Lomotil] or loperamide) should not be used because of the risk of prolonging the illness.

A child with typical dysentery that responds to initial empirical antibiotic treatment should continue taking the same drug for a full 5-day course, even if the stool culture is negative.

Drug Category: Antibiotics

Ampicillin and TMP-SMZ are effective for susceptible strains; amoxicillin is less effective than this because of its rapid absorption high in the GI tract. The oral route is preferred except for seriously ill patients. In the United States, sentinel surveillance data from 1999-2000 indicated that 54% of S sonnei and 47% of S flexneri organisms were resistant to ampicillin and TMP-SMZ. Ampicillin (but not amoxicillin) is still the drug of choice if the isolate is susceptible to this drug.

If ampicillin and TMP-SMZ resistant strain is isolated or if susceptibility is unknown, parenteral ceftriaxone sodium, a fluoroquinolone (eg, ciprofloxacin, ofloxacin), or azithromycin dihydrate are the drugs of choice. Fluoroquinolones are typically not administered to children and adolescents younger than 18 years unless other antibiotic choices are not suitable.

FOLLOW-UP

Section 8 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Deterrence/Prevention

• For individuals who travel to highly endemic areas, it is recommended that all fruits and vegetables be washed, peeled, and/or cooked (see the CDC Web site).
• In developed countries, person-to-person transmission is the most common source of infection. In developing countries, water contaminated with human waste is the most common source for infection.
• Encourage prolonged breast-feeding in infants because the incidence of disease is markedly decreased in breastfed babies.
• The following measures help prevent person-to-person transmission of Shigella species:
• • Education of families and child-care centre personnel in hand-washing techniques, especially after toilet use
  • Avoidance of food preparation by personnel who change diapers in daycare centers
  • Exclusion febrile children with diarrhea from daycare centers
  • Proper handling and refrigeration of food, even after cooking
  • Use of universal precautions and isolation of persons with diarrhea in institutions and hospitals
  • Exclusion of children with documented Shigella gastroenteritis from child-care centers until 2 stool cultures are negative

Complications

• Dehydration is the most common complication of shigellosis.
• Other reported complications include the following:
• • CNS complications
  • • Seizures previously thought to be caused by the elaboration of Stx. The etiology is presently uncertain.
    • Syndrome of inappropriate secretion of antidiuretic hormone with profound hyponatremia
    • Lethargy, meningismus, delirium, seizures, and hypoglycemia
    • Encephalopathy and meningitis (rare and may be lethal)
  • HUS associated with strains that produce Stx (eg, S dysenteriae serotype 1 and S flexneri 2a)
  • Septicemia and disseminated intravascular coagulation, particularly in malnourished children
  • Reiter syndrome and arthritis, most commonly in adults with HLA-B27 histocompatibility antigen (occurs 2-5 wk after enteritis)
  • GI complications
  • • Cholestatic hepatitis
    • Rectal prolase
    • Toxic megacolon
    • Pseudomembranous colitis
    • Protein-losing enteropathy
  • Other manifestations
  • • Conjunctivitis, iritis, corneal ulcers, cystitis, myocarditis, and vaginitis (uncommon)
    • Ekiri syndrome (rare syndrome of extreme toxicity, seizures, hyperpyrexia, headache; can be rapidly fatal due to brain edema)

Prognosis

• Most patients recover even without treatment, though illness is more prolonged and severe if not treated.
• • The fever usually defervesces within 24 hours.
  • Frequency of stool deceases within 2-3 days.
• The overall mortality rate in developed countries is less than 1%. In the Far East and Middle East, the mortality rates for infections of S dysenteriae may be as high as 20-25%.
• Severely-malnourished children with shigellosis and hypoglycemia, hypothermia, altered consciousness, and/or bronchopneumonia are at high risk of dying.

MISCELLANEOUS

Section 9 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Medical/Legal Pitfalls

• Shigella infections may be misdiagnosed as meningitis or meningoencephalitis.
• Shigella infection may be misdiagnosed or not confirmed if specimens are not processed without delay and selective media are not used for culture. More than 1 stool culture or rectal swab should be obtained and promptly inoculated onto more than 1 type of culture medium.

REFERENCES

Section 10 of 10

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

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Article Last Updated: Sep 12, 2006