Disentri Basiler (Shigellosis)

01Pathophysiology

Shigellosis is caused by Shigella, a gram-negative facultative intracellular pathogen with four clinically relevant species: S. sonnei (the most common in developed countries), S. flexneri (the most common worldwide and in developing nations), S. dysenteriae (the most virulent, producing Shiga toxin), and S. boydii (least common). Humans are the only natural reservoir, and the organism spreads through the fecal-oral route by person-to-person contact or contaminated food and water.

The hallmark of Shigella pathogenesis is its remarkably low infectious dose. As few as 10 to 200 organisms are sufficient to cause disease, which explains why it spreads so efficiently in daycare centers, institutions, and areas with poor sanitation. The bacteria survive gastric acid transit and reach the colon, where they are taken up by M cells overlying Peyer's patches. Once translocated to the subepithelial side, the organisms invade colonic epithelial cells from the basolateral surface and replicate intracellularly.

A critical virulence mechanism is actin-based motility. After intracellular replication, Shigella polymerizes host cell actin into "rocket tails" that propel the bacterium into adjacent epithelial cells, allowing direct cell-to-cell spread without exposure to the extracellular immune system. This invasive cycle causes widespread mucosal destruction, ulceration, and an intense neutrophilic inflammatory infiltrate, which is the direct reason the patient develops bloody mucoid diarrhea with fecal leukocytes.

The clinical course follows a biphasic pattern. Early in infection, a secretory enterotoxin and the initial inflammatory response produce watery diarrhea and high fever. As mucosal invasion progresses, the illness transitions to dysentery: frequent small-volume stools containing blood and mucus, accompanied by tenesmus and urgency. This watery-to-bloody progression is a classic vignette pattern.

S. dysenteriae type 1 is uniquely dangerous because it elaborates Shiga toxin (Stx), which binds the Gb3 receptor on host cells and cleaves the 60S ribosomal subunit, halting protein synthesis and causing cell death. When Shiga toxin enters the systemic circulation and damages renal glomerular endothelial cells, the result is hemolytic uremic syndrome (HUS), characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. Additionally, Shiga toxin has neurotoxic properties, which partly explains why seizures can occur in young children with shigellosis, independent of febrile seizure thresholds.

02Classification and Clinical Manifestation

03Diagnostic Workup

The initial approach to any patient presenting with bloody diarrhea and fever is to obtain a stool sample for microscopy and culture. The presence of fecal leukocytes on a wet mount or methylene blue stain immediately narrows the differential to invasive or inflammatory organisms (Shigella, Salmonella, Campylobacter, EHEC, C. difficile, Entamoeba) and distinguishes this from non-inflammatory secretory causes such as cholera or enterotoxigenic E. coli.

The confirmatory study is a stool culture on selective and differential media. Shigella colonies are identified by three defining biochemical features: they are non-lactose fermenting (colorless colonies on MacConkey agar), non-motile (unlike Salmonella), and H2S-negative (unlike Salmonella, which produces H2S on triple sugar iron agar or XLD agar). This triad of characteristics is a frequently tested discriminator. Antibiotic susceptibility testing from the culture isolate is important because Shigella resistance patterns vary geographically, particularly to trimethoprim-sulfamethoxazole and ampicillin.

When HUS is suspected (pallor, petechiae, oliguria following dysentery), the workup should include a peripheral blood smear looking for schistocytes and helmet cells, a platelet count, LDH and haptoglobin (markers of hemolysis), and serum creatinine to assess renal function. Sigmoidoscopy is almost never required to make the diagnosis but, if performed, would reveal diffuse colitis with ulceration concentrated in the rectosigmoid region, reflecting the organism's tropism for the distal colon.

04Management and Treatment

The first and most important step in managing any diarrheal illness, including shigellosis, is fluid and electrolyte replacement. In mild to moderate cases, oral rehydration with WHO-formulated oral rehydration solution is sufficient. Patients who are severely dehydrated, unable to tolerate oral intake, or hemodynamically unstable require intravenous isotonic crystalloid resuscitation.

What distinguishes Shigella from many other causes of bacterial diarrhea is that antibiotics are recommended for all confirmed or strongly suspected cases, regardless of severity. Antibiotic therapy shortens the duration of illness, reduces the period of fecal shedding (and therefore transmission risk, which is critical in daycare and institutional settings), and lowers the risk of complications. This is a testable principle because it contrasts directly with EHEC management, where antibiotics are contraindicated.

For adults, the first-line agent is a fluoroquinolone, typically ciprofloxacin 500 mg orally twice daily for 3 days. Azithromycin 500 mg daily for 3 days is the preferred alternative when fluoroquinolone resistance is suspected or the patient has a contraindication. TMP-SMX may be used if susceptibility is confirmed, but rising global resistance limits its empiric utility.

For children, fluoroquinolones are generally avoided due to concerns about cartilage toxicity in developing joints. The first-line agent is azithromycin at a weight-based dose. For children with severe disease requiring parenteral therapy, ceftriaxone is the drug of choice.

Antimotility agents such as loperamide are contraindicated. Slowing intestinal transit in the setting of invasive colitis prolongs mucosal contact with the pathogen, delays clearance of organisms and toxins, and increases the risk of toxic megacolon and perforation. This is a commonly tested "do not do" principle.

Patients who develop HUS require supportive care in an inpatient setting with careful fluid management, monitoring of renal function, correction of anemia through transfusion if needed, and in severe cases, dialysis. Platelet transfusions are generally avoided unless there is active life-threatening hemorrhage, because they may worsen microvascular thrombosis.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap in questions about shigellosis is confusing its management with that of EHEC (enterohemorrhagic E. coli). Both organisms can cause bloody diarrhea and both can lead to HUS, so students frequently apply the "do not give antibiotics" rule from EHEC to Shigella. This is wrong. Antibiotics are indicated for Shigella and contraindicated for EHEC. The reasoning is different: in EHEC, antibiotic-induced bacterial lysis releases a surge of Shiga-like toxin that increases the risk of HUS, whereas in Shigella, the benefit of reducing invasion, shedding, and transmission outweighs that theoretical concern. If a vignette describes bloody diarrhea with HUS and asks about antibiotics, the answer hinges entirely on whether the organism is Shigella or EHEC.

A second trap involves the seizure presentation in children. A vignette may describe a febrile child who seizes and also has diarrhea. Students may anchor on febrile seizure or meningitis and overlook shigellosis. The key clue is bloody or mucoid stools in the history. Shigella should always be on the differential for seizures in a febrile child with dysentery, because Shiga toxin has direct neurotoxic effects.

Third, do not forget the association between S. flexneri and reactive arthritis (the classic triad of arthritis, urethritis, and conjunctivitis in an HLA-B27-positive young adult presenting weeks after a diarrheal illness). The vignette will usually separate the diarrheal episode from the joint symptoms by a gap of 1 to 4 weeks, and the stool culture at the time of arthritis may already be negative.

Finally, remember the culture characteristics that separate Shigella from Salmonella, because a vignette may hinge on a single laboratory detail. Shigella is non-motile and H2S-negative; Salmonella is motile and H2S-positive. Both are non-lactose fermenters. When a question describes gram-negative rods that are non-lactose-fermenting and then provides one additional biochemical result, that single result is the intended discriminator.

The core competency being tested across all Shigella questions is the ability to distinguish invasive dysentery from other causes of bloody diarrhea, correctly apply antibiotic therapy (in contrast to EHEC), and recognize the organism's unique complications and culture profile.