Differential Diagnoses of Dysentery
The primary differential diagnoses of dysentery include Shigella species (most common bacterial cause), Salmonella, Campylobacter, Shiga toxin-producing E. coli (STEC), Entamoeba histolytica, and less commonly Yersinia enterocolitica, with Shigella being the most likely pathogen requiring empirical treatment when microscopy is unavailable. 1, 2
Primary Bacterial Causes
Shigella species remain the most frequent bacterial cause of dysentery worldwide and should be the initial diagnostic and therapeutic target 1, 2, 3. The dysentery syndrome is characterized by frequent, scanty stools with visible blood, fever, severe abdominal cramps, and tenesmus 4. When microscopy is unavailable or definite amebic trophozoites are not identified, empirical treatment for shigellosis is recommended 1.
Salmonella enterica subspecies causes dysentery with bloody diarrhea and systemic symptoms 1, 2. Blood cultures should be obtained in patients with signs of septicemia, as bacteremia is more common with Salmonella than other enteric pathogens 4, 5.
Campylobacter species are among the top three bacterial causes of dysentery globally 2, 6. These infections can present with bloody diarrhea and are associated with post-infectious complications including reactive arthritis and Guillain-Barré syndrome 4.
Shiga toxin-producing E. coli (STEC), particularly O157:H7, is a critical diagnosis in industrialized countries 1, 2. When clinical or epidemic history suggests STEC, diagnostic approaches must detect Shiga toxin or encoding genes and distinguish E. coli O157:H7 from other STEC serotypes 1. Approximately 65% of STEC O157 patients have peripheral white blood cell counts >10,000 cells/µL, and about 10% develop hemolytic uremic syndrome without bloody diarrhea 1.
Parasitic Causes
Entamoeba histolytica must be considered, particularly in developing countries and rural/periurban areas 1, 2. Amebic dysentery presents with persistent or chronic symptoms lasting weeks to months, with visible blood, mucus, and semiliquid stool consistency 4. Stool microscopy should identify trophozoites, with care taken to distinguish them from white blood cells, as amebic dysentery tends to be misdiagnosed 1. Visible blood appears in 39% of E. histolytica cases, but fecal leukocytes are less prominent (7%) compared to Shigella (39% with >50 leukocytes/high power field) 7.
Less Common But Important Causes
Yersinia enterocolitica should be tested in patients with persistent abdominal pain, especially school-aged children with right lower quadrant pain mimicking appendicitis (mesenteric adenitis), and those with fever and epidemiologic risk including exposure to raw or undercooked pork products 1, 2, 6.
Clostridium difficile may be considered in patients >2 years with healthcare-associated diarrhea or diarrhea following antimicrobial use, though bloody stools are not an expected manifestation 1.
Cytomegalovirus should be considered in immunocompromised patients presenting with dysentery 2, 6.
Other pathogens to consider based on epidemiological clues include Vibrio species (exposure to brackish water), Aeromonas, Plesiomonas, and Schistosoma mansoni 1, 2.
Diagnostic Approach
Stool testing should be performed for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC in patients with fever, bloody or mucoid stools, severe abdominal cramping or tenderness, or signs of sepsis 1, 5. A single diarrheal stool specimen (one that takes the shape of the container) is optimal for diagnosis 1, 5.
For STEC detection, use sorbitol-MacConkey agar or chromogenic agar to screen for O157:H7, plus Shiga toxin detection for non-O157 serotypes 1, 5. Shigella dysenteriae type 1 also produces Shiga toxin and should be considered, especially with international travel history 1.
Stool microscopy for E. histolytica trophozoites is essential when available 1. If microscopy shows no definite trophozoites after two different antibiotics for shigellosis fail, then treatment for amebiasis should be considered 1.
Critical Clinical Distinctions
Bacterial dysentery typically presents acutely with rapid onset, high fever, and pronounced systemic symptoms 4. Amebic dysentery is more insidious, with persistent or chronic presentation over weeks to months 4.
Blood cultures are indicated in infants <3 months, patients with signs of septicemia, suspected enteric fever, immunocompromised patients, and those with travel to enteric fever-endemic areas 1, 4, 5.
Common Pitfalls
Do not use antibiotics empirically in STEC infections, particularly those producing Shiga toxin 2, as this increases the risk of hemolytic uremic syndrome 4. The exception is infants <3 months with suspected bacterial etiology, or immunocompetent individuals with documented fever, abdominal pain, bloody diarrhea, and presumed bacillary dysentery due to Shigella 4.
Resistant shigellosis remains more likely than amebiasis even after treatment failure with two antibiotics 1. Periodic antibiotic susceptibility testing is advisable given widespread multidrug-resistant Shigella strains 1.