Acute inflammatory diarrhea from invasive enteric bacteria; supportive care is the backbone, antibiotics reserved for severe or high-risk cases (and avoided in EHEC).
Also known as: bacterial gastroenteritis, Salmonella, Shigella, Campylobacter, EHEC, STEC, O157:H7, dysentery, food poisoning
Overview
Acute infectious enteritis caused by invasive or toxigenic gram-negative bacilli that produce inflammatory diarrhea, typically with fever, abdominal cramps, and occasionally bloody stools. The four most board-relevant pathogens are nontyphoidal Salmonella, Shigella species, Campylobacter jejuni, and pathogenic Escherichia coli — particularly enterohemorrhagic (EHEC/STEC) O157:H7.
Epidemiology
Foodborne illness causes ~48 million US cases annually (CDC). Campylobacter and Salmonella are the most frequently confirmed bacterial enteric pathogens. Outbreaks linked to undercooked poultry (Campylobacter, Salmonella), eggs and reptiles (Salmonella), ground beef and leafy greens (EHEC), and person-to-person spread in daycares and institutions (Shigella).
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Question 1Infectious DiseaseMedium
A 6-year-old boy is brought to the emergency department with 3 days of severe crampy abdominal pain and visibly bloody diarrhea. He attended a family barbecue 4 days ago where he ate a hamburger. He is afebrile (temperature 37.1°C). Examination shows diffuse abdominal tenderness without rebound or guarding and mildly dry mucous membranes. A complete blood count and basic metabolic panel are pending. Which of the following is the most appropriate next diagnostic test?
AStool Shiga toxin EIA assay
BStool ova and parasite exam
CAbdominal and pelvic CT scan
DStool C. difficile toxin PCR
Reveal answer & full explanation
Correct answer: A — Stool Shiga toxin EIA assay
AStool Shiga toxin EIA assay✓
BStool ova and parasite exam
CAbdominal and pelvic CT scan
DStool C. difficile toxin PCR
Why Stool Shiga toxin EIA assay is correct
Bloody diarrhea without fever after eating ground beef is the classic presentation of enterohemorrhagic E. coli (EHEC/STEC O157:H7), which releases Shiga-like toxin.
IDSA guidelines recommend a stool Shiga toxin assay (EIA or PCR) on any patient with bloody diarrhea, because identifying STEC changes management: antibiotics and antimotility agents are avoided since they increase the risk of hemolytic-uremic syndrome.
Confirming EHEC also triggers monitoring of CBC and creatinine through about day 10 for HUS (microangiopathic hemolytic anemia, thrombocytopenia, acute kidney injury).
Why the others are wrong
Stool ova and parasite exam fits subacute, travel- or daycare-associated watery diarrhea from Giardia or Entamoeba; parasites rarely cause this acute, ground-beef-linked bloody illness.
Stool C. difficile toxin PCR is indicated with recent antibiotic or healthcare exposure and watery diarrhea, neither of which is present here.
Abdominal and pelvic CT scan is reserved for peritoneal signs, severe pain, or suspected toxic megacolon or perforation; this child has no peritoneal findings, so it is not the next step.
Question 2Infectious DiseaseMedium
A 6-year-old boy is brought in with 3 days of severe crampy abdominal pain and grossly bloody diarrhea that began after a backyard barbecue where he ate an undercooked hamburger. He is afebrile with a temperature of 37.1°C (98.8°F). Abdominal exam shows diffuse tenderness without peritoneal signs. A stool viral PCR panel is negative, fecal leukocytes are present, and a Shiga toxin assay returns positive. The clinician counsels the family that antibiotics will be withheld to reduce the risk of hemolytic-uremic syndrome. Which of the following best explains the findings?
AMolecular mimicry against neuronal gangliosides
BShiga-like toxin damage to vascular endothelium
CEnterocyte invasion via a type III secretion system
DDirect colonic epithelial invasion through M cells
Reveal answer & full explanation
Correct answer: B — Shiga-like toxin damage to vascular endothelium
AMolecular mimicry against neuronal gangliosides
BShiga-like toxin damage to vascular endothelium✓
CEnterocyte invasion via a type III secretion system
DDirect colonic epithelial invasion through M cells
Why Shiga-like toxin damage to vascular endothelium is correct
The vignette is classic enterohemorrhagic E. coli (EHEC/STEC, typically O157:H7): bloody diarrhea with a notably low-grade or absent fever after undercooked ground beef, plus a positive Shiga toxin assay.
EHEC adheres to colonic epithelium and releases Shiga-like toxin, which damages vascular endothelium to produce hemorrhagic colitis and, in children, hemolytic-uremic syndrome (microangiopathic hemolytic anemia, thrombocytopenia, acute kidney injury).
Antibiotics are withheld because they can increase toxin release and HUS risk; antimotility agents are also avoided.
Why the others are wrong
Enterocyte invasion via a type III secretion system is the mechanism of nontyphoidal Salmonella (invading enterocytes and Peyer patches, with bacteremia in immunocompromised or sickle cell hosts), not the toxin-mediated endothelial injury seen here.
Direct colonic epithelial invasion through M cells describes Shigella, which causes dysentery with high fever and tenesmus; this patient is afebrile and the Shiga toxin assay points to EHEC.
Molecular mimicry against neuronal gangliosides is the Campylobacter pathway linked to Guillain-Barre syndrome, an antecedent-infection complication, not the cause of HUS-prone hemorrhagic colitis.
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Ingestion of undercooked poultry, eggs, ground beef, unpasteurized dairy, or contaminated produce
Travel to areas with poor sanitation
Daycare attendance and crowded living (Shigella)
Reptile or amphibian exposure (Salmonella)
Immunocompromise, sickle cell disease (invasive Salmonella, osteomyelitis), age <5 or >65
PPI or H2-blocker use (reduced gastric acid barrier)
Pathophysiology
Salmonella invades enterocytes and Peyer patches via type III secretion, with potential bacteremia in immunocompromised hosts and sickle cell patients (osteomyelitis). Shigella invades colonic epithelium directly via M cells and produces Shiga toxin (S. dysenteriae type 1), causing dysentery. Campylobacter invades the distal small bowel and colon, sometimes triggering molecular mimicry against gangliosides (Guillain-Barré syndrome). EHEC adheres to colonic epithelium and releases Shiga-like toxin, damaging endothelium and producing hemorrhagic colitis and hemolytic-uremic syndrome (HUS).
Clinical presentation
Symptoms
Crampy abdominal pain, fever, and diarrhea developing 1-3 days after ingestion (longer incubation for Campylobacter)
Bloody or mucoid stools (dysentery pattern) more common with Shigella, EHEC, and Campylobacter
Nausea and vomiting more prominent with Salmonella
EHEC: severe abdominal cramps with bloody diarrhea and notably low-grade or absent fever
Signs / physical exam
Diffuse abdominal tenderness without peritoneal signs
Ischemic colitis — Older adults with vascular disease, sudden abdominal pain followed by bloody diarrhea, watershed areas on imaging
Appendicitis with periappendiceal phlegmon — RLQ pain, fever, anorexia; CT distinguishes from terminal ileitis (which Yersinia and Campylobacter can mimic)
Diagnostic workup
Diagnostic criteria
Clinical syndrome of acute inflammatory diarrhea plus positive stool culture or PCR for the responsible pathogen.
Labs
Stool culture or multiplex GI PCR panel — preferred initial test; identifies Salmonella, Shigella, Campylobacter, STEC
Stool Shiga toxin assay (EIA or PCR) in any bloody diarrhea — essential to identify EHEC
CBC: leukocytosis with left shift in Shigella; falling platelets and rising creatinine raise concern for HUS
BMP for renal function and electrolytes
Blood cultures if febrile or immunocompromised (invasive Salmonella)
Imaging
Not routinely required
CT abdomen if severe pain, peritoneal signs, or toxic megacolon suspected
Treatment
First-line
Aggressive oral or IV rehydration with isotonic fluids and electrolyte replacement
Empiric antibiotics generally NOT indicated for routine community-acquired bacterial enteritis; reserve for severe disease, dysentery, immunocompromise, age extremes, or confirmed Shigella
Avoid antimotility agents (loperamide) in dysentery or suspected EHEC — increased risk of toxic megacolon and HUS
Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.