Shigella Species and Fecal WBC Lab Test

Types of Diarrhea

• Acute bloody diarrhea (also called dysentery) and acute watery diarrhea (including cholera) typically last for several hours or days.
• Persistent diarrhea lasts for several weeks (2-4 weeks).
• Chronic diarrhea lasts for more than 5 weeks.

Pathogenesis/Pathophysiology

• Diarrhea occurs when there are subtle abnormalities in input or output at any level of the GI tract, resulting in excessive osmotic load, increased secretion, or diminished fluid resorption.
• The causes of diarrhea include inflammatory, osmotic, secretory, motility, and other factors.

Common Causes of Diarrheal Diseases/Food Poisoning

• Bacterial causes:
  • Staphylococcus aureus
  • Vibrio cholerae
  • Shigella spp.
  • Escherichia coli
  • Salmonella spp.
  • Campylobacter jejuni
  • Bacillus cereus
  • Clostridium botulinum
  • Yersinia enterocolitica
• Non-bacterial causes:
  • Viruses (e.g., Rotavirus, Norovirus, Adenovirus, Hepatitis A)
  • Parasites (e.g., Giardia lamblia, Entamoeba histolytica)

Invasive Diarrhea

• Caused by bacterial invasion of the bowel mucosal surface
• Organisms involved: Salmonella spp., Campylobacter spp., Shigella spp., E. coli, Vibrio spp.

Shigella Species

• Gram-negative, facultatively anaerobic rods
• Most communicable of the diarrheal bacteria
• Requires a low microbial load
• Symptoms include fever, malaise, fatigue, anorexia, and diarrhea with blood and pus
• Symptoms appear 12-50 hours after exposure

Fecal WBC Lab Test

• Observe blood, WBCs, and pus
• Culture: Colorless colonies on MAC, blue-green colonies on HE, and red/colorless on XLD

Escherichia coli

• Gram-negative, facultative anaerobic rods
• At least five different pathogenic groups cause gastroenteritis:
  • ETEC: Enterotoxigenic (cause of traveler's diarrhea)
  • EIEC: Enteroinvasive (bacteria dysentery)
  • EPEC: Enteropathogenic (diarrhea outbreaks in infants in hospital setting)
  • EHEC: Enterohemorrhagic (presence of Shiga-like toxin)
  • EAEC: Enteroaggregative (chronic diarrhea in HIV patients, travelers, and children in poor countries)

Enterotoxin-Mediated Diarrhea

• Caused by toxins produced by bacteria
• Examples: S. aureus, Clostridium botulinum, Bacillus cereus, Vibrio cholerae, Enterotoxigenic E. coli

Staphylococcus aureus

• Found on the skin, hair, noses, and throats of people and animals
• Can multiply quickly at room temperature and produce a toxin that causes food poisoning
• Sources: Salads, macaroni, bakery products, cream pies, milk, and dairy products

Clostridium perfringens

• Part of the normal flora of the vagina and GIT
• Produces multiple toxins, including alpha toxin that causes lysis of endothelial and blood cells
• Generates spores in improperly cooked foods
• Causes loss of fluid in intracellular protein

Bacillus cereus

• Gram-positive rods that are spore-forming
• Also acquired by penetrating injuries and intravenous injections
• Produces toxins (diarrheal toxin, emetic toxin)
• Incubation period: 1-6 hours
• Source: Soil, and in raw, dried, and processed foods
• Lab diagnosis: Produces hemolytic colonies on blood agar

Vibrio species

• Gram-negative rods, highly motile, facultative anaerobes, with one to three flagella at one end
• Associated with raw or undercooked seafood harvested from contaminated water
• Infects the small intestine
• Enterotoxin activates adenylate cyclase, which initiates an outpouring of fluid into the intestine
• Watery diarrhea (rice water stools) evident

Lab Diagnosis

• Utilize TCBS media (Thiosulphate Citrate Bile Salt Agar)
• Yellow and profuse colonies
• Inhibits colonic flora
• Differentiates sucrose fermenters from species of Vibrio that are non-fermenters
• Stool contains no RBCs or WBCs since it is toxin-mediated and non-inflammatory

Viral Agents

• Rotavirus: Primarily affects children < 5 years old, spread by fecal-oral route, peak incidence from December to June
• Calicivirus: Norovirus, spread by fecal-oral route, outbreaks on cruise ships, cause of stomach flu

Parasitic Infections

• Giardia lamblia: By ingestion of contaminated water or person-person spread, only lives in the host, chronic infection results in malabsorption syndrome
• Entamoeba histolytica: Exists in cyst and trophozoite form, transmitted by feco-oral with food, water, or hand, symptoms include abdominal discomfort, intense pain in the right side, and dysentery

Diagnosis

• Stool Sample: Direct saline wet mount microscopy, Trichrome Stain (for trophozoites), Biopsy
• Histologically, villi appear flattened
• Fluorescent Antibody Tests

Risk Factors

• Number of ingested organisms: Median infectious dose (ID50)
• Achlorhydria: Inadequate stomach acidity
• Reduction in normal flora: Use of antibiotics
• Age, improper food handling, personal hygiene

Laboratory Dx of Gastrointestinal Pathogens

• Specimen Collection and Handling: Collect within 4 days of onset of symptoms, process ASAP, do not refrigerate
• Fecal Leukocytes: Direct microscopic exam to detect presence of WBCs and RBCs
• Routine stool cultures: Test for Salmonella species, Shigella species, and Campylobacter jejuni

Treatment of Diarrhea

• Patients must be watched for dehydration
• Antibiotics are not effective against viral pathogens, give supportive care for hydration
• Antibiotics may shorten illness due to invasive bacteria or an enterotoxin-mediated process
• Antidiarrheal medications: Decrease bowel motility or the acid (Lomotil, Pepto-Bismol), primarily used with enterotoxin-mediated diarrhea or viral gastroenteritis

Prevention

• Key measures to prevent diarrhea include:
  • Access to safe drinking water
  • Use of improved sanitation
  • Hand washing with soap
  • Exclusive breastfeeding for the first 6 months of life
  • Good personal and food hygiene
  • Health education about how infections spread
  • Rotavirus vaccination

This quiz covers the characteristics of Shigella species, a type of diarrheal bacteria, and how to diagnose it through a fecal WBC lab test. Learn about the symptoms and laboratory tests used to identify Shigella.