19.1 Gastrointestinal Infections I: Gastroenteritis & Enterocolitis, Cholera, Typhoid & Salmonellosis, Shigellosis, Food Poisoning and Viral Hepatitis

Why this area matters

The TOS gastrointestinal area lists:

• enterocolitis and gastroenteritis;
• hepatitis;
• typhoid fever and salmonellosis;
• shigellosis;
• cholera;
• intestinal parasites, covered in the next section.

Acute diarrhea is among the leading causes of illness in Filipino children. Typhoid, cholera and hepatitis A outbreaks follow contamination of water supplies, especially after typhoons and floods. The Philippines also has a high prevalence of chronic hepatitis B. Board questions use the stool character, incubation period and food history to point to an organism, then ask about rehydration, antibiotics or vaccines.

A syndromic approach to diarrhea

Enterocolitis is inflammation of the small and large bowel, typical of Salmonella, Campylobacter, Yersinia (which can mimic appendicitis) and C. difficile. Gastroenteritis emphasizes vomiting and watery diarrhea, typical of viruses and preformed toxins.

Viral gastroenteritis

• Rotavirus:
  • a double-stranded RNA virus (Reoviridae) with a wheel-like appearance;
  • the classic cause of severe dehydrating diarrhea in infants and young children;
  • prevented by oral rotavirus vaccine.
• Norovirus: causes explosive outbreaks of vomiting and diarrhea in all ages, in schools, ships and institutions. Very few virions are infectious, and alcohol hand rub works poorly against it.
• Enteric adenovirus (types 40 and 41) and astrovirus cause milder disease in children.

Cholera

• Vibrio cholerae serogroups O1 (classical and El Tor biotypes) and O139 cause epidemic cholera.
• Patients have painless rice-water stools and vomiting, with rapid hypovolemic shock, hypokalemia and acidosis but no fever.
• Diagnosis: dark-field microscopy shows darting motility, and stool culture on TCBS gives yellow colonies.
• Rehydration follows the WHO plans:
  • Plan A (no dehydration): home fluids and ORS.
  • Plan B (some dehydration): ORS 75 mL/kg over 4 hours.
  • Plan C (severe dehydration): Ringer's lactate 100 mL/kg IV. Give 30 mL/kg in the first 30 minutes, then 70 mL/kg over 2.5 hours, in patients aged 1 year or older. Infants get the same volumes over 1 hour and 5 hours.
• Antibiotics shorten diarrhea and shedding: single-dose doxycycline in adults, and azithromycin in children and pregnant women.
• Zinc is given to children with diarrhea.
• Prevention relies on safe water, sanitation and oral cholera vaccine in outbreaks.

Typhoid fever and salmonellosis

• Salmonella Typhi:
  • infects humans only and spreads through contaminated food and water;
  • invades Peyer patches of the terminal ileum and survives within macrophages, then spreads to the liver, spleen, bone marrow and gallbladder.
• Clinical course:
  • Week 1: step-ladder fever, headache, abdominal pain, constipation or diarrhea, and relative bradycardia.
  • Week 2: rose spots on the trunk, hepatosplenomegaly and a coated tongue.
  • Week 3: complications such as intestinal perforation and hemorrhage, typhoid encephalopathy and myocarditis.
• Diagnosis:
  • blood culture is the most useful test in the first week;
  • bone marrow culture is the most sensitive, even after antibiotics;
  • stool and urine cultures turn positive later.
  • The Widal test has poor specificity in endemic areas, and rapid antibody tests are only supportive.
• Treatment:
  • azithromycin for uncomplicated disease, or ceftriaxone for severe disease, guided by local susceptibility because fluoroquinolone resistance is widespread in Asia;
  • surgery for perforation.
• Chronic carriers harbor the organism in the gallbladder, often with gallstones, and are a source of outbreaks; food handlers need clearance.
• Prevention: typhoid Vi polysaccharide or typhoid conjugate vaccine, safe water and food hygiene.
• Non-typhoidal Salmonella (from poultry, eggs and reptiles) causes enterocolitis. Bacteremia and osteomyelitis occur in infants, older adults, people with HIV and people with sickle cell disease. Uncomplicated enterocolitis is usually not treated with antibiotics, which can prolong carriage.

Shigellosis and other invasive bacteria

• Shigella:
  • a very small inoculum is enough, so it spreads person to person in crowded settings;
  • it invades colonic M cells and spreads between cells using actin;
  • S. dysenteriae type 1 makes Shiga toxin and can cause hemolytic uremic syndrome;
  • treatment is rehydration and an antibiotic chosen by local resistance, such as azithromycin, ciprofloxacin or ceftriaxone.
• Campylobacter jejuni comes from undercooked poultry and is the most common antecedent of Guillain-Barré syndrome.
• EHEC (O157:H7) comes from undercooked beef and causes bloody diarrhea with little fever. Avoid antibiotics because of the risk of HUS.
• EPEC causes infant diarrhea through attaching-and-effacing lesions.
• Vibrio parahaemolyticus comes from raw seafood. V. vulnificus comes from raw oysters or seawater wounds and is deadly in people with liver cirrhosis or iron overload.

Clostridioides difficile and Helicobacter pylori

• C. difficile:
  • follows antibiotics (clindamycin, cephalosporins, fluoroquinolones) and hospitalization;
  • toxins A and B cause watery diarrhea, pseudomembranous colitis and toxic megacolon;
  • management is to stop the offending antibiotic and give oral vancomycin or fidaxomicin;
  • alcohol does not kill the spores, so use soap and water.
• H. pylori:
  • a spiral, urease-positive organism that colonizes the gastric antrum;
  • causes chronic gastritis, duodenal and gastric ulcers, gastric adenocarcinoma and MALT lymphoma;
  • diagnosed by urea breath test, stool antigen or biopsy urease test;
  • treated with a proton pump inhibitor plus two antibiotics, or bismuth quadruple therapy.

Food poisoning by incubation period

Viral Hepatitis: Comprehensive Serological Differentiation

Viral hepatitis encompasses five distinct hepatotropic viruses, each with unique taxonomic, transmission, and chronicity profiles:

Deciphering the Hepatitis B Serology Panel

Interpretation of the standard HBV serologic panel is a high-yield competency on the PLE:

The Serologic Window Period: In resolving acute HBV infection, HBsAg becomes undetectable in the serum before neutralizing anti-HBs antibodies reach detectable thresholds. During this diagnostic "window period" (which can persist for weeks), the only detectable marker of acute infection is Anti-HBc IgM.

Philippine notes on hepatitis:

• The Philippines has a high burden of chronic hepatitis B, mostly acquired at birth or in early childhood. For this reason, the National Immunization Program gives the hepatitis B birth dose within 24 hours of birth, followed by the pentavalent vaccine series.
• Infants of HBsAg-positive mothers also receive hepatitis B immune globulin.
• Hepatitis A outbreaks follow contaminated water or food.
• Chronic hepatitis C is curable with direct-acting antivirals.