Lower Alimentary Tract Infections

Learning Objectives

• Identify the dominating genus of bacteria that constitute the normal flora in the gastrointestinal tract.

• Differentiate between intoxications and infections with emphasis on individual organisms.

• Differentiate between dysentery and gastroenteritis.

• Describe the major causes of intoxications and gastroenteritis by identifying the endotoxins and exotoxins.

• Identify common sources of gastrointestinal infections.

• Describe some clinical features of viral intestinal infections including norovirus, rotavirus, and Hepatitis A, B, C, D, and E.

Transmission of Infections/Intoxications

• Most gastrointestinal infections are transmitted via the fecal-oral route.

• Breaking the chain of transmission involves:

  • Proper disposal and treatment of sewage.

  • Disinfection of drinking water.

  • Proper food preparation and handling.

• Consideration of feasibility in Canada versus other countries.

Predominant Flora of the Intestine

• The gastrointestinal tract is primarily populated by Gram-negative bacteria, including both anaerobes and aerobes.

• The dominating species include Bacteroides, with over 500 different bacteria expected to be present, many of which are unidentified.

Types of Diseases of the GI Tract

Infections

• Pathogen multiplies in the mucosa or disseminates to other systemic organs.

• Characterized by symptoms such as:

  • Fever

  • Release of endotoxin (lipopolysaccharide, LPS).

Intoxications

• Involves the ingestion of preformed toxins, leading to:

  • Sudden onset of symptoms without fever or systemic symptoms.

• Common result of both types includes diarrhea, a major cause of infant mortality due to dehydration.

Main Pathogenic Mechanisms of Lower Alimentary Tract Pathogens

• Attachment: Bacteria attach to intestinal cells.

• Cell Invasion: Some pathogens may invade host cells.

• Loss of Microvilli: Damage to intestinal microvilli occurs, affecting absorption.

• Exotoxin Production:

  1. Increases secretion of water and electrolytes (e.g. Vibrio cholerae enterotoxin).

  2. Inhibits protein synthesis and causes cell death (e.g. Clostridium difficile cytotoxin).

• Many GI pathogens are Gram-negative and possess both endotoxins and exotoxins.

Dysentery vs Gastroenteritis

Dysentery

• Defined as severe diarrhea containing blood, often pus, and/or mucus.

  • Examples include Entamoeba histolytica and Shigella dysenteriae.

Gastroenteritis

• Inflammation of the stomach and intestinal mucosa without pus.

  • Examples include Vibrio cholerae and Cryptosporidium.

• Common symptoms include:

  • Diarrhea

  • Vomiting

  • Abdominal cramps

  • Nausea

Food Intoxications Overview

Staphylococcal Food Poisoning

• In Canada, the economic cost of food recalls due to S. aureus is estimated at 3.2 million CAD/year.

• S. aureus produces heat-stable enterotoxins, that can survive boiling for up to 30 minutes and are resistant to enzymatic destruction in the GI tract.

• Factors for growth include:

  • Salt tolerance

  • Osmotic stability due to the cell wall

  • Resistance to heat (up to 60°C for 30 minutes).

• Common foods include custards, gravy, cream pie, and ham; adequate refrigeration can help prevent growth.

Staphylococcal Enterotoxin

• Triggers the vomiting reflex.

• Causes abdominal cramps.

• Symptoms typically appear 2-4 hours after ingestion.

• Recovery is usually complete within 24 hours with no lasting side effects.

Bacillus cereus

• Associated with Bacillus cereus tainted bean curd.

• Can present in two forms:

  1. Diarrhea due to enterotoxin produced in the gut (8-16 hr - infection).

  2. Vomiting due to ingestion of the enterotoxin in food (1-6 hr - intoxication).

• Gram-positive, aerobic, spore-forming bacilli.

• Heating may not kill spores; prevalent in rice dishes.

• Considered a self-limiting disease.

Common Infections of the GI Tract

• Most common causes include:

  • Salmonella

  • Shigella

  • Campylobacter

  • E. coli (e.g. ETEC, EIEC, EPEC, EHEC)

  • Clostridium perfringens

  • Bacillus cereus (both intoxication and infection)

  • Vibrio cholerae

  • E. histolytica, Giardia, Cryptosporidium

  • Rotavirus, norovirus, sapovirus

  • Hepatitis A.

Salmonellosis (Salmonella Gastroenteritis)

• Caused by Gram-negative, non-spore-forming, motile bacilli.

• Two main types of Salmonella infections:

  • Gastroenteritis

  • Typhoid fever

• Salmonella enterica enterica and Salmonella enterica typhi are common serotypes.

Incidence and Economic Impact of Salmonellosis

• Approximately 1 in 6 people (about 48 million) get sick each year from contaminated food in the U.S.

• Salmonella infections account for an estimated 365 million CAD in direct medical costs each year.

Specific Features of Salmonella Gastroenteritis

• There are over 2000 serotypes responsible for human diseases (zoonotic).

• Symptoms typically take 12-36 hours post-exposure to develop.

• Bacteria are sensitive to gastric acid; a high bacterial load is required for infection.

• Common sources include meat products, eggs, poultry, and reptile skin; S. enteritidis can infect chickens leading to bacteria inside the eggs.

• Treatment usually does not involve antibiotics due to the risk of prolonging bacterial shedding.

• Currently, there is no vaccine available.

Typhoid Fever

• Caused specifically by S. typhi.

• Characterized by different pathogenesis and transmitted only from human to human.

• Main symptoms include:

  • High fever (≥ 40°C)

  • Headaches

  • Diarrhea beginning in the second or third week, not the primary symptom.

• Results in sepsis (enteric fever); bacteria multiply in phagocytic cells, rapidly disseminating to the bloodstream.

  • Chronic carriers can emerge post-recovery, as seen in cases like Typhoid Mary.

• Treatment involves antibiotics such as ceftriaxone.

Shigellosis

• This is an infection caused by Gram-negative bacilli, which are non-motile.

• The four primary species are:

  • Shigella dysenteriae (A)

  • Shigella boydii (B)

  • Shigella flexneri (C)

  • Shigella sonnei (D)

• Severity varies by the infecting strain; more virulent strains may have plasmids essential for attachment and entry into cells.

• Transmission occurs from person-to-person, lacking an animal reservoir.

• The bacteria produce Shiga toxin, a key virulence factor responsible for their cytotoxic effects.

Campylobacter Gastroenteritis

• A facultatively anaerobic, Gram-negative, spiral-shaped bacillus.

• It is the leading cause of foodborne bacterial illness in the U.S. and Canada, with an estimated 4 million cases in Canada annually.

• Common in poultry and cattle as commensals; the disease is typically self-limiting.

• Preventative measures include hygienic practices related to the handling of poultry and monitoring water sources.

• Complications may include Irritable Bowel Syndrome (IBS) and Guillain-Barré Syndrome.

• Currently, there is no vaccine available.

Escherichia coli Gastroenteritis

• Most strains are harmless and normal residents of the GI tract, but pathogenic strains may have specialized fimbriae enabling attachment and produce toxins.

• Pathogenic strains include:

  • Enterotoxigenic E. coli (ETEC)

  • Enteroinvasive E. coli (EIEC)

  • Enteroadhesive E. coli (EAEC)

  • Enteropathogenic E. coli (EPEC)

  • Enterohemorrhagic E. coli (EHEC)

Enterohemorrhagic E. coli (EHEC)

• This strain is increasingly recognized in developed nations and is known for causing “Hamburger Disease.”

• The associated healthcare costs in Canada are around 240 million CAD.

• The strain O157:H7 is well known for producing Shiga-like toxins, leading to hemorrhagic colitis and hemolytic uremic syndrome (HUS).

HUS Complications

• Hemolytic uremic syndrome may result in kidney failure.

• Estimated that 5-10% of small children infected with EHEC develop HUS, with a mortality rate of 5%.

• Common sources include poorly handled meat, poultry, raw alfalfa sprouts, and contaminated water.

• Most cases are self-limiting in adults, but children and the elderly are at higher risk for complications.

• Treatment generally focuses on rehydration and supportive care without a vaccine available.

Clostridium Gastroenteritis

• Clostridium perfringens (Gram-positive, spore-forming, anaerobic bacilli) can proliferate in the intestinal tract, producing an exotoxin that leads to abdominal pain and diarrhea.

• Most cases are mild, presenting 8-12 hours post-ingestion.

• Severe cases may occur in individuals lacking trypsin or consuming diets rich in trypsin inhibitors (such as soybeans), potentially resulting in Pigbel (gangrene of intestines).

• Associated with meat dishes (stews) and generally not treated with antibiotics; there is no vaccine.

Vibrio cholerae

• A facultatively anaerobic, Gram-negative 'vibrio' (comma-shaped) bacterium.

• Capable of causing over 20 liters of fluid loss per day, presenting as “rice water stool.”

• Prevalent in fecally contaminated water and a major source for infection.

• Known for causing global pandemics, including the 2010 Haiti outbreak that saw 650,000 infections and over 8,000 deaths.

• Key virulence factors include the cholera enterotoxin (heat-labile) and TCP (toxin co-regulated pili).

• A vaccine exists with 40-60% effectiveness (oral vaccine Dukoral), and treatment primarily involves rehydration with glucose and electrolytes.

Viral Gastroenteritis

• Clinical infections are noted globally, with severe implications in developing countries as a leading cause of death in children.

• Distinctions from other gastroenteritis causes are typically non-specific clinically.

• Predominant pathogens include rotavirus and norovirus, both transmitted via fecal-oral routes.

Rotavirus Gastroenteritis

• RNA virus, wheel-shaped, and non-enveloped.

• Primarily affects children, with most cases occurring in those under 2 years of age.

• Incubation period is 2-3 days with symptoms including low-grade fever, diarrhea, and vomiting for about a week.

• There are new vaccines available, e.g., Rotorix® and Rotateq®.

Norovirus Gastroenteritis

• RNA, small round, non-enveloped virus.

• Major contributor to epidemics of viral gastroenteritis, responsible for 2/3 of food poisoning cases.

• Has a 2-day incubation period and symptoms similar to rotavirus with sudden onset, typically self-limiting and lasts 1-3 days.

Protozoan Diseases

• Commonly associated with diarrhea:

  • Giardia lamblia

  • Cryptosporidium

  • Entamoeba histolytica

Helminthic Diseases

• Typically present without diarrhea:

  • Tapeworms (cestodes)

  • Roundworms (nematodes)

Hepatitis Overview

• Infections can lead to inflammation of the liver caused by various viruses:

  • Hepatitis A

  • Hepatitis B

  • Hepatitis C

  • Hepatitis D

  • Hepatitis E

• CMV and EBV can also cause infectious hepatitis.

Hepatitis A (HAV)

• RNA, non-enveloped virus, transmitted fecal-orally via contaminated water and mollusks.

• The virus is resistant to chlorine and has an incubation period of 2-4 weeks.

• Pathogenesis involves multiplication in intestinal epithelial cells, followed by infection of the liver and excretion in feces.

• Symptoms include nausea, fever, anorexia, vomiting, and jaundice, often with sudden onset.

• Generally self-limiting with no chronic form; 50% of infections are subclinical.

• Diagnosis is made by detecting HAV IgM antibodies in serum, and lifelong immunity exists after infection, for which a vaccine is available.

Hepatitis B (HBV)

• Enveloped DNA virus from the Hepadnaviridae family.

• Transmitted via sexual contact or as a bloodborne disease; capable of causing both acute and chronic diseases:

  • Acute: liver inflammation, vomiting, jaundice.

  • Chronic: cirrhosis and hepatocellular carcinoma (liver cancer).

• A vaccine is available, and treatments include antivirals such as lamivudine, Harvoni, and Sovaldi (costly lifetime treatments averaging 50,000-94,500 CAD).

Hepatitis C (HCV)

• Enveloped RNA virus with bloodborne transmission.

• Like HBV, it can induce both acute and chronic disease, with 80% of acute cases leading to chronic infection.

• Chronic manifestations are similar to those of HBV, including cirrhosis and liver cancer.

• There is no vaccine available; however, new antivirals have been approved since 2011.

Hepatitis D Virus (HDV)

• Enveloped RNA virus that acts as a satellite virus, relying on HBV for its replication.

• Transmission parallels that of HBV and disease can only occur in the presence of HBV.

Hepatitis E Virus (HEV)

• Non-enveloped RNA virus within the Caliciviridae family, transmitted via the fecal-oral route, similar to HAV.

• Endemic in areas with poor sanitation, particularly in India and Southeast Asia; does not cause chronic disease but poses a significant risk to pregnant women (up to 20% mortality).

Summary of Hepatitis Viruses