Diseases of the gastrointestinal tract

Components of digestive tract (and sites of infection)

Normal flora and other protective mechanisms

Pathogenesismany candidates: notice all of the tables in thischapter!

Structure of the digestive system

• Ingestion (mouth)• Digestion (mouth,

stomach, small intestine)• Accessory organs (liver,

pancreas, gall bladder)• Absorption (small, large

intestine• Excretion (large intestine)

The oral cavity

• Many resident bacteria

• Some bacteria cause plaque (e.g., S. mutans, Actinomyces)

• Caries- breakdown of enamel– Periodonal disease– Can cause systemic

complications

Other infections of the mouth

• Mumps– spreads from upper respiratory tract to

salivary glands; can spread to meninges and/or testes

– Vaccine available since 1967 (MMR)• Thrush (C. albicans)• Herpes simplex type 1 (cold sores)

Bacterial infections of the GI tract

“Food poisoning”- toxins, other contaminantstoxins are already formed so onset is rapidTermed intoxication)

S. aureus toxin is problematic because it is heat-stable

Symptoms: diarrhea, pain, nausea, vomitingusually no immunity established

“When in doubt, throw it out”

Bacterial (gastro-)enteritis: organism causes disease, not exotoxin

Diarrhea- small intestine affectedDysentery- large intestine (blood, pus)Enteric fever- systemic

Salmonellamany distinguishable types (serovars)very common; usually spread by improperlyperpared foodsymptoms occur about 48 hours after infectioninvades mucosa in small, large intestines

Shigella- not as invasive as Salmonella, but very contagiousbloody diarrheaS. dysenteriae produces a neurotoxin

Vibrio- cholera outbreaks occur when sanitation isdisrupted

Enterotoxin makes interstines permeable to water;patients lose massive amountsfluid replacement, vaccination

Many other organisms produce enterotoxins (E. coli,Campylobacter, etc.

Intestinal pathogens have different modes of activity

Cholera toxinInvasiveness of Shigella

H. pylori

• First cultured in 1982 (Marshall and Warren)

• Generates ammonia from urea

• Causes peptic ulcers• Linked to chronic

gastritis, stomach cancer

Most infectious enteritis probably caused by viruses

Rotavirus (esp. young children)tends to be seasonal

Norwalk virus (now norovirus)very common in adults2-day incubation periodSee Table 25.11

Poliovirus: introduced by fecal-oral route but doesinfect digestive system

Comparison of hepatitis viruses(now table 25.12)

Parasitic diseases of the digestive system

• Often transmitted from other animals– Food (beef, fish, pork, etc.)– Incidental contact (soil, insects, feces)– Helminths: Flukes, tapeworms (cestodes)

roundworms (nematodes)– Often symptoms are subtle or nonexistent– Can infect diverse tissues– Requires antihelminthic drugs for treatment

Infections by protozoans• Giardia, Cryptosporidium,

Cyclospora, Entamoeba, etc.

• Cyst is hard to eliminate• Clean drinking water is

important• Cyclospora spread on

contaminated food• Have complex life cycles• Invasion of tissues;

inflammation

Fungal toxins can be deadly

• Aflatoxins (moldy grain, peanuts)– Strong carcinogens

• Ergot (rye, wheat) can cause hallucinations; can be medicinal

• Mushroom toxins mainly produced by Amanita; toxic to liver

Summary

• Substantial opportunities for infection• Bacterial/viral: no cure or lasting immunity• Drugs for protozoan or helminthic

infections are toxic• Vaccines limited (and often inappropriate)• Avoidance• Hydration therapy