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
