iseases of the gastrointestinal tract omponents of digestive tract (and sites of infectio ormal flora and other protective mechanisms athogenesis many candidates: notice all of the tables in th chapter!
Feb 25, 2016
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