Diseases of the gastrointestinal tract Components of digestive tract (and sites of infection) Normal flora and other protective mechanisms Pathogenesis many candidates: notice all of the tables in this chapter!
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 contaminants toxins are already formed so onset is rapid Termed intoxication) S. aureus toxin is problematic because it is heat-stable Symptoms: diarrhea, pain, nausea, vomiting usually no immunity established “When in doubt, throw it out”
Bacterial (gastro-)enteritis: organism causes disease, not exotoxin Diarrhea- small intestine affected Dysentery- large intestine (blood, pus) Enteric fever- systemic Salmonella many distinguishable types (serovars) very common; usually spread by improperly perpared food symptoms occur about 48 hours after infection invades mucosa in small, large intestines
Shigella- not as invasive as Salmonella, but very contagious bloody diarrhea S. dysenteriae produces a neurotoxin Vibrio- cholera outbreaks occur when sanitation is disrupted Enterotoxin makes interstines permeable to water; patients lose massive amounts fluid replacement, vaccination Many other organisms produce enterotoxins (E. coli, Campylobacter, etc.
Intestinal pathogens have different modes of activity Cholera toxin Invasiveness 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 adults 2-day incubation period See Table 25.11 Poliovirus: introduced by fecal-oral route but does infect 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