Tuesday 31 January 2006
Infections via the gastrointestinal tract occur when local defenses are weakened or the organisms develop strategies to overcome these defenses.
Host defenses are weakened by low gastric acidity, by antibiotics that unbalance the normal bacterial flora (e.g., in pseudomembranous colitis), or when there is stalled peristalsis or mechanical obstruction (e.g., in blind loop syndrome).
Enteropathogenic bacteria elicit gastrointestinal disease by a variety of mechanisms:
While growing on contaminated food, certain staphylococcal strains release powerful enterotoxins that cause food poisoning symptoms without any bacterial multiplication in the gut.
V. cholerae and toxigenic E. coli multiply inside the mucous layer overlying the gut epithelium and release exotoxins that cause the gut epithelium to secrete high volumes of watery diarrhea.
Shigella, Salmonella, and Campylobacter invade and damage the intestinal mucosa and lamina propria and so cause ulceration, inflammation, and hemorrhage, clinically manifested as dysentery.
S. typhi passes from the damaged mucosa through Peyer patches and mesenteric lymph nodes and into the bloodstream, resulting in a systemic infection.
Fungal infection of the gastrointestinal tract occurs mainly in immunologically compromised patients. Candida, part of the normal gastrointestinal flora, shows a predilection for stratified squamous epithelium, causing oral thrush or membranous esophagitis, but may also disseminate to the stomach, lower gastrointestinal tract, and systemic organs.
The cyst forms of intestinal protozoa are essential for their transmission because cysts resist stomach acid. In the gut, cysts convert to motile trophozoites and attach to sugars on the intestinal epithelia through surface lectins.
Thereafter, there is wide species variation. Giardia lamblia attaches to the epithelial brush border, whereas cryptosporidia are taken up by enterocytes, in which they form gametes and spores.
Entamoeba histolytica causes contact-mediated cytolysis through a channel-forming pore protein and thereby ulcerates and invades the colonic mucosa. Intestinal helminths, as a rule, cause disease only when they are present in large numbers or in ectopic sites, for example, by obstructing the gut or invading and damaging the bile ducts (Ascaris lumbricoides).
Hookworms may cause iron deficiency anemia by chronic loss of blood sucked from intestinal villi; the fish tapeworm Diphyllobothrium latum can deplete its host of vitamin B12, giving rise to an illness resembling pernicious anemia.
Finally, the larvae of several helminth parasites pass through the gut briefly on their way toward another organ habitat; for example, Trichinella spiralis larvae preferentially encyst in muscle, Echinococcus species larvae in the liver or lung.
viral digestive infections
bacterial digestive infections
fungal digestive infections
parasitic digestive infections
contagious acute gastrointestinal infections
Musher DM, Musher BL. Contagious acute gastrointestinal infections. N Engl J Med. 2004 Dec 2;351(23):2417-27. PMID: 15575058