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Protozoal infections of the GI tract (Small intestine (Giardia lamblia…
Protozoal infections of the GI tract
Small intestine
Giardia lamblia
Frequent cause of travellers' diarrhoea globally
Detected in both drinking and recreational water, and can also be passed person to person
Infective dose is small (10-25 cysts)
Diagnosis by microscopy of stool samples
Life cycle
Stage 1: Trophozoite
Flagellated and bi-nucleated
Lives in the upper part of the small intestine
Adheres to brush border of epithelial cells
Stage 2: Cyst
Formed when trophozoite forms resistant wall
Passes out in stools
Can survive for several weeks
Pathogenesis
Present in the duodenum, jejunum and upper ileum
Attaches to the mucosa via ventral sucker
Does not penetrate the surface
Causes damage to the mucosa and villous atrophy
Leads to malabsorption of food (especially fats and fat soluble vitamins)
May swim up the bile duct to gall bladder
Clinical manifestations
Mild infections are asymptomatic
Diarrhoea is usually self-limiting (7-10 days)
Chronic diarrhoea presents in immunocompromised patients
Stools are characteristically loose, foul smelling and fatty
Treatment
Mepacrine hydrochloride
Metronidazole
Tinidazole
Cryptosporidium parvum
Significance grew during early years of AIDS epidemic
Opportunistic infection
Transmission through faecally-contaminated drinking water
Animal reservoir (usually cattle)
Infective dose as few as 10 oocysts
Life cycle
Asexual and sexual development within host
Ingestion of resistant oocysts
Release of infective sporozoites in small intestine
Invasion of intestinal epithelium
Division to form merozoites which re-infect cells
After sexual phase, oocytes are released
Pathogenesis
Enters cells of the microvillus border of small intestine
Remains within vacuole of epithelial cell
May multiply to give large numbers of progeny, especially in immunocompromised hosts
Clinical manifestations
Moderate to severe profuse diarrhoea
Up to 25 litres of watery faeces/ day
In HIV positive individuals with CD4+ T-cell counts of <100/mm3, diarrhoea is prolonged and may become irreversible and life-threatening
Usually left limiting disease
Treatment
Nitazoxanide
Spiramycin
Large intestine
Entamoeba histolytica
Common in tropical and sub-tropical countries (prevalent in >50% of population)
Transmission
Ingestions of contaminated food or water
Anal sexual activity
Life cycle
Cysts pass through stomach and excyst in the small intestine giving rise to progeny
These adhere to epithelial cells and cause damage mainly through cytolysis
After mucosal invasion, cysts invade red blood cells giving rise to amoebic colitis
Trophozoite stages live in large intestine and pass out as resistant, infective cysts
Pathogenesis
Adheres to epithelium and acute inflammatory cells
Resists host humoral and cell mediated immune defence mechanisms
Produces hydrolytic enzymes, proteinases, collagenase, elastase
Produces protein that lyses neutrophils, the contents of which are toxic to the host
Clinical manifestations
Small localised superficial ulcers leading to mild diarrhoea
Entire colonic mucosa may become deeply ulcerated leading to severe amoebic dysentery
Complications include intestinal perforation
Trophozoites may spread to the liver and other organs
Rarely, abscess spread to the overlying skin
Treatment
Metronidazole