Amoebiasis and Giardia Quiz

Amoebiasis

• A protozoal disease caused by Entamoeba histolytica
• High incidence in tropical countries (Mexico, South America, Southeast Asia, and Africa)
• Global incidence rate is approximately 10%
• Common in crowded urban communities
• Two types of amoeba are found:
  • E. dispar and E. coli are commensal (non-pathogenic) and primarily feed on bacteria.
  • E. histolytica is invasive, larger and actively moving, and contains red blood cells.

Transmission, Life Cycle, and Pathology

• Transmitted by ingestion of E. histolytica cysts, not trophozoites
• Convalescent and asymptomatic carriers are significant sources of infection, as patients with acute dysentery only pass trophozoites (non-infectious).
• Excystation occurs in the lower ileum and caecum.
• A cyst with 4 nuclei can divide to 8 trophozoites through binary fission
• Trophozoites invade tissue and cause tissue damage, leading to necrosis (cell death).

Clinical Manifestations

• Infection sites: Cecum, ascending colon, and sigmoid colon are commonly affected. Small intestine is rarely involved.
• Lesion types: Lesions may be patchy or continuous, starting as discrete erosions that develop into microabscesses and extending into mucosal and muscularis mucosa. This develops into typical flask-shaped ulcers.
• Blood vessels: Blood vessels can be involved, causing thrombosis or bleeding into the intestinal lumen. Portal vein involvement can lead to amoebae reaching the liver.
• Cysts: E. histolytica cysts are not found in tissues.
• Mild: Gradual onset, constipation instead of diarrhea, and presence of scant trophozoites in stool.
• Moderate: Patient unwell, passing blood-stained mucus (mucus and blood) with stools 5-15 times/day (walking dysentery). Patient may appear non-toxic.
• Severe: Toxic-appearing patient; severe diarrhea (more than 15 motions/day); dehydrated, tenesmus and numerous trophozoites and leukocytes in stool. This can lead to toxic megacolon with severe bowel ulceration and dilation.
• Chronic amoebic dysentery: Characterized by vague bouts of abdominal pain with cecum and colon full of gas. This condition includes periodic diarrhea and constipation, similar to IBS.
• Amoeboma: Asymptomatic or tender abdominal mass that can be confused with cancer.

Complications

• Peritonitis
• Bleeding
• Intestinal obstruction
• Intestinal intussusception
• Stricture of colon
• Brain or liver abscess

Diagnosis

• Definitive diagnosis involves demonstration of E. histolytica trophozoites during acute infection and cysts in remission.
• Amoebic fluorescent antibody tests are positive in approximately 96% of invasive amoebiasis cases and 100% in cases with liver abscess.

Treatment

• Metronidazole (Flagyl): Effective against tissue ameobiasis (but not cysts).
  • 200 mg after oral or intravenous administration.
  • Dose: 800 mg three times daily or 5 days.
• Tinidazole: Single-dose 2 g or sustained-release regimen.
  • Can be used in oral infections.
• Amoebic liver abscess requires aspiration to determine presence of trophozoites (which are 80% sensitive).
• Metronidazole 800 mg TDS for 10 days and chloroquine in addition to metronidazole.
• Tinidazole 2g for 3 days and chloroquine according to protocol.

Amoeba Liver Abscess

• Originates from infection of liver by E. histolytica trophozoites that circulate from the colon via the portal system, leading to cell necrosis (destruction), creating thick chocolate-brown pus.
• Only trophozoites are typically visible in the pus and walls of the abscess; cysts are not.
• 50% of patients with abscesses report a prior history of dysentery.
• Single abscess is usual, but multiple abscesses may also be observed; often seen after 5 months.

Giardiasis

• Worldwide occurrence in areas with inadequate hygiene and sanitation.
• Humans are the main reservoir.
• Infection occurs via contaminated drinking water or food (direct fecal-oral spread).
• Cysts survive for weeks outside the human body.

Giardia's Life Cycle

• Characterized by ingestion of cysts
• Cysts excyst in the small intestine, releasing trophozoites.
• Trophozoites multiply in the gut.
• Some trophozoites encyst and are passed in feces.

Other Key Points

• Metronidazole acts on amoeba, Giardia, Trichomonas, other anaerobic microbes, and has partial effects on Leishmania.
• Metronidazole is excreted unchanged via urine and inhibits liver enzymes.
• 70% excreted unchanged in urine.