Amoebiasis

Amoebiasis, also known as amoebic dysentery, is an intestinal infection caused by the parasitic amoeba Entamoeba histolytica. This condition can vary from being asymptomatic to severe, with symptoms such as lethargy, weight loss, colonic ulcerations, abdominal pain, diarrhoea, or bloody diarrhoea.

Complications can include severe colitis, colonic perforation, peritonitis, and anaemia due to prolonged gastric bleeding. The parasite can invade the intestinal lining causing bloody diarrhoea and, if it reaches the bloodstream, it can spread to other organs, most frequently the liver.

Cause and Transmission

Amoebiasis is caused by Entamoeba histolytica, which is primarily transmitted by the faecal-oral route. Infection spreads through ingestion of cysts found in contaminated food and water, and can also be spread indirectly through dirty hands or objects and by anal-oral contact. The trophozoites die quickly outside the body, while the cysts can survive for up to a month in soil or 45 minutes under fingernails.

Signs and Symptoms

Approximately 90% of infected individuals are asymptomatic. However, symptoms can range from mild diarrhoea to dysentery with blood and intense abdominal pain. Extra-intestinal complications such as liver, lung, or brain abscesses can also occur. In invasive cases, amoebae may enter the bloodstream and travel to other organs, most commonly the liver, causing amoebic liver abscesses. Symptoms take between a few days to a few weeks to develop, typically around two to four weeks.

Pathogenesis

Amoebiasis results from tissue destruction induced by E. histolytica, involving host cell death, inflammation, and parasite invasion. The parasite secretes various molecules such as lectins, cysteine proteases, and amoebapores, which aid in attachment, tissue lysis, and phagocytosis of host cells.

Diagnosis

Diagnosis involves stool examination using microscopy to identify cysts or trophozoites, although it can be challenging to distinguish E. histolytica from non-pathogenic species. Serological tests can detect specific antibodies, and immunohistochemical staining can identify trophozoites in tissue samples. More advanced methods like PCR are considered the gold standard but are not widely used due to cost.

Prevention

Preventing amoebiasis involves improving sanitation to prevent contamination of food and water with faeces. Simple measures include washing hands thoroughly, cleaning bathrooms regularly, avoiding raw vegetables in endemic areas, and treating water by boiling or with iodine tablets.

Treatment

Treatment depends on the infection's location. For tissue infections, medications like metronidazole, tinidazole, nitazoxanide, dehydroemetine, or chloroquine are used. For luminal infections, diloxanide furoate or iodoquinoline is prescribed. A combination of medications may be required for effective treatment of all disease stages. Asymptomatic infections may be treated with a single antibiotic, whereas symptomatic infections require two antibiotics.

Prognosis

Most cases of amoebiasis result in amoebas remaining in the gastrointestinal tract, with severe ulceration occurring in less than 16% of cases. Rarely, the parasite invades soft tissues, commonly the liver, leading to abscesses. Other complications include pulmonary amoebiasis, brain abscesses, and cutaneous amoebiasis. Malnutrition and stunting of growth in children are associated with E. histolytica infection.

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Self-assessment MCQs (single best answer)

What is the primary cause of amoebiasis?

How is amoebiasis primarily transmitted?

What is the most common extra-intestinal complication of amoebiasis?

Which of the following is NOT a symptom of amoebiasis?

What is the definitive diagnostic method for identifying Entamoeba histolytica?

Which of the following medications is used for luminal infections of amoebiasis?

How long can Entamoeba histolytica cysts survive under fingernails?

What percentage of individuals infected with Entamoeba histolytica are asymptomatic?

Which of the following is a preventative measure for amoebiasis?

What is a common treatment for tissue infections caused by amoebiasis?