Amebiasis (Amoebic Dysentery) | Pathogenesis, Clinical presentation, Diagnosis & Treatment

Introduction to Amoebiasis

This section provides an introduction to amoebiasis, also known as amoebic dysentery. It discusses the causative agent, Entamoeba histolytica, and its global prevalence.

Amoebiasis Overview

• Amoebiasis is caused by Entamoeba histolytica, a unicellular protozoan.

• The disease is most prevalent in developing nations with poor sanitary facilities.

• Approximately 50 million cases of invasive amoebic disease occur worldwide each year.

• Amoebiasis is the second leading cause of death due to parasitic diseases.

Life Cycle and Transmission

This section explains the life cycle of Entamoeba histolytica and how the disease is transmitted.

Life Cycle and Transmission

• The life cycle of E. histolytica involves two main stages: trophozoite and cyst.

• Trophozoites are the active stage that invades the intestinal mucosa to cause disease.

• Cysts are the infective stage that can survive in the environment for a long time.

• Amoebiasis is mainly transmitted through ingestion of fecally contaminated food and water.

• Asymptomatic individuals who shed cysts in their feces can contaminate food, while house flies can act as mechanical vectors.

• Sexual transmission is also possible, especially through oral sex.

Diagnosis and Clinical Presentation

This section discusses the diagnosis and clinical presentation of amoebiasis.

Diagnosis and Clinical Presentation

• Infected individuals and asymptomatic cyst passers shed both trophozoites and cysts in their feces.

• Both trophozoites and cysts are considered diagnostic stages.

• Ingestion of mature cysts is necessary to cause the disease.

• Trophozoites can multiply in the large intestine, but may not always cause disease.

• Amoebiasis can present as non-invasive colonization, intestinal disease, or extra-intestinal disease.

Pathogenesis and Tissue Invasion

This section explores the pathogenesis of amoebiasis and how Entamoeba histolytica invades tissues.

Pathogenesis and Tissue Invasion

• Parasitic virulence factors, such as adhesion molecules and proteolytic enzymes, enable E. histolytica to bind to host tissues and penetrate the epithelial cell lining.

• The organism reaches the submucosal layer of the colon and forms flask-shaped ulcers with unaffected submucosa in between.

Conclusion

Amoebiasis, caused by Entamoeba histolytica, is a significant global health issue. Understanding its life cycle, transmission, diagnosis, clinical presentation, and pathogenesis is crucial for effective management and prevention strategies.

Macrophages Infiltration and Immune Response

This section discusses the infiltration of macrophages into the affected area and the immune response against amoebiasis.

Macrophage Activation and Killing Process

• Macrophages infiltrate into the affected area due to the secretion of chemical mediators by epithelial cells, including interleukin-1, interleukin-8, and cyclooxygenases.

• Neutrophils and macrophages get activated by interferon gamma secreted by surrounding cells.

• Activated neutrophils and macrophages kill trophozoites through nitric oxide oxidative pathways and non-oxidative pathways.

Impact of Host Immunity on Infection

• The described processes occur only if host immunity is adequate to halt the infection.

• Severely immunocompromised individuals may not be able to contain the infection.

Invasion of Surrounding Tissues

• Trophozoites may secrete proteolytic enzymes and penetrate muscle and serous layers of the colon, reaching the peritoneal cavity.

• Contact-dependent killing of monocytes and macrophages occurs in the peritoneal cavity.

• Trophozoites invade surrounding tissues, causing extra-intestinal amoebiasis such as amoebic liver abscess, peritonitis, pleura pulmonary amoebiasis, pericarditis, and brain abscesses.

Clinical Presentation of Amoebiasis

This section describes the clinical presentation of amoebiasis, including intestinal and extra-intestinal conditions.

Intestinal Amoebiasis

• Incubation period of Entamoeba histolytica is usually two to four weeks.

• Asymptomatic infection and symptomatic non-invasive infection are common forms of intestinal amoebiasis.

• Amoebic colitis is the most common type, characterized by gradual onset, diarrhea, abdominal pain, anorexia, weight loss, and sometimes fever.

• Fulminant colitis is a rare complication with severe bloody diarrhea, abdominal pain, peritonitis, high-grade fever, and potential dehydration.

• Chronic amoebic colitis presents with recurrent episodes of bloody diarrhea and vague abdominal pain similar to inflammatory bowel disease.

Extra-intestinal Amoebiasis

• Extra-intestinal conditions include amoebic liver abscesses, peritonitis, pleura pulmonary amoebiasis, pericarditis, brain abscesses, and cutaneous amoebiasis.

• Amoebic liver abscess is the most common form of extra-intestinal amoebiasis with symptoms such as fever, right upper quadrant pain and tenderness.

• Pleural pulmonary amoebiasis occurs due to contiguous spread from a liver abscess or hematogenous spread with symptoms like coughing and dyspnea.

• Cutaneous amoebiasis occurs due to intestinal or hepatic fistulae extending to the skin surface with ulceration in perianal and urogenital areas.

Overview of Amoebiasis and Diagnosis

This section provides an overview of amoebiasis, its different forms, and the methods used for diagnosis.

Forms of Amoebiasis

• Pulmonary amoebiasis can occur when trophozoites spread hematogenously to the brain tissue, causing cerebral amoebiasis.

• Intestinal and hepatic fistula formation can lead to cutaneous amoebiasis and perianal ulceration.

Diagnosis of Amoebiasis

• Microscopy, culture, antigen detection, antibody detection, PCR assay, and imaging studies are available methods for diagnosing amoebiasis.

• Staining and microscopic examination of fresh stool samples are commonly used in developing countries. Presence of trophozoites with ingested red blood cells indicates invasive disease.

• Routine microscopy cannot reliably distinguish between pathogenic Entamoeba histolytica and non-pathogenic Entamoeba dispar or Entamoeba bangladeshi.

• Culture can be performed on stool samples, rectal biopsy specimens, and liver abscess aspirates but is technically difficult.

• Antigen detection by enzyme-linked immunosorbent assay (ELISA) has excellent sensitivity and specificity. Samples from feces, serum, and liver aspirates can be tested.

• Serum antibodies against Entamoeba histolytica are present in a high percentage of patients with symptomatic intestinal amoebiasis or amoebic liver abscesses.

• PCR assays can detect specific genes of Entamoeba histolytica in feces and liver aspirate samples.

• Imaging studies such as radiography, ultrasonography, CT scan, and MRI are usually performed to diagnose amoebic liver abscesses.

Treatment and Prevention of Amoebiasis

This section discusses the treatment and prevention strategies for amoebiasis.

Treatment of Amoebiasis

• Most people with amoebiasis are treated on an outpatient basis, but severe cases may require inpatient care.

• Luminal agents like paramomycin and iodoquinol, which are minimally absorbed in the gastrointestinal tract, are suitable for treating asymptomatic infections.

• Metronidazole is the main therapy for invasive amoebiasis. Broad-spectrum antibiotics may be given to treat superinfection.

• Surgical intervention may be required in severe conditions such as perforated amoebic colitis, massive gastrointestinal bleeding, toxic megacolon, bacterial superinfection in amoebic liver abscesses, failure to respond to metronidazole after four days of treatment, and large left-sided amoebic liver abscesses at risk of rupturing into the pericardium.

Prevention of Amoebiasis

• The best way to prevent amoebiasis is by avoiding fecal contamination of food and water.

• Improved sanitation practices, hygiene measures, water treatment methods, and early treatment of carriers can help prevent the spread of infection.

• In non-endemic areas where Entamoeba cysts are not killed by soap or low concentrations of chlorine, boiling water is recommended.