Amoebiasis: Causes, Symptoms, Diagnosis, Treatment, and Prevention

~Introduction

Amoebiasis (also spelled amebiasis) is an intestinal infection caused primarily by the protozoan parasite Entamoeba histolytica. It is a significant cause of morbidity and mortality worldwide, especially in tropical and subtropical regions with poor sanitation. The World Health Organization (WHO) estimates that approximately 50 million people are infected globally each year, with about 40,000–100,000 deaths, making amoebiasis one of the leading causes of death from parasitic diseases.

The infection can range from being asymptomatic to causing severe diarrhea, dysentery, or life-threatening extraintestinal disease, such as liver abscesses.

~History

The disease was first described in the late 19th century when Russian zoologist Fedor Lösch identified amoebas in the stools of a patient with severe diarrhea in 1875. Later, in 1903, Schaudinn named the parasite Entamoeba histolytica, referring to its tissue-destructive capability.

~Causative Agent

The pathogen responsible for amoebiasis is Entamoeba histolytica, a unicellular protozoan parasite belonging to the phylum Amoebozoa.

Morphological Forms

The parasite exists in two main forms:

Trophozoite
- Active, motile feeding stage
- Irregular shape, with pseudopodia for movement
- Found in the intestinal lumen and tissues
- Responsible for tissue invasion and disease symptoms
Cyst
- Dormant, infective stage
- Spherical, with 1–4 nuclei
- Highly resistant to environmental conditions
- Transmitted via contaminated food or water

~Epidemiology

Amoebiasis is endemic in:

Asia (India, Bangladesh, Pakistan, China, Southeast Asia)
Africa
Central and South America

It is most prevalent in areas:

Lacking clean water supply
Practicing open defecation
Where hygiene practices are inadequate

High-risk groups include:

Children under 5 years
Immunocompromised individuals
Travelers to endemic areas
People living in overcrowded conditions

~Transmission

The disease is transmitted by the fecal–oral route, mainly through:

Ingestion of cyst-contaminated water or food
Consumption of raw vegetables washed with contaminated water
Poor personal hygiene
Direct person-to-person contact (less common, but possible in institutional settings)

Flies and cockroaches can act as mechanical vectors, transferring cysts to food.

~Life Cycle

Ingestion of mature cysts in contaminated food or water.
Excystation occurs in the small intestine, releasing trophozoites.
Trophozoites migrate to the large intestine, where they:
- Multiply by binary fission
- In some cases, invade the intestinal wall
In the lumen, some trophozoites encyst, forming new infective cysts.
Cysts are passed in feces, contaminating the environment.
Invasive disease occurs when trophozoites penetrate the intestinal mucosa, enter the bloodstream, and spread to organs like the liver, lungs, or brain.

~Pathogenesis

The pathogenic process involves:

Adherence: Trophozoites attach to intestinal epithelial cells via surface lectins.
Tissue invasion: They secrete enzymes such as proteases that degrade host tissues.
Cell death: Cytolysis and apoptosis of host cells occur.
Immune evasion: The parasite can resist complement-mediated killing and survive in host tissues.
Extraintestinal spread: Occurs via the bloodstream, commonly to the liver.

~Clinical Manifestations

Amoebiasis can present in different forms:

1. Asymptomatic Amoebiasis

Common in many infected individuals
Cysts are shed in stool, but there are no symptoms
Person acts as a carrier

2. Intestinal Amoebiasis

Acute Amoebic Colitis (Amoebic Dysentery):

Gradual onset of abdominal pain
Frequent stools containing blood and mucus
Low-grade fever
Tenesmus (straining during defecation)

Chronic Amoebic Colitis:

Alternating diarrhea and constipation
Weight loss
Fatigue

Fulminant Amoebic Colitis (rare but severe):

Extensive ulceration
Perforation of the colon
Peritonitis

3. Extraintestinal Amoebiasis

The most common form is Amoebic Liver Abscess (ALA):

Right upper quadrant abdominal pain
Fever, chills
Enlarged, tender liver
Jaundice (less common)
Without treatment, abscess may rupture into the pleural cavity, pericardium, or peritoneum

Rarely, E. histolytica spreads to the lungs, brain, or skin.

~Complications

Perforated colon with peritonitis
Massive hemorrhage
Liver abscess rupture
Secondary bacterial infections
Chronic colitis mimicking inflammatory bowel disease

~Diagnosis

Accurate diagnosis is important for effective treatment.

1. Microscopy

Stool examination for cysts or trophozoites
Trophozoites containing red blood cells are diagnostic of E. histolytica
Limitation: Cannot differentiate pathogenic E. histolytica from non-pathogenic E. dispar or E. moshkovskii

2. Antigen Detection

Enzyme-linked immunosorbent assay (ELISA)
Rapid diagnostic kits

3. Serology

Antibody detection is useful for extraintestinal disease (e.g., liver abscess)
Positive in >90% of ALA cases

4. Molecular Methods

Polymerase chain reaction (PCR) for species-specific detection

5. Imaging

Ultrasound, CT scan, or MRI for suspected liver abscess

~Treatment

Treatment depends on the form of the disease.

1. Luminal Amoebicides

Eliminate cysts from the intestine
Paromomycin (first choice)
Diloxanide furoate
Iodoquinol

2. Tissue Amoebicides

Act against invasive trophozoites in tissues
Metronidazole or tinidazole
Followed by luminal agent to prevent relapse

3. Amoebic Liver Abscess

Metronidazole or tinidazole
Luminal agent afterward
Drainage in large, left-lobe, or non-resolving abscesses

Note: Asymptomatic carriers must also be treated to prevent spread.

~Prevention and Control

Prevention is aimed at breaking the fecal–oral transmission cycle.

1. Safe Water Supply

Boiling water or using filters
Chlorination (less effective against cysts unless at high concentration)

2. Sanitation

Proper sewage disposal
Avoiding use of untreated human feces as fertilizer

3. Food Hygiene

Washing fruits and vegetables thoroughly
Avoiding raw vegetables from questionable sources

4. Personal Hygiene

Handwashing with soap before meals and after defecation
Health education in endemic areas

~Public Health Significance

Amoebiasis causes:

High morbidity and mortality in endemic countries
Economic burden from lost productivity and healthcare costs
Strain on healthcare systems during outbreaks

The WHO includes amoebiasis in the list of neglected tropical diseases that need integrated control programs.

~Recent Research and Developments

Molecular differentiation between E. histolytica and non-pathogenic species improves diagnosis
Vaccine research is ongoing, focusing on parasite lectins
Newer drugs aim to be more effective with fewer side effects
Public health programs integrating amoebiasis prevention with other waterborne disease control measures

~Case Example

A 32-year-old man from rural Bangladesh presented with fever, right upper quadrant pain, and weight loss for 3 weeks. Ultrasound revealed a solitary liver abscess. Serology for E. histolytica was positive. He was treated with intravenous metronidazole for 10 days, followed by oral paromomycin. Symptoms resolved, and follow-up ultrasound showed significant reduction in abscess size.

~Prognosis

With early diagnosis and proper treatment, the prognosis of amoebiasis is excellent. Delay in treatment, particularly in liver abscess or fulminant colitis, can be fatal. Preventive measures are crucial to avoid reinfection, especially in endemic areas.

~Conclusion

Amoebiasis remains a major public health concern in many parts of the world, largely due to inadequate sanitation and unsafe drinking water. The infection can range from mild diarrhea to severe, life-threatening disease. Effective treatment exists, but prevention through clean water, proper sanitation, food hygiene, and public health education is the most sustainable way to reduce its burden. Coordinated efforts between healthcare providers, communities, and governments are essential for long-term control.

Friday, August 15, 2025

Amoebiasis