Describe pathogenesis and laboratory diagnosis of extra-intestinal amoebiasis.

Pathogenesis of Extra-Intestinal Amoebiasis

The pathogenesis of extra-intestinal amoebiasis involves several stages, beginning with intestinal colonization and subsequent dissemination.

• Intestinal Phase: Infection begins with the ingestion of *E. histolytica* cysts, typically through contaminated food or water. Cysts excyst in the small intestine, releasing trophozoites. Trophozoites colonize the large intestine, where they can cause amoebic dysentery or remain asymptomatic.
• Invasion and Dissemination: Trophozoites invade the intestinal mucosa, leading to ulceration and inflammation. From the intestinal lumen, trophozoites enter the portal circulation.
• Hepatic Phase: The liver is the most common site of extra-intestinal involvement. Trophozoites are carried to the liver via the portal vein. Here, they cause localized necrosis, forming an abscess. The abscess typically develops in the right lobe of the liver.
• Further Dissemination: In rare cases, trophozoites can escape the liver and reach other organs via the systemic circulation. Common extra-hepatic sites include the lungs, brain, pericardium, and spleen.

Laboratory Diagnosis of Extra-Intestinal Amoebiasis

Laboratory diagnosis of extra-intestinal amoebiasis relies on a combination of direct and indirect methods. The choice of tests depends on the site of infection and the clinical presentation.

1. Direct Methods

• Microscopic Examination:
  • Aspiration of Abscess Fluid: Microscopic examination of aspirated abscess fluid is the gold standard for diagnosing amoebic liver abscess. Trophozoites can be identified directly in approximately 50-80% of cases.
  • Biopsy: Biopsy of affected tissues (e.g., lung, brain) can reveal trophozoites, but is often invasive and not routinely performed.
• PCR (Polymerase Chain Reaction): PCR-based assays are highly sensitive and specific for detecting *E. histolytica* DNA in abscess fluid, biopsy samples, or even blood. It's becoming increasingly important due to its higher sensitivity compared to microscopy.

2. Indirect Methods (Serological Tests)

Serological tests detect antibodies against *E. histolytica*. They are useful when direct methods are negative or impractical.

• ELISA (Enzyme-Linked Immunosorbent Assay): ELISA is a commonly used serological test. It detects antibodies (IgG and IgM) against *E. histolytica*. However, cross-reactivity with antibodies to other amoebae can occur, leading to false-positive results.
• Indirect Hemagglutination Test (IHA): IHA is another serological test that detects antibodies against *E. histolytica*. It is less specific than ELISA.
• Immunofluorescence Assay (IFA): IFA is a more specific serological test, but it is more expensive and requires skilled personnel.

3. Imaging Techniques

While not directly diagnostic of the parasite, imaging plays a crucial role in identifying and characterizing extra-intestinal amoebiasis.

• Ultrasound: Useful for detecting liver abscesses.
• CT Scan: Provides detailed images of the liver and other organs, helping to identify abscesses and assess their extent.
• MRI: Useful for evaluating brain abscesses and other extra-hepatic lesions.

Diagnostic Method	Sensitivity	Specificity	Advantages	Disadvantages
Microscopy (Abscess Fluid)	50-80%	High	Gold standard, inexpensive	Low sensitivity, requires skilled personnel
PCR	>95%	>98%	High sensitivity and specificity	Expensive, requires specialized equipment
ELISA	70-90%	60-80%	Easy to perform, relatively inexpensive	Cross-reactivity, false positives
CT Scan	N/A	N/A	Excellent visualization of abscesses	Does not directly detect parasite