Diagnostic Tests

Anyone who develops bloody diarrhea should see a doctor as soon as possible, and notify the doctor if they have been traveling in the tropics (although bloody diarrhea is seen in many other illnesses, the diagnosis in tropical regions are typically either emoebic dysentery or shigellosis) [13].

Diagnostic tests generally involve examination of stools under the microscope [12, 13]. Microscopy is able to distinguish between E. histolytica and the nonpathogenic intestinal protozoa Entamoeba coli  (not to be confused with the bacteria Echerichia coli) by appearance: E. histolytica cysts have a maximum of four nuclei, centrally located endosomes in the nucleus, and rounded chromatoidal bodies; Entamoeba coli have a maximum of eight nuclei, endosome that is off-centered in the nucleus, and chromatoidal bodies that are more jagged [16].

However, microscopy cannot be used to distinguish the nonpathogenic Entamoeba dispar  from the pathogenic Entamoeba histolytica, since both have very similar morphologies, exhibit ~98% identity at the rRNA level, and have similar host range and cell biology [9]. This causes the problem of incorrect diagnosis in many regions of the world where E. dispar is generally ten times more common than E. histolytica. Thus, differentiating the two species relies on sophisticated methods such as isoenzyme, antigen and DNA analyses (including PCR analysis). As well, there are kits that are used to differentiate between the two species directly from stool samples [3, 9]. The WHO recommended that asymptomatic infections diagnosed by microscopy alone should not be treated—since indiscriminately treating asymptomatic individuals may lead to drug resistance, as well as using up resources that can be used in treating those who actually have the disease. Evidence supporting the possibility of drug resistance come from failed treatments with metronidazole and differences in drug susceptibilities.  Therefore, it is essential to understand mechanisms involved in multi-drug resistance among the different antiamoebic drugs available [12].  However, it is important to distinguish whether the asymptomatic individuals are infected, and treatment of these individuals are more important on a public health level to prevent the spread of the disease. 

In asymptomatic individuals, the infections are usually diagnosed by finding cysts in stool samples.  Procedures that help to recover the cysts from stools include several flotation and sedimentation methods and stains that enable clear visualization of the isolated cysts under the microscope.  Since these asymptomatic individuals do not shed cysts as frequently, a minimum of 3 stool samples are examined [16].

In symptomatic individuals, the trophozoite form can often also be seen in fresh feces (but this form does not live long since they can be killed rapidly by water or drying).  Serological tests usually show to be positive for the presence of antibodies in those who are infected, whether or not they have the symptoms.  But accuracy of this tests requires a wait of two weeks after the infection. In general, serology is positive in over 90% of patients with invasive amoebiasis [11]. Other, though more expensive tests, include a 2 types of kits: one which can detect presence of amoeba proteins in stools, and one that detects amoeba DNA.  Due to the higher cost of these kits, they’re not widely used in the poorer countries that have higher infection rates and need these tools most.

If there are complications then further investigations, such as ultrasound, will be used to confirm the diagnosis [13]. Ultrasound, CT and MRI scans of the abdomen are useful in diagnosing hepatic amoebiasis [11]. In advanced cases, colonoscopy is used to observe whether the amoeba has caused ulcers in the small intestine [15]. Furthermore, a blood test can be given to those who appear to have infection that has invaded the wall of the intestine or some other organ of the body.  The only problem with blood tests is that it may give a false positive: the test can still be positive if the person had amoebiasis in the past but is no longer infected now [8].