Amoebiasis - Diagnosis of Human Illness

Diagnosis of Human Illness

Asymptomatic human infections are usually diagnosed by finding cysts shed in the stool. Various flotation or sedimentation procedures have been developed to recover the cysts from fecal matter and stains help to visualize the isolated cysts for microscopic examination. Since cysts are not shed constantly, a minimum of three stools should be examined. In symptomatic infections, the motile form (the trophozoite) can often be seen in fresh feces. Serological tests exist and most individuals (whether with symptoms or not) will test positive for the presence of antibodies. The levels of antibody are much higher in individuals with liver abscesses. Serology only becomes positive about two weeks after infection. More recent developments include a kit that detects the presence of amoeba proteins in the feces and another that detects ameba DNA in feces. These tests are not in widespread use due to their expense.

Microscopy is still by far the most widespread method of diagnosis around the world. However it is not as sensitive or accurate in diagnosis as the other tests available. It is important to distinguish the E. histolytica cyst from the cysts of nonpathogenic intestinal protozoa such as Entamoeba coli by its appearance. E. histolytica cysts have a maximum of four nuclei, while the commensal Entamoeba coli cyst has up to 8 nuclei. Additionally, in E. histolytica, the endosome is centrally located in the nucleus, while it is usually off-center in Entamoeba coli. Finally, chromatoidal bodies in E. histolytica cysts are rounded, while they are jagged in Entamoeba coli. However, other species, Entamoeba dispar and E. moshkovskii, are also commensals and cannot be distinguished from E. histolytica under the microscope. As E. dispar is much more common than E. histolytica in most parts of the world this means that there is a lot of incorrect diagnosis of E. histolytica infection taking place. The WHO recommends that infections diagnosed by microscopy alone should not be treated if they are asymptomatic and there is no other reason to suspect that the infection is actually E. histolytica.

Typically, the organism can no longer be found in the feces once the disease goes extra-intestinal. Serological tests are useful in detecting infection by E. histolytica if the organism goes extra-intestinal and in excluding the organism from the diagnosis of other disorders. An Ova & Parasite (O&P) test or an E. histolytica fecal antigen assay is the proper assay for intestinal infections. Since antibodies may persist for years after clinical cure, a positive serological result may not necessarily indicate an active infection. A negative serological result however can be equally important in excluding suspected tissue invasion by E. histolytica.