From ages 50 onward, mature adulthood finishes off the life cycle. If humans live beyond age 75, they have entered into "late adulthood". Death is the end of the life cycle.
Evolution and Human Behavior: An Introduction to the Course
Entamoeba histolytica is a protozoan parasite that causes amebic dysentery and liver abscess. The disease is common in tropical regions of the world where hygiene and sanitation is often approximate. The epidemiology of E. histolytica has been studied around the world. However, there is a dearth of comprehensive literature on the epidemiology of this pathogen as well as its pathogenicity in the tropical and underdeveloped regions of the world where the disease is actually more common. Epidemiological figures in many endemic tropical countries are often overestimated because of inaccurate identification. Accurate data on the prevalence of the pathogenic strain(s) of E. histolytica in those regions will allow for the effective cure of patients with anti-amoebic drugs thus preventing the development of resistant types and reducing management costs.
With the advents of HIV and AIDS, several organisms have been identified as potential opportunistic pathogens. However, it is not clear whether amoebiasis is an opportunistic infection or not. Up to date, very little data has been published on the occurrence of E. histolytica in relation to HIV and AIDS. In developed countries amebiasis tends to be more common in older patients and occurs mostly among men who have sex with men or in institutions. However, in tropical regions, the epidemiology of amoebiasis is completely different and is more common among the general population and particularly among patients attending health care centers with diarrhea. Therefore, it is important to understand the epidemiology of this pathogen in tropical areas where it is responsible for most morbidity and mortality.
The recent reclassification of E. histolytica into different species now including the pathogenic Entamoeba histolytica and the non pathogenic Entamoeba dispar and Entamoeba moshkovskii has further added to the complexity of the epidemiology of amoebiasis since these three species cannot be differentiated by microscopy that is the most commonly used diagnostic method particularly in tropical countries where resources are limited, but can only be differentiated by the use of molecular methods such as the polymerase chain reaction based methodologies. Recent development of simpler but more sensitive methods such as the Loop-Mediated Isothermal Amplification (LAMP) should improve the understanding of the epidemiology of this disease.
Over the past few years we have studied the epidemiology of E. histolytica in African countries (Cameroon, Zimbabwe, and South Africa). In the present topic, we review these and other studies conducted in the African continent as well as other tropical regions in the light of new and more specific and sensitive molecular methods. The pathogenesis mechanism of amoebiasis is still not clear and recently differences in population levels of E. histolytica strains isolated from asymptomatic and symptomatic individuals have been shown to exist. One of the factors believed to be the determinant of the various clinical presentations of the disease is the organism’s virulence. The different methodologies used for the detection and epidemiology of amoebiasis will be reviewed as well as the role of E. histolytica in HIV disease. Recent advances on the pathogenesis and control of amoebiasis will also be reviewed.
Amoebiasis caused by the protozoan parasite E. histolytica was first recognized as a deadly disease by Hippocrates who described a patient with fever and dysentery (460 to 377 B.C.). With the application of a number of new molecular biology-based techniques, tremendous advances have been made in our knowledge of the diagnosis, natural history, and epidemiology of amoebiasis. Amoebiasis remains an important health problem in tropical countries where sanitation infrastructure and health are often inadequate (Ximénez et al., 2009). Clinical features of amoebiasis range from asymptomatic colonization to amoebic colitis (dysentery or diarrhea) and invasive extraintestinal amoebiasis, which is manifested most commonly in the form of liver abscesses (Fotedar et al., 2007). Current WHO estimates of 40-50 million cases of amoebic colitis and amoebic liver abscess (ALA) and up to 100,000 deaths annually, place amoebiasis second only to malaria in mortality (Stanley 2003; Ravdin 2005; WHO/PAHO/UNESCO 1997). Global statistics on the prevalence of E. histolytica infection indicates that 90% of individuals remain asymptomatic while the other 10% develop clinically overt disease (Jackson et al, 1985; Haque et al., 1999). Although all the deaths could be due to invasive E. histolytica infections, the value for the prevalence of E. histolytica is an overestimate since it dates from before the separation of the pathogen E. histolytica from the non-pathogen E. dispar (Diamond & Clark, 1993). Recently however, Entamoeba moshkovskii, a morphologically identical species, has been detected in individuals inhabiting endemic areas of amoebiasis (Ali et al., 2003, Fotedar et al., 2008, Khairnar et al., 2007, Parija & Khairnar, 2005) and could be contributing to the prevalence figures. Thus, the reclassification of E. histolytica into the three morphologically identical yet genetically different species has further added to the complexity of the epidemiology of amoebiasis since they cannot be differentiated by microscopy that is the most commonly used diagnostic method particularly in tropical countries where resources are limited. Furthermore, the worldwide prevalence of these species has not been specifically estimated. Thus, obtaining accurate species prevalence data remains a priority as there are gaps in our knowledge for many geographic regions of the tropics.
Although only a minority of E. histolytica infections - one in every four asymptomatic intestinally infected individuals – progress to development of clinical symptoms (Gathiram and Jackson, 1987; Blessmann et al., 2003; Haque et al., 2006), the exact basis for this difference remains mostly unsolved. This might be partly due to the differences in the pathogenic potential of the infecting strains (Burch et al., 1991) and/or the parasite genotype (Ali et al., 2007) or due to the variability of the host immune response against amoebic invasion (Mortimer and Chadee, 2010).
The disease mechanism and the exact prevalence and incidence of infection caused by E. histolytica are still unknown. The epidemiological data available for endemic countries however, albeit sporadic, is based mostly on the microscopic identification of the E. histolytica/E. dispar/E. moshkovskii complex, often inaccurately reported as "E. histolytica". To date many highly sensitive and specific techniques such as enzyme-linked immuno-sorbent assays (ELISA) and polymerase chain reaction (PCR) have been developed for the accurate identification and detection of E. histolytica in various clinical samples (Ackers, 2002). It is anticipated that these molecular tools will allow us to reconstruct a more reliable picture of the true epidemiology of the disease mainly in endemic regions of the world and to better our understanding of the role of the parasite and/or host factors that determine the disease outcome.
