Genetic Variability of *Entamoeba histolytica* Strains

*Shler Akram Faqe Mahmood*

### **Abstract**

*Entamoeba histolytica* is pathogenic parasite that causes asymptomatic infection mostly; however, it may also cause invasive intestinal amoebiasis and liver abscess, leading to significant rates of human mortality globally. The clinical outcome of the infection with the parasite is variable and evidence suggested the contribution of genetic diversity within *E. histolytica* to human disease. The information documented the whole-genome sequence of the *E. histolytica* reference laboratory strain (HM-1:IMSS) and the development of sophisticated molecular technique potentiate ability to identify strains of *E. histolytica* that may lead to insights into the population structure, virulence, pathogenesis, clinical outcome of the disease and epidemiology of the organism.

**Keywords:** *Entamoeba histolytica* strains, genetic diversity, inter-species variability, *E. histolytica* genetic biomarker

## **1. Introduction**

*Entamoeba histolytica* is a pseudopod-forming protozoan parasite that inhabit primarily in human gastrointestinal tract. Amoebiasis can be asymptomatic in the majority (about 90%) of infected people or can lead to severe invasive infection characterized by symptoms such as bloody diarrhoea, abdominal pain, flatulence, nausea, and vomiting. In some cases, amoebae extend out of intestinal tract forming ulceration and abscess, particularly in liver causing amoebic liver abscesses [1]. *E. histolytica* affects 50 million individuals annually [2], causing about 11,300 fatality throughout the world, making it the fourth leading cause of death due to parasitic infections [3]. Amoebiasis is mainly transmitted by faecal-oral ingestion of contaminated water or food with cystic stage and hence distributed more in developing countries [4]. The parasite life cycle is simple, converting between two stages: a cyst stage that survives in adverse environmental conditions, transmitting the disease and a trophozoite (the vegetative form), which invades the host epithelial cells causing diseases [5]. Infection begins by excystation of the cyst, cyst cell membranes are disintegrated by the effect of the gastric juice and bile salts yielded motile trophozoites in the small bowel and then followed by adherence and invasions of trophozoites to colonic mucins and colonic epithelial cells. At the lower part of the ileum, the trophozoites encysted and released from intestinal mucosa

and excreted with faeces to contaminate the environment repeating the cycle again. In some cases, the trophozoites disseminate haematogenously resisting the complement proteins and appear to have special preference for hepatic tissue [6]. The molecular pathway that the parasite followed during the processes of tissue invasion is poorly understood.

The differences reported in the clinical outcome of the infections with *E. histolytica*, which ranged from asymptomatic carriers to invasive amoebiasis and even to extra-intestinal abscess formation, may be due to genetic variability and inter-strainrelated virulence genes in the genomic sequence of *E. histolytica* [7].

It has been suggested that the presence of these genetic alterations could have either permitted the parasite to more readily evade host immune responses or be related to higher virulence, causing more severe clinical presentations [8].

### **2. Epidemiology of amoebiasis**

*E. histolytica* has been reported worldwide, with the higher prevalence rates in tropical and subtropical of underdeveloped regions where hygiene and sanitation conditions are compromised [9, 10]. Amoebic endemicity has been recorded in Africa, Mexico, Central and South America, Pacific islands, and Asia, where the major way for transmission is feco-oral. Even the expression of clinical features of amoebiasis which range from asymptomatic infections, acute dysentery and chronicdisseminated infection are varied geographically. For example, amoebic dysentery is predominate in Egypt [11], whereas amoebic liver abscess is more prevalent in both South Africa and Hue City in Vietnam [12]. In developed countries, low incidence rats are reported; and most *E. histolytica* infections are sexually acquired among homosexual community, for example, those documented in Australia, Europe, North America and parts of Asia [2, 10, 13, 14]. In the United States, the incidence of amoebic infections is quite low, yet amoebiasis-related deaths still usually occur, responsible for at least five deaths per year [15]. Most reported cases of amoebiasis in the United States are seen in immigrants or returning travellers from endemic countries [16]. Data from the international surveillance and monitoring system (GeoSentinel Surveillance Network) have documented that *E. histolytica* is the third most common pathogen isolated from returning travellers with infectious intestinal disease [16]. Travellers to the Middle East, South Asia, and South America appear to have the highest risk of amoebiasis.

Evaluating the global burden of *E. histolytica* infection is quite difficult due to limitation in diagnostic capacity and surveillance in most regions endemic with the parasite. Epidemiological studies can be affected by several factors, for example, study design, geographic area, sample size, symptom severity, incubation and the sensitivity of the diagnostic tool used. However, seroprevalence reports of *E. histolytica* in rural areas of Mexico showed as high as 42% [17]. Infant during their first year of life of Dhaka, Bangladesh, reported amoebic diarrhoea in 11% of the children [18]. Prevalence of *E. histolytica* based on PCR detection revealed that the infection rate was 13.7% in northeast states of India and 6% in Northern Iraq [19, 20]. Seropositive results for *E. histolytica* were accounted for 11% of seven provinces of China and 41% among male homosexual community in the provinces of Beijing and Tianjin [21, 22]. Depending on the antigen detection of *E. histolytica* from stool samples, the parasite reported the highest rate among the enteropathogens that related to the diarrhoea presented by 20%

*Genetic Variability of* Entamoeba histolytica *Strains DOI: http://dx.doi.org/10.5772/intechopen.106828*

among children under the age of 16 years of Jeddah, Saudi Arabia [23]. Despite widespread occurrence of amoebiasis in Africa, limited information is present about the infection rates and epidemiology of *E. histolytica*. However, a cross-sectional study in South Africa showed that the prevalence of *E. histolytica* was 8.5% of patients attending gastroenterology clinics detected by PCR [24]. *E. histolytica* is represented as the one of the top 10 causative agents of diarrhoea in children under the age of 5 years in regions of sub-Saharan Africa and South Asia, according to the large Global Enteric Multi-Center Study (GEMS) [25].

The global impact of amoebic infection and amoebiasis remains significant, nevertheless challenging to quantify with accuracy given several epidemiologic and methodologic difficulties, but prevalence rates persist as high as 40% in certain populations.

## **3. Whole-genome sequences of** *Entamoeba*

The genome assembly of *E. histolytica* (strain HM1:IMSS) contains 20,800,560 base pairs of DNA in 1496 scaffolds; about 75% of the genome are AT-rich regions and since there are 8333 annotated genes, therefore the coding sequence accounts for nearly 50% of all assembled sequences [26].

The genome shows impressive evolutionary features, in particular, the presence of substantial number of genes (at least 68) that seems to have been acquired by horizontal gene transfer from bacteria. Most of these transferred genes appear to have been ancient and implicated in metabolic processes specific for the anaerobic lifestyle of the organisms [27].

Genomic structure and architecture of Entamoeba are still not well characterized. For example, it is unknown whether there is a natural ploidy or haploid number of chromosome, although both are estimated [28]. The processes of genetic reassortment are common in *E. histolytica*, which are important to determine parasite population dynamics and parasite phenotypes through detecting factors that can trigger this genetic reassortment. Therefore, it is possible due to high levels of recombination to generate strains of *E. histolytica* with increased virulence expressing transient polygenic traits as results of association of different alleles. In spite of exhibiting of asexual reproduction by *E. histolytica* trophozoite, structural diversity exist in this species due to the union that could happen between different trophozoite strains co-infected the same host [7].

There are complex patterns of *E. histolytica* molecular karyotype, which display differences in chromosome sizes between strains and a mixture of circular and linear DNA [28, 29]. Circular DNA structures are present in *E. histolytica*, for instance, rRNA gene found in multiple copies per nucleus; these segmental duplications in the genomic DNA might be represented by the circular molecules of DNA [29–31]. However, it is unknown whether the copies of these DNA structures are present in different number from that of the 'core' chromosomes and if they segregate in the same way or not. Furthermore, the exhibition of short tandem repeats in the tRNA genes represented an unusual organization of *E. histolytica* genome, which appears in arrays of tandem duplicated combinations of genes separated by DNA [32]. The tRNA gene arrangement seems to be quite variable and has altered in the evolution of different species linages; therefore, it is used as population genetic markers to show different isolates of *E. histolytica* (and *E. dispar*) [33].
