AMEBIASIS

AMEBIASIS

Amebiasis is a parasitic disease caused by the protozoan Entamoeba histolytica, which affects primarily the colon and secondarily, in the case of extraintestinal amebiasis, the liver, lungs, brain and, in rare cases, other organs and tissues. Amebiasis continues to be one of the most widespread parasitoses in the world and is the third most common cause of death due to parasites (right after malaria and schistosomiasis).

Invasive amebiasis is a major social and health problem in some areas in Africa, Asia and Latin America.

ETIOLOGY

The etiologic agent of amebiasis is Entamoeba histolytica (E.dysenteriae), a protozoan of the phylum Sarcomastigophora (Protozoa), subphylum Sarcodina, division Rhizopoda, class Lobosea, order Amoebida, family Entamoebidae, genus Entamoeba.

Morphology. E.histolytica is a unicellular organism, without a cell wall, which has pseudopodia as organs of locomotion. We can distinguish two morphological aspects:

The trophozoite is the vegetative form of the parasite, which may be found in the intestinal tract, dysenteric stools, gastric wall abscesses and visceral metastases. It measures 10-60 (25) microns in diamete 434s1815e r. It moves in a single direction, using its pseudopodia. Its cytoplasm shows two zones: a hyaline and granule-free outer zone (ectoplasm) and a finely granular inner zone (endoplasm). The endoplasm contains numerous vesicles included in a cytoplasmic membrane. Sometimes the cytoplasmic vesicles contain ingested red cells in various stages of disintegration. The cytoskeleton is made up of microfilaments that can be generally found beneath the plasmatic membrane, where the amoeba attaches itself to the substrate and where the phagocytic canals are formed. The spherical nucleus, situated eccentrically, has a centrally located karyosome and a peripheral chromatin, uniformly distributed in fine granules on the inner side of the nuclear membrane; it measures 3-5 microns. In between the karyosome and the nuclear membrane there are fine fibrillae, radially distributed.

The cystic form represents the form of resistance, dissemination and multiplication of the amoeba. The cyst is round and has a double membrane that forms a chitinous wall, which confers its resistance to the external environment. The mature cyst contains 4 nuclei. Inside, we can see chromatoid bars, short and thick, formed due to the concentration of ribosomes in rods or clusters. In immature cysts (with 1 or 2 nuclei) a glycogen vacuole can be visible, but it becomes more diffuse as the cyst matures. The diameter of the cyst is 7-15 microns (in extreme cases 5-20 microns).

Life cycle. The cyst is expelled with the faeces in the external environment.

The cysts are resistant and may remain viable for weeks or months: for over 48 hours in food and wet stools at a temperature of 20-25 C; for a month (or even 40 days) in sewage water and in natural surface water at a temperature of 4 C. If the cyst has been ingested by a new host, along with the food or drinking water, it reaches the small intestine where the parasite excysts under the influence of the following factors: osmotic modifications and favourable pH (neutral or alkaline), enzymatic activity of the encysted microorganism, enzymatic action of the host's tissues. The cystic wall is digested probably by the digestive enzymes of the intestinal lumen. The encysted amoeba becomes very active, separates itself from the cystic wall and rapidly divides into a number of parasites equal to the number of nuclei present inside the cyst (usually 8). Eight single-nucleus trophozoites are formed, which are smaller than those present in the colon. The trophozoites are transported to the caecum where they mature completely and multiply through binary fission. Then they reach the colon where they become spherical and continue their multiplication, exercising or not pathogenic actions. The trophozoites may determine, through their pathogenic actions, characteristic lesions of the colon and, clinically, signs of the intestinal amebiasis.

Through the intestinal lesions, the trophoites may pass into the mesenteric circulation and, through the portal system, they reach the liver where they cause lesions identical to those in the colon. Through the bloodstream they may also disseminate and settle in other tissues and organs like the lung, the brain etc. Other times, from the hepatic abscess, the trophoites pass, through contiguity, to the right lung and causing the unilateral pulmonary amebiasis.

In unfavourable conditions, an attachment protein of the parasite (Gal/Gal Nac lectine), together with the glycoproteins in the mucine (the protective mucus of the intestinal mucosa) and/or other bacteria in the intestine, initiate the process of encysting, forming the chitinous cell wall. This way, the natural life cycle of the amoeba continues.

EPIDEMIOLOGY

Geographical spread. Amebiasis is a cosmopolitan disease, widely spread across the globe, more frequent in the hot and damp areas (especially in the tropical and subtropical areas) than in the temperate areas.

Morbidity Amebiasis remains one of the most widespread parasitoses in the world. According to OMS, 10% of the world population is infested with E. histolytica. The prevalence of amebiasis is dependent on a series of factors: age (increasing prevalence among the school-aged children), social and economic as well as sanitary-hygienic conditions, level of culture of the population, density, natural disasters, armed conflicts etc. Every year, there are approximately 50 million new cases of invasive disease and over 100.000 deaths caused by it.

Remarkably, the invasive amebiasis is infinitely rarer than the asymptomatic infection. Only 10% of the people infected with E. histolytica develop a symptomatic invasive amebiasis, the rest of them remaining asymptomatic carriers that expel the parasite from their intestines within 12 months.

In the endemic areas, the index of infestation with E. histolytica can be, in some communities, as high as 20-80% of the population. In Asia, especially in India and South-East Asia, the prevalence of the invasive disease is 14% and, of all the cases of amebiasis, 25% are hepatic amebiasis. In tropical Africa, the prevalence of amebiasis is 1-2% and the number of hepatic amebiasis is much lower. In North Africa, Middle East and Latin America, amebiasis is less frequent; it is also rare in Europe and USA where many of the cases are "imported" and where the asymptomatic infection is predominant.

The reservoir of infection is only the man. He alone is the only source of infection. The man passes on the infection to other people (but also to primates, dogs, cats and, probably, pigs as well) in the following hypostases:

asymptomatic cyst expeller, where the disease developed subclinically;

convalescent cyst expeller, after an untreated or insufficiently treated severe episode of disease;

patients suffering from chronic intestinal amebiasis;

asymptomatic carriers of E.histolytica.

The transmission is faecal-oral, by ingesting the cysts through direct contact (dirty hands) or indirect contact, through contaminated water and food (fruit, vegetables and other raw food, improperly washed or cooked). Flies contribute to spreading the disease by contaminating the food with E histolytica cysts (and having, therefore, a passive role in transmitting the disease). Lately, there has been an increased incidence of the sexual transmission of E.histolytica in homosexual males (as well as in bisexual males).

The disease receptivity is general. The risk of infection with E.histolytica is higher in travellers to endemic areas (especially if they stay there more than a month). In the endemic areas, the children, especially those suffering from malnutrition, develop fulminant forms of colitis.

Immunity Although the pathogenic strains of E.histolytica trigger a humoral and cellular immune response, we do not know with certainty if the host's immune response can prevent a reinfection. The recurrence of the intestinal and hepatic amebiasis was demonstrated in spite of the presence of some high seric antibody titres, which pleads for the fact that the circulant antibodies do not offer protection against the reinfection with E.histolytica. Presently, it is accepted that the immunity appears after the disease but this protective unity occurs especially in the cases of an invasive amebiasis. The antiamebiasis seric antibodies last for up to 10 years after the invasive amebiases has been cured.

DIAGNOSIS

A positive diagnosis is established taking into consideration the epidemiological data (patients from endemic areas), clinical data and laboratory examinations.

Parasitological diagnosis The microscopic examination of the feces discloses the E.histolytica trophozoites and cysts in the case of intestinal infection. Due to the fact that the amoebas are irregularly and inconstantly expelled from the intestinal tract on a daily basis, the parasitological examination of the stool must be repeated for 4-6 times, on an interval of 7-10 days.

Through rectosigmoidoscopy, biologic material is removed from the ulcerous lesions, material that may show signs of trophozoites when a direct microscopic examination is carried out.

Immunodiagnosis The serologic reactions, that reveal the serum antibodies specific to the E.histolytica, are very useful in the diagnosis of invasive amebiasis. The most frequently used tests are: ELISA, indirect hemagglutination (hemagglutinin-inhibition), indirect immunofluorescence, gel diffusion and counterimmunoelectrophoresis. These tested positive in more than 90% of the invasive amebiasis patients. Significant antibody titres continue to show for months or even years after the invasive disease has been completely cured, these antibodies being pointed out through indirect hemagglutination (which tests positive in the acute disease and stays positive for many years after the invasive disease has been efficiently cured). By contrast, ELISA, counterimmunoelectrophoresis and gel diffusion become negative few months after the invasive disease has been cured; therefore a positive test indicates, almost always, an acute disease.

The evidence of the galactose-binding lectine antigen of E. Histolytica in the serum and stool is managed through the multi-layer ELISA method. This antigen can no longer be detected in serum and stool after seven days of treatment with tissue amebicides of an invasive infection.

The rectosigmoidoscopic examination has a limited value in establishing the diagnosis. Usually the acute intestinal amebiasis shows characteristic ulcerations in the form of the "shirt-stud abscesses" but sometimes it shows unspecific lesions (congestion, oedema) or even a normal aspect of the intestinal wall. The sampling of a biopsy from the intestinal mucosa can offer conclusive information if the direct microscopic examination reveals hematophagus trophozoites of E. Histolytica.

The barium examination has little value in establishing a positive diagnosis of acute intestinal amebiasis. It can show alterations of the large intestine, without them being characteristic: parietal ulcerations and pilus deformation in acute amebiasis or alternations of spastic areas and atone, tubular areas in chronic amebiasis.

The differential diagnosis of acute intestinal amebiasis is made by comparing it to bacterial dysentery, diarrheic syndrome in balantidiosis, schistosomiasis (intestinal or urogenital), haemorrhagic rectocolitis, rectal cancer, rectosigmoid villous tumor and rectocolic poliposis.

Ameboma raises problems with the differential diagnosis when compared with the colon tumors and, sometimes, with the ileo-caecal tuberculosis.

The scaring chronic amebiasis is almost impossible to be distinguished from other chronic colitises or functional colopathies if the coproparasitologic exam is negative and the patient has no acute intestinal amebiasis episodes in the past.