MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

BIOLOGY

CHAPTER 2. BIOGEOCENOTIC LEVEL OF LIFE ORGANIZATION AND HUMAN PLACE IN IT

2.3. MEDICAL PROTOZOOLOGY

Phylum Sarcomastigophora

Class Lobozea

Amoebas are the simplest and most primitive Protozoans. The Cytoplasm is enclosed only by a Cell/33.html">Plasma Membrane, meaning The Cell lacks a fixed shape. Pseudopodia (false feet) serve as Organelles for locomotion and food capture. The Mechanism of engulfing and digesting food is known as phagocytosis. They reproduce primarily asexually by binary fission. Under unfavorable conditions, they form cysts. Within the genus Amoeba, medical significance is attributed to the pathogenic dysentery amoeba, along with several non-parasitic species (such as the intestinal amoeba, oral amoeba, etc.).

Entamoeba histolytica (Dysenteric amoeba). Entamoeba histolytica is the CAUSATIVE AGENT OF amebiasis, or amebic dysentery. It was discovered by the St. Petersburg scientist F.O. Lösch in 1875.

Humans are the sole source of infection.

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Fig. 2.1. Dysenteric amoeba (Entamoeba histolytica) and intestinal amoeba (Entamoeba coli):

a - Entamoeba histolytica forma magna with phagocytosed erythrocytes; b, c - Entamoeba histolytica forma minuta; d - tetranucleate cyst of the dysenteric amoeba; 1 - ectoplasm; 2 - endoplasm; 3 - pseudopodia; 4 - Nucleus; 5 - karyosome; 6 — phagocytosed erythrocytes in food vacuoles; Entamoeba coli: vegetative form (e) and its octanucleate cyst (f).

Fig. 2.2. Life Cycle of the dysenteric amoeba (Entamoeba histolytica):

a - healthy carrier; b - patient with amebiasis: 1 - transformation of the cyst into a vegetative form after oral ingestion into the gastrointestinal tract; 2, 3 - minuta vegetative form; 4 - cysts; 5 - tissue form in the mucosa of the Large Intestine; 6 - magna vegetative form with ingested erythrocytes.

Localization: large intestine.

Geographical distribution: worldwide, but more prevalent in countries with hot climates.

Morphophysiological characteristics and life cycle. It parasitizes exclusively in humans. The life cycle includes the following forms: 1) cyst; 2) minuta vegetative form (forma minuta); 3) magna vegetative form (forma magna); and 4) tissue form (Figs. 2.1, 2.2).

Cysts are excreted into the external environment with the feces of infected individuals. They enter The Human Body orally (fecal-oral transmission route) via unboiled Water, unwashed vegetables and fruits, or contaminated hands, and are further spread by flies and cockroaches (mechanical vectors). The infective stage is the cyst. Upon reaching the human intestine, the cyst wall dissolves, releasing a tetranucleate amoeba that rapidly divides into four single-nucleated minuta vegetative forms - f. minuta (7-15 µm in diameter). These inhabit the lumen of the large intestine, feed primarily on Bacteria, and do not cause disease (non-pathogenic). In the lower sections of the intestine, they encyst. Mature cysts contain four nuclei. Sometimes, an infected person sheds cysts for years without showing any symptoms of illness. Such individuals are called cyst carriers. They pose a significant danger because they serve as a source of infection for others. A single cyst carrier can excrete up to 600 million cysts per day.

However, under certain conditions in some individuals (such as hypothermia, overheating, avitaminosis, infections, or helminthiases), f. minuta invades the intestinal wall, where it actively multiplies. Amoebas residing in the Tissues of the intestinal wall constitute the tissue form of the dysenteric amoeba. This form is pathogenic, causing mucosal lesions characterized by ulceration (amebiasis). During this process, the walls of Blood capillaries are destroyed, leading to hemorrhages into the intestinal lumen. The invasion of the intestinal mucosa and its subsequent destruction are attributed to the ability of the dysenteric amoeba to secrete Proteolytic Enzymes that dissolve tissue Proteins.

Upon the onset of amebic intestinal lesions, the minuta vegetative forms residing in the intestinal lumen transform into the magna vegetative form (f. magna), which feeds on erythrocytes. It is characterized by large dimensions (30-40 µm) and a distinct nuclear Structure: the nuclear Chromatin forms radial arrangements, with a large chromatin mass — the karyosome — positioned centrally. The magna vegetative form is characterized by broad, blunt pseudopodia and jerky locomotion. The cessation of bleeding and the consequent inability to feed on erythrocytes trigger the transformation of the magna form back into the minuta form, which subsequently initiates encystment.

Pathogenic effect. Amebiasis is an anthroponosis, an acute protozoan infection that can transition into a chronic stage. The source of infection is an ill person or a cyst carrier. Amebiasis manifests as frequent bowel movements (up to 10-20 times per day) containing mucus and blood. By invading Blood Vessels, the amoebas can be carried via the bloodstream to the Liver, Lungs, and Brain, where they form abscesses (suppurative lesions). Untreated, the disease can be fatal.

Laboratory Diagnostics. The test material consists of fresh feces. Smears are examined under a Microscope. Microscopic analysis reveals the magna vegetative form with phagocytosed erythrocytes in acute amebiasis, and tetranucleate cysts in chronic amebiasis and in clinically healthy cyst carriers. Cyst diameter ranges from 10 to 15 µm.

Prevention. Individual prophylaxis involves washing hands before meals and after using the restroom, thermal Processing of food and drinking water, thorough washing of vegetables and fruits consumed raw, and protecting food and water from dust and flies. Public prophylaxis includes sanitary inspection of water supplies, food-processing plants, grocery stores, and public restrooms; vector control against flies; screening food service personnel for cysts; and treating patients with amebiasis as well as cyst carriers.

Entamoeba coli (Intestinal amoeba). Non-pathogenic, commensal, does not cause disease, lives in the lumen of the large intestine, and is morphologically similar to the dysenteric amoeba. It also forms vegetative stages and cysts, but it does not secrete proteolytic enzymes and does not invade the intestinal wall. It feeds on bacteria and food debris. Phagocytosed erythrocytes are absent from its cytoplasm. The endoplasm contains numerous vacuoles. The cyst typically contains 8 nuclei, with a diameter of 13-25 µm (Fig. 2.1).

Entamoeba gingivalis (Mouth amoeba). The first parasitic amoeba discovered in humans. It is frequently found in decayed Teeth and the white coating on teeth. It feeds on bacteria and leukocytes. Its pathogenic significance is doubtful.

Free-living amoebas. Among free-living aquatic amoebas (Naegleria, Acanthamoeba, Hartmanella), mutant forms occur that, upon entering the human body, cause Central Nervous system diseases (meningoencephalitis).