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. Their Cytoplasm is bounded from the external environment only by a Cell membrane, meaning the body lacks a fixed shape. The Organelles of locomotion and food capture are pseudopodia (false feet). The process of capturing and digesting food is called phagocytosis. They reproduce primarily asexually by binary fission. Under unfavorable conditions, they form cysts. Of medical importance is the order Amoebida, which includes the pathogenic dysentery amoeba and several species of non-parasitic amoebas (intestinal, oral, etc.).

Dysenteric amoeba (Entamoeba histolytica). The 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.

The sole source of infection is humans.

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

a - Entamoeba histolytica forma magna with phagocytosed erythrocytes; b, c - Entamoeba histolytica forma minuta; d - quadrinucleate 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 - small vegetative form; 4 - cysts; 5 - tissue form in the mucous membrane of the Large Intestine; 6 - large vegetative form with ingested erythrocytes.

Localization - large intestine.

Geographical distribution - worldwide, but most 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) small vegetative form (forma minuta); 3) large vegetative form (forma magna), and 4) tissue form (Fig. 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, and contaminated hands; they are also spread by flies and cockroaches (mechanical vectors). The infective stage is the cyst. In the human intestine, the cyst wall dissolves, releasing a 4-nucleate amoeba that rapidly divides into four uninucleate small vegetative forms - f. minuta (7-15 µm in diameter). These live in the lumen of the large intestine, feed mainly on Bacteria, and do not cause disease (non-pathogenic). In the lower sections of the intestine, they encyst. A mature cyst contains four nuclei. Sometimes an infected person excretes cysts for years without showing any signs of illness. Such individuals are called cyst carriers. They pose a significant hazard as a source of infection to others. A single cyst carrier can shed up to 600 million cysts per day.

However, in some individuals under certain conditions (hypothermia, overheating, avitaminosis, infections, helminthiases), f. minuta penetrates the intestinal wall, where it multiplies intensively. Amoebas localized in the Tissues of the intestinal wall constitute the tissue form of the dysenteric amoeba. This form is pathogenic, causing lesions in the mucous membrane with ulcer formation (amebiasis). This process destroys the walls of Blood capillaries, leading to Hemorrhage into the intestinal lumen. The penetration of amoebas into the intestinal mucosa and its destruction are attributed to the ability of the dysenteric amoeba to secrete Proteolytic Enzymes that dissolve tissue Proteins.

Upon the appearance of amebic intestinal lesions, the small vegetative forms residing in the intestinal lumen transform into the large vegetative form (f. magna), which feeds on erythrocytes. It is characterized by large dimensions (30-40 µm) and nuclear Structure: nuclear Chromatin forms radial arrangements, with a large mass of chromatin - the karyosome - located in the center. The large vegetative form is characterized by blunt, broad pseudopodia and jerky locomotion. The cessation of bleeding and, consequently, the inability to feed on erythrocytes leads to the transformation of the large vegetative form back into the small form, which then begins to encyst.

Pathogenic effect. Amebiasis is an anthroponosis, an acute protozoal disease that tends to transition into a chronic stage. The source of infection is a sick person or a cyst carrier. Amebiasis is characterized by frequent bowel movements (up to 10-20 times a day) containing mucus and blood. By penetrating Blood Vessels, amoebas can be carried via the bloodstream to the Liver, Lungs, and Brain, forming abscesses (suppurations) there. Without Treatment, the disease can be fatal.

Laboratory Diagnosis. Specimen - fresh feces. Smears are examined under a Microscope. The large vegetative form with phagocytosed erythrocytes is detected in acute amebiasis, while quadrinucleate cysts are found in chronic amebiasis and in practically healthy cyst carriers. Cyst size is 10-15 µm.

Prophylaxis. Personal - washing hands before meals and after using the restroom; heat treatment of food and drinking water; thorough washing of raw vegetables and fruits consumed fresh; protecting food and water from dust and flies. Public - Sanitary supervision of water supply sources, food establishments, grocery stores, and public restrooms; vector control (flies); screening food handlers for cysts; and treating individuals with amebiasis and cyst carriers.

Intestinal amoeba (Entamoeba coli). Non-pathogenic, a commensal that 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 not observed in its cytoplasm. The endoplasm contains numerous vacuoles. The cyst typically contains 8 nuclei, and cyst size ranges from 13-25 µm (Fig. 2.1).

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

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