BASICS OF MEDICAL BIOLOGY - 2012

Medical and biological foundations of parasitology. Medical protozoology. Subkingdom Protozoa. Phylum Sarcomastigophora. Class Lobosea

Parasitism is a form of antagonistic coexistence between organisms of different species, in which one (the parasite) uses the other (the host) as a source of Nutrition and habitat, causing it harm, though usually not significant enough to cause its death. The phenomenon of parasitism is extremely widespread in nature. Parasitic forms include all Viruses, many Bacteria, Fungi, animals, and some plants.

Parasitology (from Greek parasitos - a parasite, logos - science) is the science that studies the biology and ecology of parasites, their relationships with hosts and the environment, the diseases they cause, and control measures against them. Medical parasitology studies these same issues with respect to the human Organism: the biology and ecology of human parasites, the diseases they cause, and Methods for their Diagnosis, Treatment, and Prevention. It consists of three main branches: 1) medical protozoology, which studies human parasites belonging to the subkingdom Protozoa; 2) medical helminthology, which investigates parasitic worms (helminths) of humans; 3) medical arachnoentomology, which studies Representatives of the phylum Arthropoda that are of medical significance.

There is a large group of diseases caused by living organisms. Among them, infectious and invasive diseases are distinguished. Human diseases caused by animal parasites (pathogenic protozoa, helminths, or Arthropods) are called invasive or parasitic diseases (parasitoses), in contrast to infectious diseases, which are caused by non-animal organisms such as viruses, bacteria, rickettsiae, and fungi. The names of parasitic diseases are formed by adding suffixes such as "-iasis", "-osis", or "-esis" to the ROOT of the species (or genus) name (e.g., amebiasis, trichinosis, Scabies).

Among infectious and invasive diseases, we distinguish anthroponoses, anthropozoonoses, and zoonoses. Anthroponoses are diseases specific only to humans (e.g., amebiasis). Zoonoses are diseases characteristic of animals. Anthropozoonoses (zooanthroponoses) are diseases whose causative agents affect both animals and humans (e.g., leishmaniasis, echinococcosis).

Parasitism is divided into facultative and obligate. Facultative parasitism is typical of organisms that live freely in nature, but upon accidentally entering the body of another species (the host), adopt a parasitic lifestyle (such as certain Roundworms). Obligate parasitism is characteristic of organisms that are incapable of living freely in nature.

Class="center">Classification of Parasites

Parasites are organisms that use organisms of another species (the host) as a source of nutrition and habitat, causing them harm and shifting The regulation of their environmental relationships entirely or partially onto the host.

Parasites are classified as follows:

- According to the site of localization: 1) Ectoparasites (external parasites) live On the surface of the host's body: on the Skin surface, within the skin layers, or in cavities communicating with the external environment (e.g., lice, mites). 2) Endoparasites (internal parasites) live inside the host's Internal Organs, Tissues, cavities, or body Cells (e.g., dysentery ameba, Ascaris).

- Depending on the duration of contact with the host, parasites can be temporary or permanent. Temporary parasites are typically associated with the host only during feeding, spending most of their life cycle in the external environment (e.g., mosquitoes, argasid ticks). Among permanent parasites, a distinction is made between relatively permanent (spending only a specific developmental stage on the host) and strictly permanent ones, which spend their entire life cycle on or inside the host and cannot survive in the external environment at any stage of their development (e.g., malaria plasmodia, lice).

- Depending on the number of potential hosts, they are classified as: monoxenous, adapted to life in a single host species (e.g., human roundworm, which parasitizes humans exclusively), and heteroxenous, adapted to life in multiple species (e.g., Trichinella, a parasite of over 100 mammal species).

Classification of Hosts

Hosts can be definitive, intermediate, or reservoir. A definitive (final) host is an organism in which the parasite reaches the sexually mature stage and reproduces sexually. An intermediate host is an organism in which the parasite resides in a larval stage or reproduces asexually. If There are two intermediate hosts, the second one is called supplementary (or second intermediate). A reservoir host is an organism in which the parasite maintains viability and accumulates, but does not undergo development.

The host develops a complex of defensive reactions in response to parasite invasion. It has been established that a parasitic infection does not always develop into disease. Susceptibility to invasion is influenced by age, the physiological state of the organism, and the status of The Immune System. The host's impact on the parasite involves cellular, tissue, and humoral reactions. The host organism serves as a first-order environment for the parasite (according to E.N. Pavlovsky). The external environment in which the host lives is the second-order environment. The external environment affects the parasite through biotic and abiotic factors not directly, but through the host organism.

A parasitic system is a parasite population together with all host populations that directly sustain its existence.

Parasites of different species can coexist within a host organism. The aggregate of parasites simultaneously living within a host organism or in its individual organs is called a parasitocenosis. This term was proposed by E.N. Pavlovsky (1952). Different species of parasites interact with one another, either amplifying or mitigating their combined harmful effect on the host. For example, bacterial dysentery in patients suffering from ascariasis and other helminthiases is more severe and less responsive to treatment. It is important for pharmacists to understand the relationships within parasitocenoses. The aggregate of organisms (both parasites and non-parasites) together with the host organism was termed a symbiogeocenosis by the Ukrainian parasitologist O.P. Markevych.

Morphophysiological adaptations to parasitism: 1) the presence of attachment organs in parasites (suckers, hooks, attachment grooves); 2) enlargement of the Digestive System volume due to the appearance of blind intestinal cecal pouches (e.g., medicinal leech, ticks); 3) high fecundity resulting from the extensive Development of the Reproductive System; 4) simplification of body Organization through the regression or underdevelopment of certain Organ Systems (e.g., lack of locomotive organs and digestive system in tapeworms, loss of wings in lice and fleas).

Life Cycle of a parasite is the totality of developmental stages through which a parasite reaches sexual maturity and becomes capable of generating the next generation. A parasite carrier is a human or animal in whose body parasites live without causing clinical manifestations of disease. The source of the infectious agent (parasite) is an organism that serves as its natural habitat and site of reproduction.

The mechanism of transmission of the agent (The entry of parasites into the host organism) is the method by which the pathogen moves from an infected to a susceptible organism. The MAIN MECHANISMS OF transmission are: 1) fecal-oral — the parasite, at a certain stage of its development, is excreted outside with the host's feces, and its infective stage enters the host's Mouth via unwashed hands or contaminated food (e.g., dysentery ameba); 2) transmissible (vector-borne) — the parasite enters the host organism through a Blood-sucking vector (e.g., malaria plasmodium); 3) contact — the host becomes infected through direct contact with a sick person or with the personal belongings of the infected person.

Routes of infection refer to the pathways by which a parasite enters the host organism. The main routes of infection are: peroral — via the mouth (through food, Water, etc.); percutaneous — through the skin; airborne — via the respiratory tract; contact-household — through contact with a sick person or their belongings; vector-borne (transmissible) — through vector bites; transplacental — through the Placenta from mother to fetus; sexual — through sexual contact; hemotransfusion — via accidental transfusion of contaminated blood; contaminative — through passive entry of the pathogen into the host organism.

Autoinfection is the repeated self-infection of a host by a parasite already parasitizing within its body. Reinfection is a repeated infection of a person by parasites with which they were previously infected, but after recovering. Autoreinfection is repeated self-infection with one's own parasites without their prior exit into the external environment.

Pathogenicity is the potential ability of a given parasite species to induce an invasive process or a parasitic disease. Pathogenicity is a specific trait, meaning that a particular parasite causes only a specific parasitic disease (e.g., the whipworm causes trichuriasis). The infective stage is the stage of the parasite's life cycle that, upon entering the host organism, causes disease. The invasive process is the aggregate of all defensive and pathological Reactions of the organism that arise in response to the penetration and action of the parasite. Virulence is the degree of pathogenicity. An invasive (parasitic) disease is the extreme degree of development of the invasive process, manifested by specific clinical signs. The parasite's impact on the host is mechanical and toxic, and it feeds at the host's expense. Extensity of infection is the relative number of individuals infected by a parasite within a population. Intensity of infection is the number of parasites present in a single individual, organ, or Cell.

Vector-Borne Diseases

Pathogens of vector-borne diseases, both infectious and invasive, are transmitted from one host to another through vectors. A distinction is made between obligately vector-borne and facultatively vector-borne diseases. Pathogens of obligately vector-borne diseases are transmitted exclusively via vectors (such as malaria, epidemic and relapsing fevers). Pathogens of facultatively vector-borne diseases are transmitted both by vectors and through other, alternative routes (such as plague, tularemia). The plague pathogen can be transmitted to humans vector-biologically (through flea bites), as well as by contact (when skinning infected commercial rodents) or airborne droplet transmission (in the pneumonic form of plague). Vectors can be specific or mechanical. Specific (obligate) vectors are those in whose bodies the pathogen undergoes a developmental cycle, predominantly blood-sucking arthropods. For example, the Anopheles mosquito is a specific vector of the malaria pathogen. In the bodies of mechanical vectors, the pathogen does not undergo a developmental cycle; rather, it is merely transported through space. Thus, pathogens of various diseases can be carried on the body surface, legs, or in the gut of the common housefly.

Natural focal diseases are infectious and invasive diseases that can persist in specific geographic areas for long periods, independently of humans. The Doctrine of natural focal diseases was developed in the 1930s by Academician Ye.N. Pavlovsky. A natural focus is an area with a specific biocenosis whose obligatory components are: 1) the disease pathogen; 2) the natural reservoir of the pathogen, represented by wild animals; and 3) the vector. A natural focus poses a potential danger to humans. If a person enters this area (during expeditions or hunting), they may contract the pathogen. Initially, natural focality was established for vector-borne diseases: taiga (spring-summer) encephalitis, tick-borne relapsing fever, and leishmaniasis. It was later discovered that non-vector-borne diseases, including helminthiases (such as opisthorchiasis, paragonimiasis, diphyllobothriasis, trichinosis, and alveolar echinococcosis), are also naturally focal. In addition to natural foci, there are synanthropic (rural, settlement, urban) and natural-synanthropic foci of invasive and infectious diseases. The doctrine of natural focality is incorporated into the WHO action program for combating parasitic diseases. In developing such measures, the biological CHARACTERISTICS OF THE pathogen, the vector, and the animal reservoir hosts are taken into account.

Parasitic diseases (parasitoses) are among the most widespread conditions globally. According to WHO data, the number of ascariasis cases worldwide reaches 1,000 million people. Ancylostomiases rank second with 900 million cases, with infection rates reaching up to 90% in certain foci. Trichuriasis (500 million cases), amebiasis (400 million), giardiasis (370 million), and enterobiasis (350 million) are distributed practically everywhere.

Prevention of parasitic diseases. Preventive measures are directed along the following lines: 1) direct or indirect elimination of parasites and their germs in the environment and The Human Body; 2) blocking the pathways of human infection by specific parasites; and 3) raising the general level of sanitary culture.

Methods for preventing parasitic diseases include biological, immunological, ecological, and social approaches. Biological methods involve using living organisms to combat parasites and their vectors; for instance, gambusia fish are used to destroy malaria mosquito larvae. Immunological methods for preventing human parasitic diseases are insufficiently developed. Ecological methods entail protecting the environment from contamination with invasive stages of parasites (such as contamination by human feces that may contain protozoan cysts or helminth eggs). Social methods of prevention comprise medical and sanitary-hygienic measures: detecting and treating the infected, organizing sanitary supervision over water supply sources and food preparation technologies, and conducting veterinary-sanitary inspections. The effectiveness of preventive measures depends on public participation and adherence to personal hygiene rules.

Medical Protozoology

This is a branch of parasitology that studies human parasites belonging to the subkingdom Protozoa, the diseases they cause (protozooses), and control measures against them.

Subkingdom Protozoa

Protozoa are single-celled animal organisms, the vast majority of which are microscopic in size. The total number of species exceeds 65,000. They inhabit diverse environments, including fresh and marine water, as well as soil. Many species lead a parasitic lifestyle.

The body of a protozoan consists of Cytoplasm, a Nucleus, and a cell membrane (a thin pellicle preserving The properties of living cytoplasm or a dense cuticle). The cytoplasm is divided into two layers: 1) an outer, transparent, denser layer (ectoplasm) and 2) an inner, granular layer (endoplasm). The endoplasm contains general-purpose Organelles (Mitochondria, Endoplasmic reticulum, Ribosomes, etc.) and specialized organelles that perform Functions of locomotion (pseudopodia, flagella, cilia), Digestion (digestive vacuoles), excretion (contractile or pulsating vacuoles), and defense (trichocysts in Ciliates). Contractile vacuoles expel excess water and liquid dissimilation products, maintain a constant osmotic pressure, and supply The Cell with oxygen. Protozoa may have one or several nuclei. The Nucleus has the typical Structure of a Introduction/5.html">Eukaryotic Cell nucleus. Protozoa reproduce asexually and sexually. Some species exhibit an alternation of Selection/8.html">Asexual and sexual reproduction (metagenesis). Nutrition is heterotrophic, involving the uptake of food via phagocytosis and pinocytosis, or osmotically, as well as mycotrophic.

The most characteristic property of protozoa is their ability to form cysts under unfavorable conditions (encystment). In this process, protozoa cease movement, assume a spherical shape, lose their locomotor organelles or retract them into the body, and become covered by a dense protective shell. Metabolic processes within the cyst slow down. In the encysted state (anabiosis), protozoa can withstand drastic environmental changes (desiccation, cooling, exposure to chemicals) while remaining viable. Upon the return of favorable conditions, the cysts rupture (excystment), and the protozoa emerge as active vegetative forms. During its period of active metabolic life, a protozoan cell is called a trophozoite.

CLASSIFICATION OF PROTOZOA. According to the recommendations of the International Committee on Taxonomy of Protozoa (1980), all protozoa are grouped into the subkingdom Protozoa, which comprises 7 phyla. Parasitic protozoa affecting humans belong to three phyla: Sarcomastigophora, Apicomplexa, and Ciliophora. The classification is based on The structure of locomotor organelles and reproductive characteristics.

Phylum Sarcomastigophora.

Class Lobozea.

Amoebas are the most simply organized protozoa. Their cytoplasm is separated from the external environment only by a membrane; consequently, the body lacks a permanent shape. The organelles of locomotion and food capture are pseudopodia (false feet). The Mechanism of capturing and digesting food is called phagocytosis. Amoebas reproduce primarily asexually by binary fission. Under unfavorable conditions, they form cysts. Of medical significance is the order Amoebida, which includes the pathogenic dysenteric amoeba and several non-parasitic species (intestinal, oral, etc.).

Entamoeba histolytica is the CAUSATIVE AGENT OF amebiasis, or amoebic dysentery, discovered by the St. Petersburg scientist F.O. Lösch in 1875.

The sole source of infection is humans.

Fig. 44. 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 within digestive vacuoles; Entamoeba coli: vegetative form (e) and its octanucleate cyst (f).

Geographic distribution - cosmopolitan, though more frequent in countries with hot climates.

Localization - Large Intestine.

Morphology. It parasitizes exclusively in humans. Its life cycle includes the following forms: 1) cyst; 2) small vegetative form (forma minuta); 3) large vegetative form (forma magna); and 4) tissue form.

Life cycle. Cysts are released into the external environment with the feces of infected individuals. They enter the human body orally (via a fecal-oral mechanism and alimentary route) through unboiled water, unwashed vegetables and fruits, and contaminated hands, and are further disseminated by flies and cockroaches (mechanical vectors). The infective stage is the quadrinucleate cyst. In the human intestine, the cyst wall dissolves, releasing a 4-nucleate amoeba that rapidly divides into eight uninucleate small 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. Sometimes an infected person sheds cysts for years without exhibiting any symptoms of illness. Such individuals are called cyst carriers. They represent a significant public health hazard as a source of infection for others. A single cyst carrier can shed up to 600 million cysts per day.

However, under certain conditions in some individuals (such as hypothermia, overheating, avitaminosis, concurrent infections, or 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 accompanied by ulceration (amebiasis). This process destroys the walls of blood capillaries, resulting in hemorrhages into the intestinal lumen. The invasion of the intestinal mucosa and its destruction are attributed to the ability of the dysenteric amoeba to secrete Proteolytic Enzymes that dissolve tissue Proteins.

When amoebic intestinal lesions develop, the small vegetative forms dwelling in the intestinal lumen transform into the large vegetative form (*f. magna*), which feeds on erythrocytes. It is characterized by a large size (30–40 µm) and nuclear structure: the nuclear Chromatin forms radial structures, with a large chromatin granule—the karyosome—situated at the center of the nucleus. The large vegetative form is distinguished by blunt, broad pseudopodia and jerky movement. 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 one, which then begins to encyst.

Pathogenic effect. Amoebiasis is an anthroponosis, an acute protozoan disease that tends to become chronic. The source of infection is a sick person or a cyst carrier. Amoebiasis manifests as frequent stools (up to 10–20 times a day) mixed with blood and mucus. 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 Diagnostics. The sample consists of feces, which must be fresh. Smears are examined under a Microscope. The large vegetative form with phagocytosed erythrocytes is detected in acute amoebiasis, whereas quadrinucleate cysts are found in chronic amoebiasis and in apparently healthy cyst carriers. The size of the cysts ranges from 10 to 15 µm.

Prevention. Personal: washing hands before eating and after using the toilet; thermal Processing of food and drinking water; thorough washing of raw vegetables and fruits; and protecting food and water from dust, flies, and cockroaches. Public: Sanitary supervision of water supply sources, food processing plants, grocery stores, and public restrooms; control of flies and cockroaches; screening of food service workers for cysts; and treatment of amoebiasis patients and cyst carriers.

Colonic amoeba (Entamoeba coli). Non-pathogenic, a commensal that does not cause disease; it lives in the lumen of the large intestine and is morphologically similar to the dysenteric amoeba. It also forms vegetative forms 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, with a size of 13–25 µm.

Gingival amoeba (Entamoeba gingivalis). The first parasitic amoeba discovered in humans. It is frequently found in decayed Teeth and dental plaque. It feeds on bacteria and leukocytes. It does not form cysts. Its pathogenic significance is questionable.

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