Practical Class N 19: Biological Basis of Parasitism PDF

PRACTICAL CLASS N 19 THEME: BIOLOGICAL BASIS OF PARASITISM. THE TOPIC RELEVANCE: The interaction of organisms with each other and with the environment is of great importance for their vital activity. The relations of organisms can be mutuall...

PRACTICAL CLASS N 19 THEME: BIOLOGICAL BASIS OF PARASITISM. THE TOPIC RELEVANCE: The interaction of organisms with each other and with the environment is of great importance for their vital activity. The relations of organisms can be mutually beneficial, mutually harmful, useful-neutral, useful-harmful. For medicine, useful- harmful relationships, which include parasitism, are of special importance. For effective treatment of parasitic infestations, as well as for determining preventive measures, a comprehensive study of parasites is necessary: their morphology, life cycle and other characteristics. Knowledge about the parasite effects on the host helps in the rapid diagnosis of the invasion. Knowledge about the natural sources of parasitic diseases has a great importance for preventive measures in the population. A natural hearth is a place of certain types of parasites localization, depending on natural and climatic conditions. If you know the prevalence of infestations in different climatic zones it will be possible to carry out specific preventive measures in time, as well as notifying the population about possible ways of infection by specific parasites. THE AIM: 1. To study the biological basis of relations between living organisms in nature. 2. To study the main types of organism relations in ecosystems. 3. To study the evidence of parasitism. 4. To determine the biological role of parasites in evolution of organic world. THE PLAN FOR THEME STUDY: 1. The main types of biotic interactions in nature. 2. Parasitism as the way of living organism existence. 3. Parasitology: subject-matter, goals. 4. The main terms used in parasitology. 5. The ways of parasites circulation in nature. 6. Ecological aspects of parasitism. 7. Medical aspects of parasitism. 8. Systematics of parasites. THE NOTES: 1. The main types of biotic interactions in nature. It is known that organisms in an ecosystem are always doing something i.e., interacting with each other as well as with the non-living environment. Plants and animals associate temporarily or permanently with organisms belonging to different species. An association between organisms, belonging to different species, which live together for mutual benefit, or one partner gets benefit and the other is neither benefited or harmed is called symbiosis. 1  There are two types of symbiosis: mutualism and commensalism. Mutualism is the symbiotic relationship in which both the partners get benefit and neither can survive without the other. In some cases of mutualism two partners remain continuously associated, e.g.: 1) Bacteria and Ruminants. Bacteria live in the gut of ruminants such as cow etc. The cow can’t digest cellulose, a main component of its food, however, bacteria can do so, because they produce a cellulose breaking enzyme called cellulase. 2) Termites and Flagellate Protozoa. The termites eat wood but cannot digest cellulose present in wood. The flagellates in the termite gut can digest and make food available for both. The symbiotic association when one partner gains benefit without any harm to the other is called commensalism. In certain cases, two partners remain permanently associated with another e.g.: 1) Barnacles attached to the backs of whales and turtles, get a free ride to better feeding places. 2) Certain crabs live in the mantle cavities of sea mussels for protection. So, commensalism is the symbiotic association in which only one partner gets benefit while in mutualism both the partners get benefit. An animal which captures and readily kills live animals for its food is called predator, the animal killed and eaten is called prey while this phenomenon is known as predation. Predator - prey relationships have a significant influence on the distribution and abundance of organisms: 1. Maintaining of the ecosystem stability. The predator - prey interaction helps maintaining the ecosystem stable, e.g., if a prey population increases, it will support more predators as a result the population of the latter also increases. If the prey is killed in large numbers, then its population size will be reduced. As a result, the population of predators also decreases. 2. Maintaining of the biological fitness in two populations. The relationship between predators and prey helps maintain the biological fitness of the two populations e.g., when prey is in short supply, predators which are weak are likely to go without food. In this way only the fittest of the prey and predators survive. Thus, less fit individuals are removed and inferior genes are eliminated from the population or gene pool. An association between organisms of different species in which one individual is benefited (parasite) and the other though harmed (host) is not killed till the parasite has completed its reproductive cycle, is called parasitism. In parasitism, the parasite derives shelter and protection from the host. 2. Parasitism as one the way of living organism existence. Parasitism includes any reciprocal association in which a species depends upon another for its existence. This association may be temporary or permanent. The term parasite, however, is ordinarily applied to a weaker organism that obtains food and shelter from another organism and derives all the benefit from the association. The harboring species, known as the host, may show no harmful effects or may suffer from various functional and organic disorders. 2  Various descriptive names denote special types or functions of parasites. An ectoparasite lives on the outside (infestation) and an endoparasite within the body of the host (infection). Parasites are termed facultative when they are capable of leading both a free and a parasitic existence and obligatory when they take up a permanent residence in and are completely dependent upon the host. Obligatory parasite can’t exist without a parasitic life. An incidental parasite is one that establishes itself in a host in which it does not ordinarily live. A temporary parasite is free-living during part of its existence and seeks its host intermittently to obtain nourishment. Temporary parasite visits its host for a short period. A permanent parasite remains on or in the body of the host from early life until maturity, sometimes for its entire life. It leads a parasitic life throughout the whole period of its life. A pathogenic parasite causes injury to the host by its mechanical, traumatic, or toxic activities. A pseudoparasite is an artifact mistaken for a parasite. A coprozoic, or spurious, parasite is a foreign species that has passed through the alimentary tract without infecting the host. Occasional or Accidental parasites attack unusual hosts. Parasites often lack the necessary organs for assimilating raw food materials and depend upon the host for predigested food. An adequate supply of moisture is assured inside the host, but during the free-living existence of the parasite inadequate moisture may either prove fatal or prevent larval development. Temperature is likewise important. Each species has an optimal temperature range for its existence and development. Both high and low temperatures are detrimental and even lethal. Differences between parasitism and predation: 1. Parasites usually have rapid rates of reproduction than that of the predators. 2. Parasites are more specialized in structure, metabolism, host specificity and life history. These characters are necessary for their transfer from one host to other. 3. Parasites show great diversity in ways and means to exploit their hosts. So, they may be viral parasites, microbial parasites, phytoparasites or zooparasites. 3. Parasitology: subject-matter, goals. Parasitology is the science that deals with organisms that take up their abodes, temporarily or permanently, on or within other living organisms for the purpose of procuring food, and with the relationship of these organisms to their hosts. In the restricted sense employed here the term is applied only to animal parasites belonging to the protozoa, helminthes, and arthropods. Medical parasitology deals with the parasites, which infect man and the diseases they produce. The goals of medical parasitology are: 1. To study the features of the parasites structure at all stages of development for accurately determination of the species. 2. To study the parasites systematics, their classes and classifications. 3. To study the parasite-host relationship 3  4. To study the features of the parasites and vectors life-cycles for establishment of parasites circulation ways in nature and ways of human infection. 5. Development of scientific basis for the diagnosis and treatment of parasitic diseases, methods of prevention and control of parasitic diseases. 4. The main terms used in parasitology. Parasites are organisms that infect other living beings. They live in or on the body of another living being, the host, and obtain shelter and nourishment from it being physically and physiologically dependent on it for at list part of its life cycle. They multiply or undergo development in the host. Parasitism arose early in the course of biological evolution. The discipline of medical parasitology deals only with parasites belonging to the animal kingdom. Though bacteria, fungi and viruses are also parasitic, they are excluded from the purview of ―parasitology‖. Human parasites may be either unicellular (Protozoa), or larger organisms (Metazoa), some of which may be many meters in size. The term parasite means: a living organism which receives nourishment and shelter from another organism where it lives. Host is an organism which harbors the parasite. Ectoparasite (Ectozoa): Lives outside on the surface of the body of the host. Lice, ticks, mites are examples of ectoparasites. They are important as vectors transmitting pathogenic microbes. Endoparasite (Entozoa): Lives onside the body of the host: in the blood, tissues, body cavities, digestive tract and other organs. The term infestation is often employed with ectoparasites in place of the term infection used with reference to endoparasites. All protozoan and helminthic parasites of man are endoparasites. Classes of hosts are: Definitive Host. Either harbors the adult stage of the parasite or where the parasite has the sexual method of reproduction. In the majority of human parasitic infections, man is the definitive host; in malaria and hydatid disease, however, man acts as the intermediate host. Intermediate Host: harbors the larval stages of the parasite or where the parasite has the asexual method of reproduction. In some cases larval developments are completed in two different intermediate hosts which are been referred to as first and second intermediate host respectively. Paratenic Host (a carrier or transport host): a host where the parasite remains viable without further development. 5. The ways of parasites circulation in nature. The endemicity of a parasite depends upon the presence and habits of a suitable host, upon easy escape from the host, and upon environmental conditions favoring survival outside the host. Parasites with simple life cycles are more likely to have a cosmopolitan distribution than those with complicated life cycles. Economic and social conditions affect the distribution of the parasites of man. Parasites get to their hosts in different ways. Often hosts are infected by vectors - usually blood-sucking arthropods. This method of transmitting the pathogen is called transmissive. There are two variants of it: inoculative and contaminative. 4 In the first case, the pathogen penetrates into the host's blood through the oral apparatus of the carrier, in the second case, it is released by the carrier with feces or otherwise onto the skin or mucous membranes and from there enters the host's body through a wound from a bite, scratch, scratching, etc. Another way of infection is through intermediate hosts. Each parasite, according to the mode of its existence, selects a particular place of abode in the host. The parasite, after entering into the body, may directly establish itself in the place, where it is introduced or the parasite, on entering the body through a particular route, may travel through different organs till it reaches its normal abode for its growth to sexual maturity. The general structure of the parasite and its various stages are studied. The parasite may pass its life-cycle in one and the same host or it may change its host. Transmission of the parasite from one host to another is hold by certain forms, which are known as infective stages. The means by which different infective agents are transferred from one host to another and the routes through which they enter the human body require careful consideration. In an endemic area a parasitic infection is continually kept up by the presence of hosts acting as reservoirs of infection. Such hosts may be either animals or man. Where an insect intermediate host (vector) plays the part in dissemination of the disease. The transmission of parasites involves three factors: source of infection; mode of transmission; the presence of a susceptible host. The combined effect of these factors determines the prevalence of the parasite at any given time and place. Since parasitic infections often tend to run a chronic course with few or no symptoms, an infected individual may become a carrier without showing clinical manifestations, thus serving as a potential source of infection to others. This probably represents the normal state of infection in which there is an equilibrium between the host and the parasite. Thus, infection does not always result in disease. The methods whereby parasites reach susceptible hosts from their primary sources are varied. Some parasites require only direct contact. Others with more complicated life cycles must pass through various developmental stages, either as free-living forms or in intermediate hosts, before becoming infective. Transmission may be through direct and indirect contact, food, water, soil, vertebrate and arthropod vectors, and rarely, from mother to offspring. The chances of infection are increased by environmental conditions favoring the extracorporeal existence of the parasite and by lack of sanitation and communal hygiene. A human being, when infected by a parasite, may serve as its only host; its principal host with other animals also infected, or its incidental host with one or several other animals as principal hosts. In addition to the natural adaptability of the parasite in respect to its host, the ease of transmission depends upon the habits and communal associations, as well as the resistance, of the host. Migrations of populations have spread parasitic disease throughout the world. 5 Human malaria was very likely introduced to the Western hemisphere by explorers from the Old World, as were hookworms. Slaves probably brought schistosomes and filarial parasites from Africa to the New World, immigrants from the Baltic countries introduced the fish tapeworm into North America. Although many important species of parasites have a worldwide distribution, tropical countries, where optimal conditions of temperature and humidity are present, are most favorable for the survival, larval development, and transmission of parasites. The short summer season in the temperate zones prevents the development of many species that require high temperatures during their larval stages. Intense dry heat or direct sunlight may destroy the larval forms. On the other hand, low temperatures arrest the development of eggs and larvae and may even destroy them. Freezing temperatures and snow force humanity to use privies and prevent general soil pollution. Moisture is essential for the development of free- living larvae, and it is also necessary for the propagation of intermediate hosts, such as arthropods, snails, and fishes. Even in the tropics dry plateaus, because of lack of humidity, are practically free from parasites except for resistant species or those that are transferred directly from host to host. 6. Ecological aspects of parasitism. Through millennia parasites have evolved their life-cycle that involves survival and development in the external environment and in one or more hosts. This life- cycle may be relatively simple or it may be incredibly complicated, with numerous morphologic forms and developmental stages. At times, generally when in the external environment, the parasite is quiescent in the form of resistant eggs or cysts; when taken up by an appropriate host it may undergo active growth and metamorphosis. As the life cycle becomes more complicated the chance of parasite survival decreases, but highly developed reproductive organs and multiplication at some stage of the parasite cycle offset the hazards of a complex life history. The final or definitive host harbors the adult or sexual stage of the parasite. Part or all of the larval or asexual stage may take place in another animal, known as an intermediate host, such as the snail for the schistosoma. Certain species of trematodes and cestodes have two such hosts, known as primary and secondary intermediate hosts. A paratenic host is an animal that harbors the parasite in an arrested state of development; however, the parasite is capable of continuing its cycle in a subsequent suitable host. A human being may be a definitive host for some parasites (such as the beef and pork tapeworms), an intermediate host for others (hydatid tapeworm), or an incidental host. The last instance refers to the situation in which the infected individual is not necessary for the parasite's survival or development, e.g., the human being in the case of trichinosis. The disease in this case is also referred to as a zoonosis. A zoonotic infection is one that is normally transmitted only among animals. The human, an incidental host in these situations, is also sometimes called a dead-end host, if the cycle for transmission of the parasite is such that it cannot be transmitted further. Other animals that harbor the same parasite are known as reservoir hosts. Such reservoir hosts ensure continuity of the parasite's life-cycle and act as additional sources of 6 human infection. Human infections are only accidental events and may not profit the parasite because the chain of transmission is usually broken with human infection. The term anthroponosis has been applied for infections with parasitic species that are maintained in man alone. Examples are malaria and filariasis The term zooanthroponosis refers to infections in which man is not merely an incidental host, but an essential link in the life cycle of the parasite. Beef and pork tapeworms are examples of zooanthroponosis. A large group of parasitic and infectious diseases is characterized by natural foci (hotbed). They are characterized by the following signs: 1) parasites circulate in nature from one animal to another independently of humans; 2) wild animals serve as the reservoir of the parasite; 3) diseases are not widespread everywhere, but in a limited area with a certain landscape, climatic factors and biogeocenoses. The components of the natural foci (hotbed) are: 1) the parasite; 2) animals susceptible to the parasite — reservoirs: 3) the corresponding complex of natural and climatic conditions in which this biogeocenosis exists. A special group of natural focal diseases are vector-borne diseases, such as leishmaniasis, trypanosomiasis, tick-borne encephalitis, etc. Therefore, an obligatory component of the natural focus of a vector-borne disease is also the presence of a vector. 7. Medical aspects of parasitism. From a medical standpoint, knowledge of the life-cycle of a parasite is important, since it indicates how human beings become infected and the stages at which preventive measures can be most effectively applied. The actual mechanisms by which parasites can damage the host include: 1) mechanical effects, such as pressure from an enlarging cyst or obstruction of vessels or hollow viscera, 2) invasion and destruction of host cells by the parasite itself, 3) inflammatory reaction to the parasite or parasite products, 4) competition for host nutrients. The timing of appearance of signs and symptoms will vary with the incu- bation period for the particular parasite involved and the activity and the location of the parasite at various stages of its development and life-cycle in the host. The prevention of parasitic diseases depends upon the erection of barriers to the spread of parasites through the practical application of biologic and epidemiologic knowledge. Almost every parasite at some time in its life-cycle is susceptible to special destructive measures. Thus, barriers such as sanitary excreta disposal may be established by breaking such weak links in the life cycle as may exist at the departure of the parasite or its egg from its host, during its extracorporeal existence, or at the time of its invasion of the human host. The control of parasitic diseases includes the following procedures: 1) reduction of the sources of infection in human beings by therapeutic measures, 7  2) education in personal prophylaxis to prevent dissemination of infection and to reduce opportunities for exposure, 3) sanitary control of water, food, living and working conditions, and waste disposal, 4) destruction or control of reservoir hosts and vectors, 5) construction of biologic barriers to the transmission of parasites. The therapeutic reduction of human sources of infection is a practical measure, but usually it is not applicable to animal reservoir hosts. Education of the general public in personal prophylaxis and knowledge of the precautions necessary to escape infection and to prevent its transmission to others, including a safe water supply and thorough cooking of food, is an effective means of combating parasitic diseases. Public health education, however, is a slow process, particularly in countries with limited educational facilities. Sanitary measures of waste disposal include the establishment of sewage systems, the installation of screened sanitary latrines, and the prohibition of untreated night soil as garden fertilizer. Food handlers, who may be carriers, require careful supervision and training in personal hygiene. The reduction in number of intermediate hosts or vectors has made possible the control of many parasitic diseases. Insect vectors may be controlled by the destruction of their breeding grounds, the application of insecticides, and the protection of the susceptible host by screens and repellents. Snails, the intermediate hosts of trematodes, may be destroyed, if sufficiently segregated, by chemical and physical agents, but the destruction of such intermediate hosts as mammals and fishes is usually impractical. Depending on the nature of the parasite, the following materials should be collected for specific laboratory diagnosis: blood, feces, urine, sputum, biopsy material. Many of the parasitic infections can be cured by specific chemotherapy, especially – protozoan diseases. For the direct treatment of intestinal helminthiasis drugs are given orally. Pathogenic effects may be different according to the nature of parasitic infections. In protozoan infections, the lesions are greatly influenced by proliferation, multiplication and metastasis to distant organs. In the majority of helminthic infections, the adult parasites are found inside the human body and no multiplication occurs. In certain helminthic infections, the normal secretions and excretions of the growing larvae and the products liberated from dead parasites behave like foreign proteins and give rise to various allergic manifestations. Prophylactic measures for parasitic infections may be following: 1. Therapeutic Prophylaxis. The parasite is attacked within the host, thereby preventing the dissemination of the infecting agent. 2. Drug (Clinical) Prophylaxis or Suppressive therapy. Measures are adopted not to prevent the infection but to abort the clinical manifestations by specific drug therapy. 8  3. Eradication of the infection in the reservoir hosts and destruction of intermediary hosts. 4. Personal prophylaxis, which further may be ensured by preventing the susceptible individuals coming in contact with infecting agents. 8. Systematics of parasites. Each parasite possesses two names: a generic and a specific. The genus begins with an initial capital and the species with an initial small letter. The generic and specific names are in Latin. For example, the common intestinal roundworm of man is named Ascaris lumbricoides. This means that it belongs to the Genus Ascaris and the name of the species is lumbricoides. The animal parasites, which doctors have to deal with diseases are divided into three main groups: 1. Phylum Protozoa – Medical Protozoology. 2. Phylum Plathyhelminthes and Phylum Nemathelminthes – Medical Helminthology. 3. Phylum Arthropoda – Medical Arachnoentomology. Each phylum may be further subdivided as follows: Phylum  Subphylum  Class  Order  Family  Genus  Species E.g.: Kingdom - Animalia Subkingdom – Protozoa Phylum - Sarcomastigophora Class - Sarcodina Order - Amoebida Genus - Entamoeba Species - Entamoeba histolytica, E. coli, E. gingivalis PRACTICAL WORKS: Work №1. Study of the main types of biotic interactions in nature. THE TASK: Complete the chart. Value for interacting № Type of organisms (+, 0, -) interaction 1-st 2-nd Definition Examples species species 1 Symbiosis 2 Mutualism 3 Commensalism 4 Parasitism 5 Predation 6 Competition 7 Neutralism 9  Work №2. Study of the principles of the parasite classifications. THE TASK: Complete the chart. № Type of parasite Definition Examples 1 Ectoparasite 2 Endoparasite 3 Permanent 4 Temporary 5 Facultative 6 Obligatory 7 Occasional 8 Aberrant/Wandering Work №3. Study of the routes by which humans acquire parasitic infections. THE TASK: Check the picture and suggest methods of the diagnosis and measures of prevention for the parasitic diseases. Route of infestation Methods of the diagnosis Measures of prevention 10  Direct contact, through the skin By vector Penetration of the eyes Inhalation Ingestion Sexual contact HOMEWORK: To study theme № 20: «MEDICAL PROTOZOOLOGY. CLASS SARCODINA. CLASS FLAGELLATA.». 11