Medical Parasitology PDF 2024-2025 - Lec 1

Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 1 Medical Parasitology Overview This programme provides training in the theoretical and practical aspects of medical parasitology, covering the...

Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 1 Medical Parasitology Overview This programme provides training in the theoretical and practical aspects of medical parasitology, covering the protozoan and metazoan parasites of humans and the vectors which transmit them. Students will gain skills enabling them to pursue a career in research, control or teaching related to medical parasitology. By the end of this programme students will be able to:  demonstrate detailed knowledge of the biology, life cycles, pathogenesis, diagnosis of parasitic infections in humans and their effects for human health and strategies for control.  demonstrate detailed knowledge and understanding of the biology and strategies for control of the vectors and intermediate hosts of human parasites  carry out practical laboratory identification of the various parasite stages both free and in tissues and diagnose infections  demonstrate specialised skills acquired through taking modules on: advanced diagnostic, molecular, immunological, genetic, chemotherapeutic, ecological and/or control aspects of the subject Introduction and Definitions - Medical parasitology is a branch of biology deals with the parasites which cause human infections and the diseases they produce. 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. -Parasitism is a relationship between two organisms: a parasite, usually the smaller of the two, and a host, upon which the parasite is physiologically dependent. The relationship may be permanent, as in the case of tapeworms found in the vertebrate intestine, or temporary, as with female mosquitoes, some leeches, and ticks, which feed intermittently on host blood. Such parasites are considered obligatory parasites because they are physiologically dependent upon their hosts and usually cannot survive if kept isolated from them. Facultative parasites, on the other hand, are essentially free-living organisms that are capable of becoming parasitic if placed in a situation 1 Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 1 conducive to such a mode. An example of a facultative parasite is the amoeba Naegleria. While there are numerous systems for classifying host-parasite relationships, the one used here distinguishes between two major types of parasites, endoparasites and ectoparasites, according to location. Endoparasites live within the body of the host at sites such as the alimentary tract, liver, lungs, and urinary bladder; ectoparasites are attached to the outer surface of the host like: lice, ticks, mites. According to its role, the host may be classified as (1) a definitive host (final host), if the parasite attains sexual maturity therein. (2) an intermediate host, if it serves as a temporary, but essential, environment for the development of the parasite and/or its metamorphosis short of sexual maturity; and (3) a transfer, if it is not necessary for the completion of the parasite’s life cycle but is utilized as a temporary refuge and a vehicle for reaching an obligatory, usually the definitive, host in the cycle. The term zoonosis can be used if parasitic infections which humans acquire from animals "are known as zoonotic". Person-to-person transmission does not normally occur in zoonosis. An example of a zoonotic disease of significant medical importance is trichinellosis, caused by the nematode Trichinella spiralis, humans become infected by consuming raw or undercooked meat from infected animals. Parasites’ Effect upon the Host There are many pathological effects of selected parasites upon their hosts. In general, the causes and types of damage done by parasites to their hosts are: 1- Mechanical damage mainly due to blockages. 2- Migration through tissues, penetration into cells. 3- Nutrition; depleting the host of nutrients or competing with the host for essential nutrients. 4- Toxins; very often the metabolic waste products of the parasite’s metabolism accumulate in the host tissues and become toxic to the host. 5- Immunosuppression; most parasites are relatively long-lived and continually present an antigenic challenge to the host, reducing the effectiveness of the immune response. 2 Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 1 6- Tissue damage; if parasites been in the ‘wrong host’ are in general more pathogenic. Most endoparasites have a preferred site within the host. This is not normally at the point of entry, hence the need for migration. Parasites in the wrong host continue their migrations, ‘lose their way’ and end up in situations where they cause considerable necrosis of tissues. 7- The physical presence of a parasite and its metabolic waste products can stimulate a host response, e.g. production of alkaline phosphatase, corticosteroids, etc. An older parasite may die and the presence of the dead parasite may become problematic. -Mechanical Damage Once a parasite has actually penetrated a host, whether it is intracellular or extracellular or in one of the many body cavities (lumens), its physical presence will disturb the local homeostatic balance. Physical features of the parasite such as hooks, protruding hardened structures and biting mouth parts damage the cells immediately surrounding the parasite. Body fluids — including blood — and cellular debris accumulate around the parasite. The immediate effect is swelling and redness. Apart from the gut and some of the lung-dwelling parasites, very few parasites are at their preferred site immediately after infection. The parasite then attempts to migrate from the site of infection to its predetermined site. - Blockages; the filarial nematodes Wuchereria bancrofti and Brugia pahangi live in the subcutaneous tissue of the lower limbs. The result is oedema of the lower limbs and also the enlargement of the genitalia. Heavy infections of Ascaris lumbricoides in children can block the small and large intestine. Erythrocytes infected with Plasmodium falciparum can become sequestered in the blood capillaries of the brain, causing cerebral malaria. The eggs of schistosomes can damage small blood vessels and block ducts within the portal system and blood vessels of the bladder. -Migration within the Host Parasites can move through the intercellular matrix between the cells and cell walls. The parasitic helminths progress through the tissues by active body movements, use of their mouth parts, hooks etc, and/or by the secretion of proteolytic enzymes. The larger the parasite the greater will be the physical damage to the tissue caused by the movement of the parasite. The motile protozoan parasites use flagellae, cilia or an amoeboid type of locomotion to move through body fluids or tissue. Most protozoa generally travel via body fluids and often depend upon the blood circulation to reach 3 Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 1 their destination. The intracellular parasites, once they have invaded their host cell, remain there until they reach a reproductive phase. To release the next phase into the host, the parasites burst out of the cell, which invariably destroys the host cell. -The Parasite’s Feeding Activity Further aspects of parasite nutrition can have an effect on the host by depleting the host of essential nutrients and in the process causing damage to surrounding tissues. Those parasitic helminths that have a gut and mouth feed upon the surrounding tissues, body fluids or even host cell debris which may have been caused by their presence. The flukes absorb their nutrients either through their cell membrane or via the outer tegument. They absorb molecules into their tissues from the surrounding host fluid and could deprive the host of certain essential nutrients. In some instances, especially with gut parasites, they often compete with the host for essential nutrients. The tapeworm Diphyllobothrium latum can absorb vitamin B12 from the contents of the host’s gut against a concentration gradient and deprive the host. Schistosoma mansoni feeds mainly on blood proteins; and a heavy infection can lead to the host suffering from malnutrition. -Toxins Many of the toxins produced by parasites are waste products of their metabolism and are simply excreted and deposited into the host’s fluids and tissues. Haematin appears to be a metabolic waste product produced by Plasmodium feeding on haemoglobin. The haematin is deposited in liver and spleen where it apparently does have some harmful effects. Amyloid deposits could be a reaction to certain parasite waste products. -Immunosuppression Most parasitic infections are chronic and there is a continuous release of antigens into the host. The host’s immune system has to either continuously react or become tolerant. This condition can make the host more susceptible to normally lesser infections. Sheep with heavy infections of Fasciola hepatica often die because of the bacterium Clostridium oedematiens rather than as a direct result of the parasite. Parasite Life-Cycles Two reproductive phases can occur during a parasite’s life history: 4  Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 1 # An asexual phase which results in the accumulation of numerous individuals of a particular species in one host. # A sexual phase: the mature phase of the parasite’s life history. There are certain parasites which can undergo the processes of both sexual and asexual multiplication.Very seldom do both processes occur at the same time and they represent different phases of the life history. It is often referred to as ‘alternation of generations’ but unlike a similar phenomenon in plants there is no change in the chromosome number. GLOSSARY Endemic: a characteristic of a particular population, or region. Examples of endemic diseases include hydatid cysts in Iraq. The disease is present in a community at all times but in relatively low frequency. By contrast, there are the related terms "epidemic" and "pandemic":  An epidemic is a sudden severe outbreak within a region or a group.  A pandemic occurs when an epidemic becomes very widespread and affects a whole region, a continent, or the entire world (covid-19). Infections. Invasions of the body by a pathogenic organism (except arthropods), with accompanying reaction of the host tissues to the presence of the parasite. Infestation. The establishment of arthropods on or within a host (including insects, ticks, and mites). in vitro. is Latin for " within the glass" i.e. in a test tube or other nonliving system. in vivo. is Latin " within the living" i.e. within the living body. parasitemia. The presence of parasites in the blood (e.g., malaria schizonts in red blood cells). parasitism. The association of two different species of organisms in which the smaller species lives on or within the other and has a metabolic dependence on the larger host species. pathogenic. Production of tissue changes or disease. pathogenicity. The ability to produce pathogenic changes. serology. The study of antibody-antigen reactions in vitro, using host serum for study. 5 Medical Parasitology Prof. Dr. Malak M. Almusawi M.Sc. Course/ 2024-2025 Lec. 1 CLASSIFICATION OF PARASITES Medical Parasitology: Covers Protozoology, Helminthology & Medical Entomology. Therefore, we can divide the parasitic organisms as below: Protozoa: unicellular eukaryotic microorganisms. It is subdivided into: A. Phylum Sarcomastigophora 1. Class Lobosea: organisms that move by means of pseudopodia 2. Class Zoomastigophorea: organisms that move by means of flagella B. Phylum Ciliophora 1. Class Kinetofragminophorea: organisms that move by means of cilia C. Phylum Apicomplexa 1. Class Sporozoea: organisms with both sexual and asexual reproductive cycles; apical complex can be seen with an electron microscope. Metazoan organisms include: i. Helminthes—wormlike invertebrates as in below: A. Phylum Platyhelminthes: flatworms 1.Class Cestoda: tapeworms (body flattened and segmented). 2.Class Digenea: trematodes, flukes (body flattened, leaf-shaped, and nonsegmented). B. Phylum Nemathelminthes 1. Class Nematoda: roundworms (body round in cross-section). ii. Arthropods possess a hard exoskeleton and jointed appendages. Only those that are parasitic to humans and those that transmit parasitic diseases are included: Phylum Arthropoda 1. Class Insecta: flies, mosquitoes, bugs, lice, and fleas 2. Class Arachnida: ticks & mites. References - Ruth, Leventhal & Russell, F. (2012). Medical Parasitology. 6 th edition. - BURTON J. B.; CLINT, E. C. & THOMAS, N. OELTMANN. (2013). Human Parasitology. 4 th edition. 6