MLS 312 2024-2025 Session: Parasitology PDF

**DEPARTMENT OF MEDICAL LABORATORY SCIENCE** **FACULTY OF ALLIED HEALTH SCIENCES** **BENSON IDAHOSA UNIVERSITY, BENIN CITY** **COURSE CODE: MLS 312** **Course content:** - **Introduction to, parasitism, and other animal associations, adaptation to parasitic way of life.** - **How paras...

**DEPARTMENT OF MEDICAL LABORATORY SCIENCE** **FACULTY OF ALLIED HEALTH SCIENCES** **BENSON IDAHOSA UNIVERSITY, BENIN CITY** **COURSE CODE: MLS 312** **Course content:** - **Introduction to, parasitism, and other animal associations, adaptation to parasitic way of life.** - **How parasites invade their host.** - **The ineffective agents of parasites.** **INTRODUCTION TO PARASITOLOGY** The study of creatures that establish temporary or permanent residences on or within other living things in order to obtain nourishment is known as **parasitology**. In a wide sense plants like bacteria and fungi, animals like protozoa, helminths, and arthropods, and borderline taxa like spirochetes are all included in parasitology and viruses. In the more restricted sense employed here the term applies only to animal parasites. **PARASITE** A parasite is an organism that is entirely dependent on another organism, referred to as its host, for all or part of its life cycle and metabolic requirements. Strictly speaking, the term parasite can be applied to any infectious agent but, by convention, it is generally restricted to infections caused by protozoan and helminthes and excludes the viruses, bacteria and fungi. Parasites may be simple unicellular Protozoa and complex multicellular Metazoa **TYPES OF PARASITES** Parasite is of two types: 1. **Microparasite**: It is small, unicellular and multiplies within its vertebrate host, often inside cells. Protozoa are microparasites. 2. **Macroparasite**: It is large multicellular and has no direct reproduction within its vertebrate host this category includes helminthes. Various descriptive names denote special types or functions of parasites. Parasite can be classified according to their habitat or location into two: 1\. **Ectoparasites** are organisms (e.g., fleas, lice, ticks) that live on the outside of their hosts, usually attached to the skin, feathers, hair, gills etc., such forms can never lead a completely parasitic existence, but utilize oxygen from outside the host. Many maintain only periodic contacts with their hosts and, according to the definition given earlier, cannot be considered parasites but essentially special kinds of predators. 2\. **Endoparasites** are organisms that live within the body of the host are known as endoparasites. All protozoan and helminthic parasites of man are endoparasites. The invasion by endoparasites is known as infection. The itch mite (scabies) for example, burrows in tunnels in the skin and could satisfy the criteria of either an ectoparasites or an endoparasite. These can be further subdivided into the following types according to their dependence on the host 1. **Temporary (facultative) parasite:** The parasite is capable of independent existence in addition to parasitic life. E.g. *Strongyloids stercolaris*, *Naegleria fowleri*, etc or Facultative when they are capable of leading both a free and parasitic existence. 2. **Permanent (obligate) parasites:** The parasite depends completely upon its host for metabolites, shelter, and transportation. This parasite cannot live outside its host. E.g. *Plasmodium* species, *Trichmonas vaginalis* etc. 3. **Incidental** parasite is one that establishes itself in a host in which it does not ordinarily live. 4. **Occasional or Periodic** parasite seeks its host intermittently to obtain nourishment. 5. **Pseudoparasites** are artifacts mistaken for parasites. 6. **Coprozoic or spurious parasites** are species that have passed through the alimentary tract without infecting the host. **According to their Pathogenicity:** 1\. Pathogenic parasites:- It causes disease in the host. E.g., *E. histolytica* 2\. Non-Pathogenic (commensal) parasite:-The parasite derives food and protection from the host without causing harm to the host. E.g. *Entamoeba coli* 3\. Opportunistic parasites:- Parasites which cause mild disease in immunologically healthy individuals, but they cause severe disease in immuno-deficient hosts. E.g. *Pneumocystis carnii*, *Toxcoplasma gondii,* *Isospora belli* **Host:** Hosts are organism which harbors the parasite. Types of Hosts:- **Definitive host:-** Depending on the parasitic species, it is either a host which harbors the adult stage of a parasite or most highly developed form of the parasite occurs; or sexually mature stages of a parasite and fertilization takes place in it, e.g., man is the definitive host of *Taenia saginata*. When the mature or most highly developed form is not obvious the definitive host is the mammalian host, e.g., human is the definitive host for trypanosomes that cause African trypanosomiasis. **Intermediate host:** Is a host harboring sexually immature or larval stage of a parasite and in which no fertilization takes place in it. E.g. Cow is the intermediate host for *Taenia saginata* **Amplifier host**- Intermediate hosts in which parasites undergo multiplication. **Reservoir host**: A wild or domestic animal which harbors a parasite and acts as sources of infection to humans. **Carrier host**: A host harboring and disseminating a parasite but exhibiting no clinical sign. Accidental (Incidental) host: Infection of a host other than the normal host species. A parasite may or may not continue full development in this host **Infection**: The parasitism by internal parasites. E.g. Tapeworm in the host. **Infestation**: The parasitism by external harassing parasites. E.g. Lice on the animals and man. **Superinfection**: When an individual harbouring a parasite is reinfected with the same species of parasite, is called superinfection. Animals infected with *Fasciola* sp. **Autoinfection**: When an infected person is his own direct source of reinfection, is called autoinfection. It may be external or internal mode -- *Enterobious vermicularis* in man. **Vector:** "a living carrier (e.g. an arthropod) that transports a pathogenic organism from an infected to a non-infected host". A typical example is female Anopheles mosquito that transmits malaria. A. **Biological vectors**:-Those vectors that complete the life cycle a parasite E.g. Anopheles (Vector of Plasmodium), Phlebotomus (Vector of Leishmania), *Glossina* (vector of *Trypanosoma), Simulium* (Vector of Onchocerca), etc. B. **Mechanical (Parathenic or transport) Vectors**: They are passive carriers of parasites, not essential in the life cycle. E.g. House fly and Cockroach as a mechanical vector for Amoebae, Giardia, etc **ADAPTATION TO PARASITIC WAY OF LIFE** **INTRODUCTION** A parasite is an organism which lives in or on another organism (called host) and benefits by deriving shelter and nutrients from them. The parasitism is a type of negative ecological / biological interaction in nature where one organism gets benefited (the parasite) and the other is harmed (the host). i.e. parasites may be microbes such as bacteria, virus and Mycoplasma, or animals such as liver like, worms, nematodes, some insects and plants such as Loranthus, Cuscuta etc. All types of parasites show peculiar adaptations to survive in or on the host system and to maximum benefit from them. Parasites show three levels of adaptations, they are:- 1\. Structural Adaptations (Morphological and Anatomical Adaptations) 2\. Physiological Adaptation 3\. Reproductive Adaptation Definition: Any feature of an organism or its part which enables it to exist under conditions of its habitat is called adaptation. The adaptations are mainly to withstand the adverse conditions of the environment and to use the maximum benefit of the environment. STRUCTURAL MODIFICATION (MORPHOLOGICAL AND ANATOMICAL) 1\. Feeding organs are usually absent in endoparasites. 2\. Fluid feeding insects such as aphids have highly specialized mouth parts for the easy absorption of cell sap from the host. 3\. The presence of well-developed piercing device in some parasites enables them to invade into the host tissue. Example the stylet in nematodes. 4\. Since endoparasite show restricted movement the locomotory organs are generally absent or highly reduced in them. Ex:- gut parasites such as *fasciola* and *taenia.* 5\. The outer covering of the endoparasite is resistant to the enzymatic digestion of the host. Example *Fasciola.* 6\. Sensory organs are highly reduced in internal parasites since the environmental conditions of endoparasites are relatively constant. 7\. The nervous system is highly reduced in most of the parasites. 8\. Parasites usually have the reduced body size to occupy the niche in the internal or external surface of the host. 9\. Most of the internal parasites are dorsi-ventrally flattened, which help the easy attachment to the host surface without any friction. 10\. Some endoparasites such as *Ascaris* have highly muscular pharynx for the easy absorption of food materials from the host. 11\. Development of huastoria in some parasitic plants for the absorption of nutrients from the host. Example cuscuta 12\. The haustoria in partial plant parasites (such as Loranthus) are attached to the xylem of the host to absorb minerals and water from them. 13\. The huastoria in complete plant parasites (such as cuscuta) are attached to the phloem of the host to absorb the prepared food material from them. PHYSIOLOGICAL ADAPTATIONS OF PARASITES 1. Parasites producing hydrolyzing enzymes to digest the host tissue. E.g. Fungi produce a variety of hydrolyzing enzyme such as cutinase, cellulose, hemicelluloses, and pectinase to digest cell wall of the host plant. 2. The hydrolyzing enzymes are exoenzymes, produced in the external surroundings of the parasites. 3. Blood feeding parasite produce anticoagulant to prevent the clotting of blood in the gut of parasites. (e.g. hirudinaria) 4. Internal parasites have high chemo sensitivity to find the best location in the host. 5. Internal parasite produce digestive enzymes for the penetration of host tissue or organs 6. Internal parasites can respire anaerobically in absence of oxygen or in anaerobic condition 7. There is a progressive tendency towards the reduction of the content of gut in some endoparasite since; many internal parasites have the capacity to absorb nutrients through the body surface. 8. Those internal parasites which don't absorb the nutrients through body surface will develop thick cuticle around the body from host enzymes. 9. All parasites especially endoparasite have high resistance to toxins or metabolites of the host origin. 10. Internal parasites have the ability to escape from the immunological reactions of the host. 11. Parasites generally attach host proteins to its external surface to escape from the host immunological responses. 12. Some parasites such as HIV and plasmodium often change their antigenic components to escape from the immunological reactions of the host. 13. Strategies by which parasites escape from the immunological reactions of the host antigen, antigenic variation, occupation of immunologically privileged sites, disruption of the host's immune response, molecular mimicry and loss or masking of surface antigens. 14. Endoparasite produce some antienzyme compound which will neutralize or inactive the digestive enzymes of the host. 15. Endoparasite keeps their osmotic concentration equal or slightly lower than the host cell for easy absorption of water and nutrients. REPRODUCTIVE MODIFICATION OF PARASITES 1. Some are hermaphrodites. This allows them to self-fertilize in the absence of the opposite sex in the internal conditions 2. The fecundity rate of parasites is very high since there is only a remote chance to find the correct host by the progenies. 3. Most of the parasites produce a large number of reproductive bodies such as eggs or sperm or cysts than their free living relatives. Example:- *Taenia soium* lays about 35000 eggs/day. *Fasciola hepatica* lays about 10000 eggs/ days, *Aascaris lumbricoides* lays about 200000 of eggs/ day 4. Reproductive bodies are tough and resistant when they are external to the host 5. The reproductive particles that secrete out of the host body are protected with very thick coat to prevent desiccation as in cysts. 6. Some parasites have highly complicated and specialized reproductive cycle. 7. Some parasites use of secondary hosts as vectors to increase the chance of their survival. 8. Most of the endoparasite sow one or more larval stages with more multiplication capacity (due to polyembryony) to increase the number of progenies Example:- nematodes 9. Rapid maturation capacity and extended life span in most of the endoparasite is another reproductive adaptation. **HOST PARASITE RELATIONSHIPS** The majority of animals live independently in their natural habitats, seeking their own food materials and utilizing free water and oxygen for their metabolic processes. Between some animals, however, a variety of patterns of association has developed, and these may be broadly divided into two groups: 1. Homogenetic associations -- those between individuals of the same genotype; and 2. Eterogenetic associations -- those between individuals of different genotype. 3. Heterogenetic associations- are in general much more complex and a number of terms have been developed to describe them. Like many terms used in biology, these are essentially operations words which are definable only within broad limits and not in absolute terms. They do, nevertheless, serve a useful role in enabling us to file data into convenient, though not water-tight, compartments. Terms such as animals when living in a close association develop a variety of pattern of associations. Host parasite relationships are of the following types: 1\. **Parasitism:** It is an intimate association between two specifically distinct organism in which one parterner usually smaller (Parasite) lives on or with in the other usually larger organism (Host) in order to obtained nourishment and at the same time it cause harm to the host. One which is benefited is called parasite and other which is loser is called host. 2\. **Commennsalism**: The literal meaning of commensalisms is eating at same table. It is loose association. Organisms of different species live together, one animal may receive some benefit called commensal but the other neither get benefited nor harm from the other. It is not obligatory for their existence. E.g. Remora and Shark. Many intestinal protozoa such as *Entamoeba coli* and *Trichomonas* sp. 3\. **Mutualism:** It is an intimate association or relationship of two animals or individuals in which both the partners are benefited. However, this relationship is not permanent and obligatory. Either of the partners can break it without any loss to either of them. e.g. Hermit crab and sea anemone. -Scorpion and Hydroids. 4\. **Symbiosis**: It comes from Greek word symbioum means to live together. it is an intimate or close association or relationship of two individuals in which both the partners are benefited like mutualism but in this, the participating species are dependent on each other for existence and are obligatory e.g. Termites and their intestinal protozoa. 5\. **Phoresis**: The term means 'travelling together' it is temporary association between two individuals or living beings in which there is no metabolic dependence. e.g. Bacteria and amoebae on the leg of fly or fungal spores on the legs of the beetle. **Predation**: It is usually a short term relationship or association between two individuals in which one (the predator) benefits at the expense of other (the prey). **Predator**: It is an individual which temporarily attacks and destroy animals or plants in order to obtain food, usually feed on smaller or weaker organisms which are their pray. So predators live on capital while parasites live on income. E.g. Cats for rats prey: They are smaller or weaker organisms or animals. These are usually eaten up by the Predators. Rats for cats. **Parasitosis**: It is an association between two individuals in which the infective agent injures the host and causes symptoms and lesions of the diseases. In this type of Parasitism, the disease of infection is denoted by putting suffix --osis (plural --oses) to the name of the parasite E.g. Trypanosoma + osis = Trypanosomosis Fasciola + osis = Fasciolosis Theileria + osis = theileriosis. **Parasitiasis**: It is an association between two individuals in which one is potentially pathogenic but does not causes symptoms of the diseases. In this type of Parasitism, the disease of infection is denoted by putting suffix --iasis (plural --iases) to the name of the parasite, E.g. Trypanosoma + iasis = Trypanosomiasis, Fasciola + iasis = Fascioliasis, Theileria + iasis = Theileriasis. **S/N** **Parasitiasis** **Parasitosis** --------- --------------------------------------------- ---------------------------------------------- 1 The host is capable to repair the damage The host is not capable to repair the damage 2 The symptoms are not noticeable The symptoms are noticeable 3 It is a state of balance It is a state of imbalance 4 Trypanosomiasis, Fascioliasis, Theileriasis Trypanosomosis, Fasciolosis, theileriosis A. EFFECT OF PARASITES ON THE HOST The damage which pathogenic parasites produce in the tissues of the host may be described in the following two ways; a. Direct effects of the parasite on the host Mechanical injury - may be inflicted by a parasite by means of pressure as it grows larger, e.g. Hydatid cyst causes blockage of ducts such as blood vessels producing infraction. Deleterious effect of toxic substances- in Plasmodium falciparum production of toxic substances may cause rigors and other symptoms. Deprivation of nutrients, fluids and metabolites -parasite may produce disease by competing with the host for nutrients. b. Indirect effects of the parasite on the host: - Immunological reaction: Tissue damage may be caused by immunological response of the host,e.g. nephritic syndrome following Plasmodium infections. Excessive proliferation of certain tissues due to invasion by some parasites can also cause tissue damage in man, e.g. fibrosis of liver after deposition of the ova of Schistosoma. The host parasite relationship defined from the definition of parasitism it is clear that it involves 2 partners, a parasite and a host and also that parasitism affect both the partners. **Host Parasite relationship** is defined as the influence of each partner by the activities of the other. In general the host-parasite relationship can be studied under two heads: 1\. **Effect of nutrition**: The kind of nutritive material ingested by parasites affects their development. A diet consisting largely of milk has an adverse effect on intestinal helminths or protozoan fauna, because it lacks p-aminobenzoic acid which is necessary for the parasite growth. A high protein diet has been found to be unfavourable for the development of many intestinal Protozoa. On the other hand, a diet low in protein favours the appearance of symptoms of amoebiasis. It has also been shown that carbohydrate rich diet favours the development of certain tapeworms. In fact the presence of carbohydrate in the diet is known to be essential for some of the worms. The nutritional status is of an increased importance both in determining whether or not a particular infection will be accompanied by symptoms and in influencing their severity if present. Nutritional disturbances may also influence resistance through its effects upon the immune mechanisms of the host. 2\. **Effect of hormones**: Hosts hormones have direct effect on the growth and in many cases sexual maturity of parasites e.g., *Ascaridia galli* attains greater lengths in hyperthyroid chickens whereas *Heterakis gallinae* attains greater length in hypothyroid host, the two worms apparently respond differently to the hormone thyroxin. The dog nematode *Toxocara canis* develops into adult only in the female dogs i.e., bitches, during their pregnancy as hosts sex hormones are necessary for its maturity and growth. 3\. **Effect of host age:** Human schistosomes usually infect young persons, and adults over thirty generally do not become infected on exposure. Age resistance does not appear to depend on immune reactions but rather to changes in the host tissues that render them as unsuitable environment for the parasite. 4\. **Effect of immunity**: The host produces one or more substances known as antibodies that are chemically antagonistic to the parasite or its products. These antibodies may stunt the growth of the parasite or kill it or prevent its attachment to the host tissues or they may precipitate or neutralize parasitic products. Primary infection with Leishmania seems to confer a degree of immunity to reinfection while many protozoa and helminthic infections confer no long lasting immunity to reinfection. They do seem to stimulate resistance during the time that the parasites are still in the body. This resistance to hyperinfection is known as premonition. 5\. **Effect of host specificity**: The host specificity varies greatly among helminths. Even closely related helminths may exhibit great differences in host requirements. It is usually supposed that a helminth requires a very specific environment complex for its development and this is found only in proper hosts. 6\. **Effect of parasite density**: When a number of helminths of one species is present in one host, the worms are usually stunted and of low reproductive capacity. This stunting effect seems to result not from insufficient food supply but from some action of the parasites on each other. 7\. **Effect of host sex**: An influence of host sex is evidenced in the development of some helminths e.g., Cysticercus fasciolaris is more frequent in male than in female rats, as a consequence of the action of sex hormones; gonadectomy lowers the resistance of females and increases that of males to infection, and injection of female hormones into males also increases the resistance of the latter, whereas injection of females with male hormones lowers their resistance to the Cysticercus. Also *Toxocara canis* develops only in pregnant bitch and not in males and others. B. **EFFECTS ON THE HOST:** The parasites bring about some changes within their hosts that may be interpreted as affecting the host's welfare. It is necessary to give some consideration to types and degrees of changes caused by the parasitic animals. However, some classical types of parasite effects on the host can be sighted: **1. Utilization of host's food**: Utilization of host's food to a detrimental point by a parasite is probably the first kind of damage that comes one's mind. Although in the past some biologists had doubts as the impact of parasites in this regard, since the amount of food microscopic parasites can utilize seems to be negligible. But recent physiological studies of nutritional requirements especially endoparasites have indicated that they robe the host of a good amount of nutrition resulting in serious consequences. *Diphyllobothrium latum* in man has been known to cause an anemia similar to pernicious anemia, because of the affinity this tapeworm has for Vitamin B12. This tapeworm can absorb 10-50 times as much vitamin B12 as other tapeworms. Since B12 plays an important role in blood formation, its uptake by *D. latum* causes anemia. 2\. **Utilization of host's non-nutritional materials**: In some cases parasites also feed on host's substances other than nutrients. The endo and ectoparasites that feed on the hosts' blood are examples. The exact amount of blood utilized by these parasites is difficult to measure during the survival in the host. Approximately e.g., due to blood feeding parasitic hookworms, the haemoglobin may drop 30% below normal. It is obvious that the blood lost through parasitic infections can become an appreciable amount over a period of time. 3\. **Destruction of host tissue**: Not all parasites are capable of destroying the host's tissue, and even among those that do so the gradation in the degree of damage is large. The parasites destroy the host's tissue in two ways: \(a) Some parasites destroy (injure) the host's tissue when they enter the host, and \(b) Others inflict tissue damage after they have successfully entered. A combination of these two types of injury may occur. The *hookworms* -- *Necator americanus* & *Ancylostoma duodenale*. The cercariae of certain *Schistosomes* that cause "Swimmers Itch" while penetrating the host skin causes inflammation and damage to the surrounding host tissues. 4\. **Abnormal growth**: One of the possible consequences of parasitism associated with cell and tissue parasites is a change in the growth pattern of the affected tissue. Some of these are serious changes, whereas others are structural and have no serious systemic importance to the whole organism. Such changes can be broadly divided into four main types: \(a) Hyperplasia (Increase in the rate of cell division): It is an accelerated rate of cell division resulting from an increased level of cell metabolism. This results to a greater total number of cells, but not necessarily an increase in their absolute size, e.g., the presence of trematode *Fasciola hepatica* in bile ducts is known to effect rapid division of the lining epithelial cells. The eggs of *Schistosoma haematobium* with their spiny projections are known to irritate the transitional epithelium of human urinary bladder causing hyperplasia. \(b) Hypertrophy (Increase in cell size): This condition is commonly associated with intracellular parasites e.g., during the erythrocytic phase of *Plasmodium vivax*, the parasitized red blood cells are commonly enlarged. \(c) Metaplasia (Transformation of one type of tissue into another): Metaplasia describes the changing of one type of tissue into another without the intervention of embryonic tissue. When the lung fluke -- *Paragonimus westermani* parasitizes man and carnivores, the normal cuboidal cells lining the bronchioles commonly undergo both hyperplasia and metaplasia and are transferred to stratified epithelium. \(d) Neoplasia (Cancer or tumour formation): It is the growth of cells in a tissue to form a new structure, for example, a tumour. The neoplastic tumour: (i) is not inflammatory, (ii) is not required for the repair of organs, & (iii) does not conform to a normal growth pattern. Neoplasms may be benign or malignant. **5. Effect** **of toxins, secretions, excretions, poisons**: Specific toxins or poisons, egested, secreted or excreted by parasites have been site in many cases as the cause of irritation and damage to hosts. Example of irritating parasite that elicits an allergic reaction in the host is that of *Schistosoma cercaria* which causes cercarial dermatitis. 6\. **Mechanical interference:** Less is known about the injuries to the host resulting from mechanical interference by parasites. The best-known case of the type of damage is elephantiasis. In persons infected with the filarial nematode *Wucheraria bancrofti*, the adult worms become lodged in the lymphatic ducts and thus obstruct the lymph into abnormal channels resulting in the swelling of affected parts, thus causing elephantiasis. Other example is *Ascaris lumbricoides* in the intestine and bile ducts of their host. 7\. **Biological effects in the host** (Sex reversal): Other interesting and challenging aspect of host parasite relationship is the biological effects on the host. Among the most important and interesting of these are the secondary manifestations resulting from damage to the specific organs. E.g. the host's genital tissue is invaded by this crustacean causing drastic changes in males, but not in females. 70% of the parasitized male crabs undergo degeneration of testes and acquire secondary female characteristics. **8. Host tissue reaction**: In instances of host tissue reaction, certain host cells and cell products aggregate around the invading parasite forming what is commonly known as host cyst, although cysts are not always of host origin e.g., when the metacercaria of the yellow grubs, *Clinostomum marginatum* encysts in the skin of fish, two cyst walls are formed around the parasite. The inner one is secreted by the parasites, the outer one is laid down by the host in response to the parasitic invasion. Although the double wall of Trematode metacercarial cysts is a common occurrence, single walled cysts also occur. capsule around the coiled worm (larvae) hence the cyst is completely of host origin. At times, the capsule becomes calcified, with calcium, coming primarily from the host, although there is some evidence that the parasite also contributes some calcium. **9. Immunological interaction between host and parasite:** As the host has immune system which has efficiency to destruct the parasites by producing antibodies (IgG, IgE, etc.) against the specific parasites or their products. Host's Immune system usually performs Antigen-Antibody reactions, such as toxin neutralization, Agglutination, Precipitation, Lyses, Compliment fixation, Increased Phagocytosis (Opsonification) & allergic sensitization. With the help of these host can resist against the parasite or can destroy it. Parasites also have evolved mechanisms to evade their host's immune systems. Usually the mechanisms used by parasites for defending them against the host response are: Induce Immunosuppressions, Becoming hypoantigenic, Change their surface antigens rapidly & repeatedly (e.g., *Trypanosoms rhodiense*, *T. gambiense*),