Lecture 1 Introduction and Definitions - PowerPoint Presentation

1. Lecture 1Introduction and DefinitionsMedical ParasitologyProf. Dr. Ahmed Ali Mohammed

2. IntroductionParasitology is the branch of biology concerned with the phenomenon of dependence of one living organism on another. Medical parasitology deals with the parasites which infect man, the diseases they produce, the response generated by him against them, and various methods of diagnosis, prevention and treatment. Parasitology is the study of invertebrate animals capable of causing disease in humans and other animals. It is concerned with 200 or so species of helminth worms and about 80 species of protozoa that infect humans. Many of these are rare and accidental parasites, but about 100 species of protozoa and helminths are commonly found in humans. They range in size from tiny protozoa as small as 1-2 μm in diameter to tapeworms that may measure up to 10 meters in length.

3. The ParasiteIs 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 protozoa and helminths and excludes the viruses, bacteria and :1. Microparasite: small, unicellular and multiplies within its vertebrate host, often inside cells. Protozoa are microparasites.2. Macroparasite: large, multicellular and has no direct reproduction within its vertebrate host. This category includes helminths.

4. On the basis of their location, parasites may be divided into:1. Ectoparasites: which live on the surface of the body, e.g., the human louse, Pediculus humanus. The infection by these parasites is known as infestation. They are important as vectors transmitting pathogenic microorganisms.2. Endoparasites: which live within the body of the host in the blood, tissues, body cavities, digestive tract and other organs. All protozoan and helminthic parasites of man are endoparasites. The invasion by endoparasites is known as infection. This group can be further subdivided into the following types:a. Obligate parasites: organisms that cannot exist without a host (e.g., Toxoplasma gondii).

5. b. Facultative parasites: organisms that under favorable circumstances may live either a parasitic or free-living existence e.g., Naegleria fowleri and Acanthamoeba spp.c. Accidental parasites: organisms that attack an unusual host e.g. Echinococcus granulosus in man.d. Aberrant parasites: organisms that attack a host where they cannot live or develop further e.g. Toxocara canis in man.e. Free-living: it describes the nonparasitic stages of existence which are live independently of a host, e.g. hookworms have active free-living stages in the soil.

6. The HostThe host is the organism which harbors the parasite and provides the nourishment and shelter to the latter. It is of the following types:1. Definitive host: the host which harbors the adult parasite, the most highly developed form of a parasite or where the parasite replicates sexually. When the most highly developed form is not obvious, the definitive host is the mammalian host.2. Intermediate host: this is the host which alternates with the definitive host and harbors the larval or asexual stages of a parasite. Some parasites require two intermediate hosts for the completion of their life cycle. These are referred to as first and second intermediate hosts respectively.

7. 3. Paratenic host: it is a host in which the larval stage of a parasite survives but does not develop further. It is often not a necessary part of the life cycle.4. Reservoir host: It is a host that harbors the parasite and serves as an important source of infection to other susceptible hosts. It can harbor a pathogen indefinitely with no ill effects. Once discovered, natural reservoirs elucidate the complete life cycle of infectious diseases, providing effective prevention and control.5. Compromised host: it is the one in whom normal defense mechanisms are impaired e.g. AIDS, absent (e.g. congenital deficiencies) or bypassed (e.g., penetration of skin barrier). Such hosts are extremely susceptible to a variety of common and opportunistic pathogens.

8. The VectorAn agent, usually an insect transmits the infection from one human host to another. It is of two types:1. Mechanical vector: the term used to describe a vector which assists in the transfer of parasitic forms between hosts but is not essential in the life cycle of the parasite, e.g. a housefly and Cockroaches in the case of Entamoeba which transfers the cysts of the parasite from the infected feces to food that is eaten by humans. 2. Biological vector: in which the pathogenic organism develops and multiplies before being transmitted to the next host, therefore, it is essential in the life cycle of the parasite.

9. The vector is only helping in the transfer of the pathogen to complete its life cycle, while intermediate host existence is essential for the completion of some parts of the life cycle (asexual only).Host-Parasite relationships1. Symbiosis: an association in which both host and parasite are so dependent upon each other that one cannot live without the help of the other. Neither of the partners suffers from any harm from this association.2. Commensalism: an association in which only the parasite derives benefit without causing any injury to the host. A commensal lives on food residues or waste products of the body and is capable of leading an independent life, as in the case of Entamoeba coli in the large intestine of man (One partner benefits, the other is not hurt).

10. 3. Parasitism: Parasitism is a relationship in which a parasite benefits and the host provides the benefit. The host gets nothing in return and always suffers from some injury because the parasite lives on the expense of the host. The degree of dependence of a parasite on its host varies.ZoonosisThis term is used to describe an animal infection that is naturally transmissible to humans either directly or indirectly via a vector. Examples leishmaniasis, South American trypanosomiasis, rhodesiense trypanosomiasis, japonicum schistosomiasis, trichinosis, fascioliasis, hydatid disease and cryptosporidiosis.It is also defined as a communicable disease from animals to humans (enzootic infection acquired by man) under natural conditions.

11. Portals of entry into the body1. Mouth The commonest portal of entry of parasites is oral, through contaminated food, water, soiled fingers or fomites. Many intestinal parasites, e.g. Entamoeba histolytica, Giardia lamblia, Balantidium coli, Enterobius vermicularis, Trichuris trichiura, Ascaris lumbricoides, Trichinella spiralis, Taenia solium, Taenia saginata, Diphyllobothrium latum, Fasciola hepatica, Fasciolopsis buski, Clonorchis sinensis and Paragonimus westermani, enter the body in this manner.

12. 2. Skin Entry through the skin is another important portal for the entry of parasites. Infection with Ancylostoma duodenale, Necator americanus and Strongyloides stercoralis is acquired when filariform larvae of these nematodes penetrate the unbroken skin of an individual walking over faecally contaminated soil. Schistosomiasis caused by Schistosoma haematobium, S. mansoni and S. japonicum is acquired when the cercarial larvae in water penetrate the skin. A large number of parasites, e.g., Plasmodium spp. Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, Trypanosoma brucei gambiense, T. b. rhodesiense, T. cruzi, Leishmania spp. and Babesia spp. are introduced percutaneously when bloodsucking arthropods puncture the skin to feed.

13. 3. Sexual contact Trichomonas vaginalis is transmitted by sexual contact. E. histolytica and Giardia lamblia may also be transmitted through sexual practices.4. Kissing Entamoeba gingivalis is transmitted from person to person by kissing or from contaminated drinking utensils.5. Congenital Infection with Toxoplasma gondii and Plasmodium spp. may be transmitted from mother to foetus transplacentally.

14. 6. Inhalation Airborne eggs of Enterobius vermicularis may be inhaled into the posterior pharynx leading to infection.7. Iatrogenic infection Malaria parasites may be transmitted by transfusion of blood from a donor with malaria containing asexual forms of erythrocytic schizogony. This is known as trophozoite-induced malaria or transfusion malaria. Malaria parasites may also be transmitted by the use of contaminated syringes and needles. This may occur in drug addicts.

15. Classification of animal parasites and vectors Kingdom/ Phylum/ Subphylum/ Class/ Order/ Family/ Genus/ SpeciesAll of the names must be in Greek or Latin. The Genus is a group of closely related species. The species designates a population, the members of which have essentially the same genetic characteristics and are capable of continuous reproduction of their kind, but usually cannot interbreed with individuals of other species. The scientific designation of a species is a combination of the genus and species name. This is referred to as binomial nomenclature. Ex. Entamoeba histolytica.

16. The groups of the parasitesProtozoa Protozoa are unicellular eukaryotic organisms, have all the essential organelles that help them in their essential activities. All of them are microscopic; most of them live singly, but many others live in colonies. Each cell unit performs all the necessary functions of life. Classification of the Protozoa Human parasites in the kingdom Protista, subkingdom Protozoa are classified under four phyla: 1. Phylum Sarcomastigophora (containing amoeba and flagellates).This phylum is subdivided into two subphyla:

17. a. Subphylum Sarcodina, and b. Subphylum Mastigophora 2. Phylum Apicomplexa (containing Sporozoa). 3. Phylum Ciliophora (containing Ciliates).4. Phylum Microspora.1. Subphylum Sarcodina (Amoebae) Amoeboid organisms use pseudopodia for both locomotion and feeding. Only Entamoeba histolytica in this group is of medical importance.Entamoeba histolyticaUndoubtedly, the best-known species of amoebae parasitizing humans is E. histolytica, the causative agent of amoebic dysentery or amoebiasis, therefore it is called dysentery amoeba. The parasite has a cosmopolitan distribution (worldwide

18. distribution) especially in warm areas. It is, nevertheless, important to remember that amoebiasis is not restricted to the tropics and subtropics where it is found also in temperate and even in Arctic and Antarctic zones. This parasite infects the human as well as the cats, pigs and monkeys. Entamoeba histolytica can produce extreme illness and even death. Since side effects from chemotherapy may be pronounced, it is of great importance that diagnosis of the condition should be precise and accurate in order to assure treatment only when absolutely necessary, not merely to eliminate a protozoan that resembles E. histolytica. The parasite has two distinct stages trophozoite and cyst, which are commonly recognized in the feces of the patient, but only the trophozoite stage is found in the tissue. The trophozoite lives in the last part of the small

19. intestine and in the large intestine stuck on the mucosa, especially in the caecum and sigmoidorectal area; it varies from 15-60µ in diameter, however, trophozoite up to 90µ in diameter have been observed in the dysenteric stool. It has a finely granular, somewhat viscous endoplasm and a clear ectoplasm. Pseudopodia are broadly finger-like (lobopodia) and extend from the ectoplasm. In addition, there are many food vacuoles containing parts of epithelial cells, Bacteria and sometimes many R.B.Cs. and leukocytes found in the cytoplasm of the parasite. The nucleus is rounded, vesicular, surrounded by a delicate nuclear membrane studded on its inner surface with minute regular chromatin granules. In the center of the nucleus, there is a single dense bead-like chromatin body, the karyosome (centric karyosome). Unfortunately, other species of Entamoeba, notably E. dispar, show similar nuclear morphologies.

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21. Habitat: the trophozoites reside in the mucosa and submucosa of the large intestine of man. The trophozoite grows and multiplies continuously in the intestine, but sometimes it is encysting in the intestine; whenever, the trophozoite will discharge the undigested food and become spherical, then it secretes a delicate solid membrane and become a cyst. The cyst contains a nucleus (the same one of the trophozoite), glycogen mass and some chromatoid bars or bodies with hazy margins and rounded ends. The chromatoid bars are considered to be deposits of nucleic acids such as RNA. During encystation, the nucleus firstly divides into two nuclei then each of the two daughter nuclei divides once again, so, the mature cyst typically has four nuclei (quadrinucleated cyst). The cyst ranges between 10-20µ in diameter, spherical or may have an oval shape. A viable cyst is highly resistant to dryness and freezing and even to certain chemicals e.g. chlorinated compounds and fluorides.

22. Cysts in water can survive for a month, while those in feces on dry land can survive for more than 12 days; they tolerate temperatures up to a thermal death point of 50˚C. However, it is affected by bacterial putrefaction of the medium, hypertonicity, direct sunlight and heat. Greater intestinal motility and/or large volumes of ingested food reduce the potential for the establishment of the amoebae.Life cycle The cyst is the infective stage of this parasite, when swallowed with the foods or drinks, excystation occurs and the freeing of the young trophozoites will occur in the duodenum where the pH is neutral or weakly alkaline, as well as the effects of the digestive enzymes, which destroy the cyst wall. These young freeing trophozoites will arrive then

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24. to the large intestine and some of them will be in contact with the mucosa. When these cysts are evacuated in the feces of the infected patient they arrive to the environment and the cycle will be repeated again, see the figure below. Multiplication of this species is thus seen to occur at two stages during the life cycle: by binary fission in the intestine (mature trophozoite stage) and by nuclear division followed by binary fission (in the metacystic stage).PathogenicityThe trophozoite destroys the host epithelial cells causing their lysis (the cause of the name). It may reach the submucosa, start in feeding and attack the blood capillaries and feed on the R.B.Cs.

25. After an incubation period of 1-4 weeks, the trophozoite invades the colonic mucosa and during its growth, it secretes proteolytic enzymes producing flask-shaped ulcers that cause profuse bloody diarrhea (amoebic dysentery). The blood is flown to the lumen of the intestine and exited with the stool, which is the first important symptom of the infection (the bloody stool). The resulting ulcers may be deep or superficial. E. histolytica may also cause amoebic appendicitis and amoebomas (pseudotumoral lesions associated with necrosis, inflammation and oedema). In the individual who develops dysentery, the mucosal ulceration may penetrate deeper into the intestinal tissue, causing vast areas of tissue to be destroyed. The overlying mucosal epithelium then may be sloughed off, exposing these necrotic areas.

26. This destructive process is usually followed by a regenerative period, resulting in a thickening of the intestinal wall as a result of the deposition of fibrous connective tissue.Extraintestinal amoebiasis: About 5-10% of individuals with intestinal amoebiasis develop hepatic amoebiasis after 1-3 months of the disappearance of dysentery. In this case, the trophozoites are carried from the ulcer in the large intestine and multiply in the liver, lead to cytolytic action then small abscesses merge to form big liver abscesses and a disease called hepatic amebiasis which causes liver dysfunction. The abscesses may grow in various locations if the parasite enters into general circulation. For instance, it may reach the lungs and causes pulmonary amebiasis and pneumonitis, or it can reach the brain and causes encephalitis or to the spleen, heart, joints, bones, muscles, urogenital system and even the skin.

27. Entamoeba histolytica (flask shaped ulcer in the intestine). Entamoeba histolytica (amebic-abscess).

28. Pathogenicity depends on: 1. Virulence of strain 2. Resistance of the host (depends on the innate immunity). 3. State of nutrition of the host. 4. Infection with other agents (free of other infections mean less susceptible to infection). 5. Some drugs may irritate the intestinal wall so irritated intestine is more susceptible to infection. 6. Bacterial flora (metabolic processes can enhance the invasiveness).

29. SymptomsA wide spectrum from asymptomatic infection “luminal amebiasis” to invasive intestinal amebiasis which causes dysentery, colitis, appendicitis, toxic megacolon, amebomas, to invasive extraintestinal amebiasis represented by liver abscess, peritonitis, pleuropulmonary abscess, cutaneous and genital amebic lesions. Yet, some people may have only mild abdominal discomfort or no symptoms at all. However, symptoms can mainly be divided to:Acute cases: Frequent dysentery with necrotic mucosa and abdominal pain. In the former type, severe diarrhea (i.e., blood and mucus in liquid feces) usually develops after an incubation period of 1 to 4 weeks and is commonly accompanied by a fever.

30. Chronic cases: Abdominal discomfort or soft stool for variable periods, may be suddenly developed into dysentery or acute abdominal pain. Recurrent episodes of dysentery with blood and mucus in the feces. Interfering gastrointestinal disturbances and constipation. Cysts can be found in the stool.Diagnosis Entamoeba histolytica must be differentiated from other intestinal nonpathogenic amebae. The nonpathogenic Entamoeba dispar is morphologically identical to E. histolytica, and differentiation must be based on isoenzymatic or immunologic analysis. Molecular methods are also useful in distinguishing between E. histolytica and E. dispar.

31. Microscopic identification of feces samples is the common method for diagnosing E. histolytica to search for cysts and trophozoites in the stool This can be accomplished using wet mount and permanently stained preparations such as iodine or trichrome or by flotation or sedimentation method for stool samples. The typical stool in amebic dysentery consists of exudates, mucous, blood and maybe little fecal material; however, we are mainly looking for the cyst stage. In liquid stool, trophozoite may also be found, but only the cyst stage is present in the solid stool. The blood examination shows moderate leukocytosis. In the serological tests, in the later stages of invasive amoebiasis antibodies appear. Tests including ELISA, IHA and IFA.

32. In the histological examination, the trophozoites can be identified in the aspirates or biopsy samples obtained during colonoscopy or surgery. The molecular methods include the DNA probe and PCR.Treatment Appropriate chemotherapy should be employed to destroy the trophozoites, relieve the symptoms and control secondary bacterial infections. The drug of choice for the entire spectrum of symptoms is metronidazole (Flagyll) or tinidazole. In addition, a bland diet, low in carbohydrates such as sugar and high in liquids and proteins, is recommended. To combat secondary bacterial infections, antibiotics such as tetracycline are used in combination with either metronidazole or tinidazole. Hepatic amoebiasis also responds well to metronidazole, although the treatment is not totally effective. Failure of metronidazole therapy may be an indication of surgical intervention.