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parasitology
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In collaboration with the Ethiopia Public Health Training Initiative, The Carter Center, the Ethiopia Ministry of Health, and the Ethiopia Ministry of Education 2004
Funded under USAID Cooperative Agreement No. 663-A-00-00-0358-00.
Produced in collaboration with the Ethiopia Public Health Training Initiative, The Carter Center, the Ethiopia Ministry of Health, and the Ethiopia Ministry of Education.
Important Guidelines for Printing and Photocopying Limited permission is granted free of charge to print or photocopy all pages of this publication for educational, not-for-profit use by health care workers, students or faculty. All copies must retain all author credits and copyright notices included in the original document. Under no circumstances is it permissible to sell or distribute on a commercial basis, or to claim authorship of, copies of material reproduced from this publication.
©2006 by Dawit Assafa, Ephrem Kibru, S. Nagesh,, Solomon Gebreselassie, Fetene Deribe, Jemal Ali
All rights reserved. Except as expressly provided above, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without written permission of the author or authors.
This material is intended for educational use only by practicing health care workers or students and faculty in a health care field.
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The writers are indebted to the Ethiopian Public Health Initiative (EPHI) for encouragement and financial support. We thank all who contributed in the write up of this lecture note and those involved in giving the secretarial service in all colleges and Universities. Included in the acknowledgment are also the reviewers of the draft material, Dr. Habtamu and Ato Asrat Hailu who are currently staffs of AAU-MF, Microbiology, Immunology, and Parasitology department. Their comments were quiet constructive and well taken up.
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Topic Page Preface ............................................................................................. ......... i Acknowledgement............................................................................. .........ii Table of Contents.............................................................................. ........ iii About the Authors ............................................................................. ....... vii List of Boxes and Tables .................................................................. ...... viii Abbreviations and Acronyms ............................................................ ........ix
UNIT ONE: General Parasitology ................................................... ........ 1 Association between parasite and host ........................................ ........ 1 Effect of parasites on the host...................................................... ........ 4 Basic concepts in medical parasitology ....................................... ........ 5 Classification of medical parasitology .......................................... ........ 8 General characteristics of medically important parasites ............. ...... 11 (1) Protozoa ............................................................................ ...... 11 (2) Heliminths .......................................................................... ...... 13 (3) Arthropods ......................................................................... ...... 14
UNIT TWO: Medical Protozology ................................................... ...... 17 Introduction ....................................................................................... ...... 17 Classification of protozoa .................................................................. ...... 20
UNIT THREE: Amoebiasis .............................................................. ...... 22 Introduction ....................................................................................... ...... 22 1.1. Entamoeba Histolytica .......................................................... ...... 22 1.2. Other Amebae inhabiting the alimentary canal ..................... ...... 27 1.3. Pathogenic free-living amoebae............................................ ...... 35
UNIT FOUR: Pathogenic Flagellates ............................................. ...... 37 Introduction .................................................................................. ...... 37 2.1 Luminal Flagellates ................................................................ ...... 37
- 2.1.1. Giardia Lamblia - 2.1.2 Trichomonas vaginalis - 2.1.3 Dientamoeba Fragilis - 2.1.4 Other flagellates inhabiting the alimentary canal viii
Box 1: different kinds of parasites -------------------------------------------------------- Box 2: different kinds of Hosts ------------------------------------------------------------ Table 1: classification of pathogenic protozoa----------------------------------------- Table 2: differentiating features of helminthes------------------------------------------
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CNS: Central nervous system CSF: Cerebro-spinal fluid DEC: Diethyl carbamazine ELISA: Enzyme linked immunosorbent assay PO: Per Os (through mouth) HIV: Human Immunodeficiency Virus AIDS: Acquired Immune Deficiency Syndrome
Most of the parasites which live in/on the body of the host do not cause disease (non-pathogenic parasites). In Medical parasitology we will focus on most of the disease causing (pathogenic) parasites. However, understanding parasites which do not ordinarily produce disease in healthy (immunocompetent) individuals but do cause illness in individuals with impaired defense mechanism (opportunistic parasites) is becoming of paramount importance because of the increasing prevalence of HIV/AIDS in our country.
There is a dynamic equilibrium which exists in the interaction of organisms. Any organism that spends a portion or all of its life cycle intimately associated with another organism of a different species is considered as Symbiont (symbiote) and this relationship is called symbiosis (symbiotic relationships).
The following are the three common symbiotic relationships between two organisms:
Mutualism - an association in which both partners are metabolically dependent upon each other and one cannot live without the help of the other; however, none of the partners suffers any harm from the association. One classic example is the relationship between certain species of flagellated protozoa living in the gut of termites. The protozoa, which depend entirely on a carbohydrate diet, acquire their nutrients from termites. In return they are capable of synthesizing and secreting cellulases; the cellulose digesting enzymes, which are utilized by termites in their digestion.
In medical parasitology, each of the medically important parasites are discussed under the standard subheadings of morphology, geographical distribution, means of infection, life cycle, host/parasite relationship, pathology and clinical manifestations of infection, laboratory diagnosis, treatment and preventive/control measures of parasites. In the subsequent section some of these criteria are briefly presented.
Morphology - includes size, shape, color and position of different organelles in different parasites at various stages of their development. This is especially important in laboratory diagnosis which helps to identify the different stages of development and differentiate between pathogenic and commensal organisms. For example, Entamoeba histolytica and Entamoeba coli.
Geographical distribution - Even though revolutionary advances in transportation has made geographical isolation no longer a protection against many of the parasitic diseases, many of them are still found in abundance in the tropics. Distribution of parasites depends upon: a. The presence and food habits of a suitable host:
Once we are clear about the geographical distribution and conditions favoring survival in relation to different parasites, effective preventive and control measures can more easily be devised and implemented.
Life cycle of parasites - the route followed by a parasite from the time of entry to the host to exit, including the extracorporeal (outside the host) life. It can either be simple, when only one host is involved, or complex, involving one or more intermediate hosts. A parasite’s life cycle consists of two common phases one phase involves the route a parasite follows inside the body. This information provides an understanding of the symptomatology and pathology of the parasite. In addition the method of diagnosis and selection of appropriate medication may also be determined. The other phase, the route a parasite follows outside of the body, provides crucial information pertinent to epidemiology, prevention, and control.
Host parasite relationship - infection is the result of entry and development within the body of any injurious organism regardless of its size. Once the infecting organism is introduced into the body of the host, it reacts in different ways and this could result in: a. Carrier state - a perfect host parasite relationship where tissue destruction by a parasite is balanced with the host’s tissue repair. At this point the parasite and the host live harmoniously, i.e. they are at equilibrium. b. Disease state - this is due to an imperfect host parasite relationship where the parasite dominates the upper hand. It can result either from lower resistance of the host or a higher pathogenecity of the parasite. c. Parasite destruction – occurs when the host takes the upper hand.
Laboratory diagnosis – depending on the nature of the parasitic infections, the following specimens are selected for laboratory diagnosis: a) Blood – in those parasitic infections where the parasite itself in any stage of its development circulates in the blood stream, examination of blood film forms one of the main procedures for specific diagnosis. For example, in malaria the parasites are found inside the red blood cells. In Bancroftian and Malayan filariasis, microfilariae are found in the blood plasma.
Treatment – many parasitic infections can be cured by specific chemotherapy. The greatest advances have been made in the treatment of protozoal diseases. For the treatment of intestinal helminthiasis, drugs are given orally for direct action on the helminthes. To obtain maximum parasiticidal effect, it is desirable that the drugs administered should not be absorbed and the drugs should also have minimum toxic effect on the host.
Prevention and control - measures may be taken against every parasite infectiving humans. Preventive measures designed to break the transmission cycle are crucial to successful parasitic eradication. Such measures include: Reduction of the source of infection- the parasite is attacked within the host, thereby preventing the dissemination of the infecting agent. Therefore, a prompt diagnosis and treatment of parasitic diseases is an important component in the prevention of dissemination. Sanitary control of drinking water and food. Proper waste disposal – through establishing safe sewage systems, use of screened latrines, and treatment of night soil. The use of insecticides and other chemicals used to control the vector population. Protective clothing that would prevent vectors from resting in the surface of the body and inoculate pathogens during their blood meal. Good personal hygiene. Avoidance of unprotected sexual practices.
Parasites of medical importance come under the kingdom called protista and animalia. Protista includes the microscopic single-celled eukaroytes known as protozoa. In contrast, helminthes are macroscopic, multicellular worms possessing well- differentiated tissues and complex organs belonging to the kingdom animalia. Medical Parasitology is generally classified into:
Describing animal parasites follow certain rules of zoological nomenclature and each phylum may be further subdivided as follows:
Super class Super family
Phylum Subphylum Class Order Family Genus Species
Subclass Subfamily