UNIT – ONE

1. INTRODUCTION
TO
Medical Parasitology
 OUTLINE
 Definition of Medical Parasitology
 History and scope of Medical Parasitology
 Importance of Parasitology
 Concepts related to Medical Parasitology
 Epidemiology of parasites.
 General life cycle of parasites
 Parasitic diseases
 Host immunity & immune - evasion of parasites
 Taxonomy and nomenclature of parasites
 Learning Objectives
At the end of this chapter the student will be able to:
 Define common terms used in Medical Parasitology
 Describe scope of Medical parasitology
 Explain host-parasite relationship
 Discuss the epidemiology of parasites
 Explain the general life cycles of parasites
 Explain briefly host immunity & immune – evasion
mechanisms by parasites
 Describe classification of medically important parasites
Introduction to Medical Parasitology
1.1. Definition
 Medical Parasitology (Greece: Para = beside, Sitos = food)
The study of the parasites of man and their medical
consequences.
 It is a subject that researches:
 The biological features of medically important parasites,
 The relationship between the human being and the
parasites,
 The diagnosis, treatment and prevention & control of the
parasitic diseases.
1.2. History and scope of Medical Parasitology
I. History of Parasitology
 Clear documentation of A.lumbricoides, D.medinensis &
Taenia spp was found in Papyrus Ebers (1550BC) & other
ancient Egyptian writings; also aware of schistosomes
 Aristotle familiar with helminths involving dogs, fish & pigs
 Aristophanes recognized A.lumbricoides, Taenia spp & E.
vermicularis
 Arabs- familiar with D.medinensis
History….
 Aretaeus- human hydatidosis
 Anthony Fitzherbert(1470-1538)- F.hepatica
 Ancient Chinese believed a man should harbor at least
3worms to remain in good health
 18thc Europe many regarded the presence of worms in
children as being beneficial to their health
 Reports of fanciful or imaginary worms causing all
manner of disease(s)
 Parasites were implicated in the 17th c in the etiology of
many diseases, including syphilis & plague
 II. Scope of Medical Parasitology

 According to the very broad definition of Parasitology,

parasites should include:-
 Viruses, bacteria, fungi,
 Protozoa and metazoa (multi-celled organisms) which
infect their host species.
However, for historical reasons the first three have been

incorporated into the discipline of Microbiology.

 Cont…
 Therefore, Medical Parasitology consists of :

Medical Protozology
(single celled animals)

Medical helminthology
(worms)
Medical Parasitology

Medical Arthropodology
Importance ….
 Six major tropical diseases to which WHO pays great
attention include:
 Malaria,
 Schistosomiasis,
 Filariasis,
 Leishmaniasis,
 Trypanosomiasis and
 Leprosy.
 Five of them are parasitic diseases except leprosy.
 All the above diseases are prevalent in Ethiopia
Burden

 Schistosomiasis - 200,000,000 infected

500,000-1,000,000 deaths/year
 Malaria - 500,000,000 infected

2,500,000 deaths/year
 Filariasis - 250,000,000 infected
 Trypanosomiasis - 25,000,000 infected

65,000 deaths/year
 Leishmaniasis - 1,200,000 infected
 1.4. Concepts related to Parasitology

1.4.1. Symbiosis
 Any association between two different organisms, in which
one partner lives in or on another one’s body and interact.
 Each organism is called a symbiont.
 3 types:

 Mutualism
 Commensalism
 Parasitism
Mutualism
 Permanent association between two different
organisms that life apart is impossible (one cannot
survive in the absence of the other.
 Two partners benefit each other,
 The mutuals are metabolically dependent on one
another;
Commensalism
 Close relationship between two different organisms from
which one benefits and the other neither benefits nor
suffers.
 E.g. E. Coli and man.
Parasitism
 The association between two different organisms in
which one (the parasite) derives benefit from the other
(the host), usually to obtain food or physical support.
 Parasitism can have minor or major effects on the
survival of the host.
 E.g. Ascaris lumbricoides and man.
1.4.2.Parasite and types of parasites

I. Parasite
 It is a small organism which lives in or on the host (larger
organism)
 In parasitism, parasite is the benefited partner which
obtains nourishment and shelter (niche) from the host
 The parasite harms the host and has a much higher
reproductive capability compared to its host.
 II. Types of Parasites

Parasite can be typed according to:

I. Habitat
 Endoparasite

 Parasite that lives inside the body of its host

 May be just under the surface or deep in the body
E.g. Tapeworms, flukes, protozoan
 Ectoparasite

 Parasite that lives on the outside surface of its host;

E.g. Flea.leeches, ticks
II. Dependency on the host
 Obligate Parasite
A parasite that is entirely dependent upon a host for its
survival.
 Facultative Parasite
A parasite that may be parasitic upon another
organism but can exist independently.
III. Amount of life time spent on the host
 Permanent Parasite
 Live the entire adult life stage in or on a host
 Usually endoparasites
 One exception is eyelash mite
 Temporary Parasite
 Spends only a short life time on a host
 Usually ectoparasites
IV. Pathogencity:
 Pathogenic parasites – parasites that have the
potential to cause disease.
 Non-Pathogenic – parasites that do not cause disease
under normal circumstances.
 Opportunistic parasites – parasites that can cause
diseases when the immune system of the host is
suppressed.
V. Life cycle
 Monoxenous parasites:
 Requiring only a single host to complete the life cycle.
 Have a direct life cycle (E.g. Ascaris lumbricoides)
 Heteroxenous parasites:
 Those which require two or more hosts to complete
their life cycle (E.g. Plasmodium species)
 Have indirect life cycle (require an intermediate host)
 Heterogenetic parasites:
 One with alteration of generations e.g., Coccidial
parasites and Strongyloides
VI. Host ranges

 Euryxenous parasites:
 Those with a broad host range.
 Stenoxenous parasites:
 Those with a narrow host range
Other types of parasites

Aberrant parasite:
 Found in locations in the host where they normally do
not occur.
 E.g. Ascaris larvae may migrate to the brain

Incidental parasite:
 Occurs in hosts where it does not normally occur;
 E.g., Fasciola normally does not occur in man but is
incidental if found in human’s liver.
1.4.3.Hosts and types of hosts

 Host: An organism that harbors and provides sustenance

for another organism (the parasite).
 Types of Hosts:
Definitive host: the host which usually harbors the
adult/sexual stage of the parasite
Intermediate host: the host which usually harbors the
larval/asexual stage of the parasite
 Characteristics of definitive (primary) host:
 Where sexual reproduction takes place.
 Where the adult parasites lives
 The larger of the hosts, usually a vertebrate.
 Host range (specificity) - frequently, a large number
of host species can act as intermediate host and
only one or a few can act as a definitive host
 Intermediate host:
 Harbors immature (larval) stage of a parasite
 Asexual multiplication takes place
 May harbor different immature developmental
stages of a parasite
E.g. Snail harboring cercaria, Redia and
Sporocysts (all immature stages of Fasciola)
 Some parasites require more than one
intermediate host which are then designated as
first, second intermediate,
 Paratenic Host
 An animal acting as a substitute intermediate host of a
parasite, usually having acquired the parasite by
ingestion of the original host;
 Also termed: transfer host; transport host.
 No development of the parasite occurs but parasite
remains alive and aids in the transmission of infection.
 May go dormant
 May cause damage to the host
E.g. Toxoplasma species in cattle
 Accidental or Incidental Host
 An animal host to a parasite which is not the usual host
species for that parasite.
 Parasite is in the “wrong” host
 Parasite usually wanders around and causes great
damage and then dies.
 Reservoir Host
 Any animal that carries a parasite that can cause
infections in humans.
 Carrier host:
 Carries parasites and can pass them on to others,
although not necessarily themselves having symptoms
of the disease.
 An important source of infection in epidemiology

E.g. Human being harboring cyst form of E. histolytica

 1.4.4.Host specificity

 The number of species the parasite can use as a

definitive or intermediate host.
 Parasites show varying degrees of host specificity
 Most parasites will infect a few closely related species
 Some parasites can infect a large group of animals
A few parasites will infect only one species
A few parasites have little or no host specificity
 1.4.5.Vector and types of vectors

Vector: an organism (usually an arthropod) which transfers

infective forms of a parasite from one host to the

other.
 Classification

1. Biological vectors:

2. Mechanical (Parathenic or transport) Vectors:

1. Biological vectors:
 An arthropod in whose body the infecting organism
develops or multiplies before becoming infective to the
recipient individual.
 Types of Biological vectors:
I. Propagative - multiplication of the pathogen without
developmental change.
E.g. Yersinia pestis in fleas
III. Cyclodevelopmental - developmental change without
multiplication.
E.g.Onchocerca volvulus in black flies.
 II. Cyclopropagative - multiplication and developmental
change of the pathogen
E.g. Plasmodium vivax in Anopheles mosquitoes
2. Mechanical vector
 An arthropod that transmits the infective organisms
from one host to another but is not essential to the life
cycle of the parasite.
 No parasitic development or reproduction occurs
1.4.6. Other terminologies

 Infective Stage : stage of the parasite that can invade

human body and continue to live there.
 The infective stage of ascarid is the embryonated egg.
 Infective Route - the specific entrance through which the
parasite invades the human body.
 Hookworms invade human body through skin.
 Infective Mode – a means how the parasite invades human
body, such as the cercariae of the blood fluke actively
penetrate the skin of a swimming man
 Geohelminthes
 Refers to the helminthes which complete their life cycles
not requiring the processes of the development in
intermediate hosts.
 They have only one host and a simple life cycle, such as
ascarid, hookworm, pinworm etc.
 Biohelminthes
 Refers to the helminthes which have to undergo the
development in intermediate hosts to complete their life
cycles, such as filaria, liver fluke, pork tapeworm.
 Alternation of Generation:
 In life cycles of some parasites, there is the regular
alternation of sexual and asexual reproductions,

E. g. Plasmodium vivax
 Trophozoite - the active, motile feeding stage of an
apicomplexan parasite, and the motile stage of flagellate
protozoa.
E.g. Giardia spp.
 Cyst – A stage in the life cycle of certain parasites
(protozoan), during which they are enveloped in a
protective wall.
 It is usually the infective stage. Its functions are
protection, transmission and multiplication.
1.5. Epidemiology of parasite
 Epidemiology: The study of the patterns of diseases
within populations
 For parasites, this includes:
 Host range – what can it infect?
 Geographic range – where is it?
 Is it a zoonotic agent?
 Can it infect humans?
 Does it have a reservoir?
 Does it have a nidus?
1.5.1. Distribution of parasites and its determinant factors

 Distribution
 Parasite occur globally
 The majority occur in tropical regions,
 Factors

 Favorable environmental conditions

 Poverty, poor sanitation and personal hygiene
The burden of some major parasitic infections
Parasite Diseases No. people infected Deaths/yr
Plasmodium Malaria 273 million 1.12 million

Soil transmitted 2 billion 200,000

helminths:
Pnemonitis, intestinal
 Roundworm (Ascaris) obstruction

 Whipworm (Trichuris) Bloody diarrhoea, rectal

 prolapse
 Hookworm
(Ancylostoma and Coughing, wheezing, abdominal
Necator) pain and anaemia
Schistosoma Renal tract and intestinal 200 million 15,000
disease

Filariae Lymphatic filariasis and 120 million Not fatal but 40 million
elephantiasis disfigured or
incapacitated
Trypanasoma cruzi Chagas disease (cardiovascular) 13 million 14,000

African trypanosomes African sleeping sickness 0.3 – 0.5 million 48,000

Leishamania Cutaneous, mucocutaneous and 12 million; 2 million new 50,000

visceral leishmaniasis cases/yr
Factors for the distribution of parasites:
I. Presence of a suitable host
II. Habits of the host
III. Escape from the host
IV. Favorable conditions outside of host
V. Economic and social conditions
1.5.2. Transmission of parasites

 Three key links for parasitic disease transmission:

 1. Source of infection
 2. Mode of transmission
 3. Susceptible host (people)
1.5.2.1. Sources of exposure
The sources of exposure could be:

1. Contaminated soil:
 Soil polluted with human excreta is commonly
responsible for exposure to infection with
geohelminthes
2. Contaminated water: may contain
a. Cercarial stages of blood flukes,
b. Viable cysts of Amoeba, flagellates etc…
c. Cyclops containing larva of D. medinensis
d. Fresh water fishes which are sources for fish
tape worm, and intestinal flukes infection
e. Crab or cray fishes that are sources for lung fluke
f. Water plants which are sources for F. buski.
3. Raw or Insufficiently cooked meat of pork, beef and fish
• Trichinella spiralis, Taenia species, D.latum.

4. Blood sucking arthropods:

 Malaria – female anopheles mosquito,
 Leishmania - sand flies
 Trypanosoma – female tsetse fly

5. Animals (domestic or wild animals harboring the parasite)

 Dogs- the hydatid cyst caused by E. granulosus
6. Human beings:
A person, his/her clothing, bedding or the immediate
environment that he/she contaminated
 Autoinfection: Spread of infection from one part of the
body to another with in a single host
 It is used to describe a host which is both intermediate
and definitive without parasite transmission from other
animals
 It may be particularly damaging due to the large
number of offspring the parasites produces.
E.g.S. stercoralis, E. vermicularis, and T. solium
 1.5.2.2. Modes of Transmission
1. Direct mode of Transmission:

Classified as:

I. Horizontal: Transmission is mainly effected through:

 Feco-oral route: most intestinal parasites
 Sexual intercourse
 Blood transfusion
 Direct skin penetration
II. Vertical: transmission of the parasite is from the

mother to child through:

 Congenital / transplacental
 Transmammary (breast milk)

2. Indirect Mode of Transmission:

If the parasite:
 Has complex life cycle,
 Requires biological vectors and/or
 One or more intermediate hosts
Route of Transmission
I. Ingestion of infective stage of parasites:
 In food, water or hands contaminated with faeces,
E.g. E. histolytica, E. vermicularis, etc.
 In raw or undercooked meat,
E.g. T. saginata, T. solium, T. spiralis
 In raw or undercooked fish, crab, or water vegetation
E.g. intestinal flukes
 Water containing Cyclope e.g., D. medinensis
II. Skin penetration when in Contact with:
 Faecally polluted soil,

E.g. S.stercoralis, Hook worms

 Water containing infective stages of the parasite

E.g. Cercaria of Schistosome species

III. Through Insect Bite,

E.g. Filarial worms, Trypanosoma spp., Plasmodium spp.

IV. Sexual Contact
E.g. Trichomonas vaginalis
V. Transmammary
E.g. S. stercoralis
VI. Inhalation of contaminated air
E.g. E. vermicularis, P. carinii
VII. Transplacental
E.g. T. gondii
VIII. Kissing
E.g. Trichomonas gingivalis, T. tenax
1.6. General Life Cycles of parasites
 Describes the cycle of development of the parasite, which
may involve passing through a number of developmental
stages & environment
 Parasitic and non-parasitic stages may occur in the cycle
 Phases:
 Growth and maturation,
 Reproduction (sexual and asexual) and
 Transmission

 All phases are vitally important for the successful survival of

the parasite.
 2

1 3
1.6.1. Simple or direct life cycle

 Also called monoxenous (only one host is required to

complete its cycle)
 The parasite often spends most of its life, usually as an
adult, and reproduces
 Transmitted from one host to another through the air, by a
fomite, or in contaminated food or water.
1.6.2. Indirect life cycle
 Also called heteroxenous (requires 2 or more hosts to
complete its life cycle)
 Frequently this may involve passing through a number of
developmental stages & environment.
1.6.3. Why do we study life cycles?
• Control.
• Treatment.
• Epidemiology.
• Fundamental research
 1.7. Parasitic infections & diseases

 Not all parasitic infections cause disease of clinical

significance.
 Both host and parasitic factors are involved for the
parasitic infection to cause disease or not
 1.7.1.Host factors
1. Genetic factors
E.g. Black population who lack Duffy antigen resist P.vivax
2. Age
3. Sex : e.g. T.vaginalis
4. Immunity: natural and acquired
5. Nutrition (malnutrition or under nutrition)
6. Intensity and frequency of infections
7. Presence of co-existing disease or conditions, which
reduces immune response. e.g. Pregnancy, HIV
8. Life style and occupation
1.7.2.Parasite factors
1. Strain of the parasite and adaptation to human host
2. Parasite load ( number of parasite )
3. Site (s) occupied in the body
4. Metabolic processes of the parasite, particularly the
nature of any waste products or toxins produced by the
parasite during its growth and reproduction
 1.7.3.Pathogenesis

 Competition for the host’s nutrients

 D. latum absorbs vitamin B-12
 Other tapeworms absorb proteins and sugars
 Hook worm ingests blood
 Destruction of host tissues
 Some injure upon entry
E. g. Swimmers itch, cercariae penetrate and cause
inflammation
 Some others cause injury after establishment
 Intestinal worms cause small lesions in gut and possible
 Tissue changes
 May cause serious consequences to host
 Hyperplasia - parasite causes host to produce more
cells
Eg Liver fluke simulating enlargement of bile duct
 Hypertrophy
 Metaplasia, change of tissue cell type to another type.
Eg. Paragonimus westermani (lung fluke)
 Neoplasia, growth of cell to form a new structure.
Eg. Tumors
 Toxins and secretions
• Some may cause pathogenic response, some may inhibit
immune function
E. g. Mosquito saliva
 Mechanical interference
• Elephantiasis (filarial worms) blocks lymphatic system
• Tapeworms in large numbers can block intestine
• Plasmodium can cause RBC’s to stick together and clog
capillaries
1.8. Host Immunity & Immune – evasion of parasites

1.8.1. Host Responses

I. Nonspecific immunity
 Macrophage endocytosis
 Common for bacteria and small protozoa
 Inflammation

 Acute – edema and increment in no of leukocytes

 Subacute – monocytes and lymphocytes present, with
fibrocytes binding parasite with collagen.
 Chronic – plasma cells present and form a granuloma
II. Specific Immunity
 Humeral response: Formation of antibodies or
immunoglobulins(Ig) by B cells.
 IgE fights helminthes
 IgM and IgG important against protozoan
 Cell mediated response: uses T-cells
 Cytotoxic T cells inject invading parasites
 Also release cytokines, which promote nonspecific
immunity.
 1.8.2. Immune-evasion of parasites
Mechanisms:
 Antigenic variation
 Change surface glycoproteins regularly
 Being poorly antigenic
 Don’t induce a response, or a mild one
 Hide within host cells
 Host can’t kill what it can’t find
Mechanisms…
 Camouflage
 Use bits of host cells and attach to parasite’s surface
 Depress host’s immune response
 Modulate suppression of host T cell production
1.9. Taxonomy and nomenclature of parasites

1.9.1. Taxonomy
1.9. Taxonomy and nomenclature of parasites

1.9.1. Taxonomy
Taxonomy of metazoa
Nematodes (round worms)
 Appear round in cross section,
 Have body cavities, a straight alimentary canal and an anus

Platyhelminthes (flat worms)

 Dorsoventrally flattened,
 No body cavity and, if present, the alimentary canal is blind

ending
Cestodes (Tapeworms):
 Adults found in the intestine of their host
 Have a head (scolex) with sucking organs, segmented
body but no alimentary canal
 Each body segment is hermaphrodite
Trematodes:
 Non-segmented, usually leaf-shaped, with two suckers

but no distinct head

 They have an alimentary canal and are usually

hermaphrodite and leaf shaped

 Schistosomes are the exception. They are thread-like,

and have separate sexes

Taxonomy of Protozoa
Taxonomy…

Sarcodina: Move by pseudopoda

Mastigophora: move by flagella
Apicomplexa: no locomotary organelle
Ciliophora: move by cilia
Microspora: spore former
1.9.2. Nomenclature of parasites

 Common name vs scientific name

 Parasites named by binomial nomenclature
 Genus (capitalized)
 Species (not capitalized)
 Binomial name underlined or separately italicized

Example: Ascaris lumbricoides, Ascaris lumbricoides

Summary

 List factors contribute for the distribution of parasites

 What are the important links for parasitic disease
transmission
 List the major sources of exposure for parasitic diseases
 Write the routes of transmission for parasitic diseases
 Write the major components of parasite lifecycle
 What is the importance of studying life cycle
 Write the mechanisms of immune evasion of parasites.
Reference
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Diseases. Elsevier Inc. 2nd 2006.
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Parasitology. John Wiley & Sons Ltd. 2001.
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Countries. 2nd edition. Part one. 2005
4. Awole M., Cheneke W. Medical Parasitology for Medical
laboratory Technology students. Upgraded lecture Notes
Series .2006.
5. Jaffeey and Leach. Atlas of Medical Helminthology and
Protozoology 2nd edition.
6. Murray P.R., Rosenthal K.S., Kobayashi G.S., Pfaller M.A.,
Medical Microbiology, 4th edition. Mosby, 2002.
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