1

Medical
Parasitology
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5/11/2024
 By Shimeles Adugna
2

 Assistant Professor of Medical Parasitology

 E-mail:shimeadu39@gmail.com
 Cellphone:+251911720233

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 OUTLINE
 Definition of terms 3

 Scope of medical parasitology

 Concepts related to medical parasitology

 Epidemiology of parasites

 General life cycle of parasites

 Parasitic diseases

 Host immunity & immuno – evasion of parasites

 Nomenclature and classification of parasites

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 Specific Learning Objectives
At the end of this chapter the 4student will be able to:
 Define common terms used in medical parasitology
 Describe Scope of Human parasitology
 Explain host-parasite relationship
 Discuss the geographical distribution, mode of transmission,
source of infection, and portal of entry of parasites
 Explain the general life cycles of parasites
 Discuss the general pathogenesis of parasites
 Explain briefly host immunity & immuno – evasion mechanisms
by parasites
 Describe classification of medically important parasites
 Laboratory diagnosis of parasites (stool examination, blood film,
serology)

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 Parasitology
5

 the branch of biology dealing with parasites and the

effects of parasitism
 is the study of parasites, their hosts, and the
relationship between them
 the branch of biology that is concerned with the
study of parasites’ Meaning, Pronunciation,
Translations and Examples
 Area of biology concerned with the phenomenon of
dependence of one living organism on another

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 7 fields/branches of parasitology
6

A. Medical parasitology
B. Veterinary parasitology
C. Structural parasitology
D. Quantitative parasitology
E. Parasite ecology
F. Conservation biology of parasite and
G. Taxonomy and phylogenetics

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 Medical parasitology
7

 Concerned with parasites of humans and their

medical significances, importance in human
communities

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 Veterinary parasitology
8

 Study of animal parasites, relationship bet parasite

and animal hosts

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 Structural parasitology
9

 Study of physical forms and structures of parasitic

proteins

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 Quantitative parasitology
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 Study of quantifying parasites in a sample of hosts

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 Parasite ecology
11

 Study of parasites on their interactions wih their

hosts and environment

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 Conservation biology of parasite
12

 Study of parasite conservation

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 Taxonomy and phylogenetics
13

 Study of classification, identification and naming of

parasites

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 14

 It is a subject that researches:

 the biological features of human parasites,
 the relationship between the human being and the
parasites,
 the prevention and treatment of the parasitic diseases

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 1.2. Scope of Medical Parasitology
15

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 Cont…

F 16

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 • Phylum Sarcomastigophora
• Amoeba
17
Medical • Flagellates
Human Parasitology

• Phylum Apicomplexa
Protozoology
• Phylum Microsporodia
• Phylum Ciliophora
• Class Nematoda
Medical • Class Trematoda
• Class Cestoda
Helminthology

• Class Insecta
Medical • Class Arachnida
• Class Crustacea
Arthropodology • Class Chilopoda(centipede)
• Diplopoda (millipede)

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 The importance of parasitology
 Five major tropical diseases
18 to which WHO pays
great attention include:
 malaria,
 schistosomiasis,
 filariasis,
 leishmaniasis,
 trypanosomiasis and

 All the above diseases are prevalent in Ethiopia

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 Features of Parasites
19
1. Smaller than their host

2. Out number the host

3. Have shorter life span than the host

4. Have greater reproductive potential than the host

5. Could be unicellular or multicellular

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 1.3. Concepts related to medical
parasitology
20

 Any association more or less permanent is called a

symbiosis, with each member a symbiont.

 Phoresis

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 22

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 Phoresis
23

 Theterm phoresis is derived from the Greek word

meaning “to carry.”
 In this type of symbiotic relationship, the phoront,
usually the smaller organism, is mechanically carried by
the other, usually larger, organism, the host.
 Unlike commensalism, there is no dependency in the
procurement of food by either partner.
 Phoresis is a form of symbiosis in which no
physiologic interaction or dependency is involved.

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 Phoresis…
24

 Both commensalism and phoresis can be

considered spatial, rather than physiologic,
relationships
 Examples of phoresis are the numerous sedentary
protozoans , algae, and fungi that attach to the
bodies of aquatic arthropods, turtles, etc.

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 1.3.1. Parasite and types of
parasites
25

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 Parasite...
26

 In another words
 A small organism (Parasite) has the potential to harm a larger
organism (Host), and relies on said host for nutrients and
shelter (a Niche).
 The parasite generally has a much higher reproductive
capability compared to its host.

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 Types of Parasites
27

Parasite can be Classified

I. According to their habitat
 Endoparasite
 Lives inside the body of the host
 May be just under the surface or deep in the body
 Tapeworms, flukes, protozoans

 Ectoparasite
 Stays on outside surface of the host
 leeches, ticks, fleas, brood parasites

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 Types of Parasites...
29

 II. Based on dependency on the host

 Obligate Parasite
 Requires finding and invading the host to complete its
life cycle
 Most of the parasites we will cover are obligate parasites

 Facultative Parasite
 May become parasitic if it is given the chance but does
not require a host.

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 Types of Parasites...
30

III. Amount of time spent

 Permanent Parasite
 Lives entire adult life stage on or in a host
 Usually endoparasites
 Temporary Parasite
 Spends only a short time on a host

 Usually ectoparasites

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 Types of Parasites...
31

IV. According to their Pathogenicity:

 Pathogenic parasites
 Non-Pathogenic (commensal)
E. Dispar,E. Coli

 Opportunistic parasite
T. gondi, Cryptosporidium spp, Cyclospora
spp

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 V. Based on their life cycle
32

 Monoxenous parasites:
 Those with direct life cycles (i.e., with one host).
 Heteroxenous parasites:
 Those with inderect life cycles requiring an intermediate host
(i.e., involves 2 or more hosts).
 Heterogenetic Parasites:
 One with alteration of generations e.g., Coccidial parasites and
Strongyloides

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 VI. Based on host ranges
33

 Euryxenous parasites:
 Those with a broad host range.

 Stenoxenous parasites:
 Those with a narrow host range;

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 Other terminology
34

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 man’s liver.

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 1.3.3. Hosts and types of hosts
35

 Host:-Hosts are organism which harbors the

parasite.

 Types of Hosts: -
 Definitive host:-
 Intermediate host:-

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 Definitive host:-
 What characterizes the primary
36 host?

 Where sexual reproduction takes place

 Normally where the adult parasites live

 Normally the larger of the hosts, usually a vertebrate.

 Convention - (parasites which only reproduce

asexually)

 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

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  Intermediate host:- 37
 sexually immature or larval stage of a
parasite
 Asexual multiplication takes place
 may harbor many immature stages of a
parasite;
 e.g., Cercaria, Redia and Sporocysts which
are all immature stages of Fasciola in the
snail intermediate host.
 Some parasites:
 require more than one intermediate host
which are then designated as first, second
intermediate,

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  Reservoir Host
 Any animal that carries a parasite that can cause
infections in humans.
 Even if it is the normal host for that parasite.
 Related to the medical perspective of parasitology
 Carrier host: -
 A person who harbors parasites has no any clinical
symptom
 He is an important source of infection in epidemiology
 e.g. human beings harboring cyst form of E.histolytica

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 Other host
39
 Paratenic or Transport Host
 No development occurs but parasite remains alive and
infective to another host
 May go dormant
 May cause damage
 e.g., Toxoplasm species in cattle

 Accidental or Incidental Host

 Parasite is in the “wrong” species.
 Parasite usually wanders around and causes great
damage because it doesn’t know where to go then dies.

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 1.3.4. Host specificity
 The number of species the
40 parasite can use as a

definitive or intermediate host.

 Parasites show varying degrees of host specificity

 A few parasites will infect only one species
 Most parasites will infect a few closely related
species (or similar anatomy)
 Some parasites can infect a large group of animals
 A few parasites have little or no host specificity

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 1.3.5. Vector and types of vectors
41

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:

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 1. Biological vectors:-
 A living obligate host in which
42
the parasite
undergoes morphologic change and/or
multiplication
 There are three classes of biologic vectors:
Ø (1) Propagative: in which the parasite multiplies
but does not undergo morphologic change
E.g. Yersinia pestis in fleas
Ø(2) Cyclopropagative : in which the parasite
undergoes cyclic, morphologic changes and also
multiplies
E.g. Plasmodium vivax in Anopheles mosquitoes
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Ø (3) Cyclodevelopmental: in which the parasite

undergoes cyclic morphologic change, but does
not multiply
E.g. Onchocerca volvulus in black flies
2. Mechanical vector
 no parasitic development or reproduction
occurs

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 1.3.6. Other terminologies
44
 *Infective Stage : it is a stage when a parasite can
invade human body and continue to live there. The
infective stage of ascaris is the embryonate egg.
 *Infective Route: is the specific entrance through
which the parasite invades the human body.
Hookworms invade human body by skin. Man gets
infection with ascaris by mouth.
 Infective Mode means how the parasite invades
human body, such as the cercariae of the blood
fluke actively penetrate the skin of a swimming
man and the infective ascaris eggs are swallowed
by man.
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 Geohelminth
 refers to the helminths 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
ascaris, hookworm, pinworm and etc.
 Biohelminth
 refers to the helminths which have to undergo the
development in intermediate hosts to complete their life
cycles, such as filaria, liver fluke, pork tapeworm and so on.

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 Alternation of Generation:
 In life cycles of some parasites, there is the regular
alternation of sexual and asexual reproductions,
eg.Plasmodium vivax

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 *Trophozoite is a living stage of protozoa when they

can move, take food and reproduce. (It is usually the
pathogenic stage.)
 *Cyst is the resting stage of a protozoa with a
protective wall. It is usually the infective stage. Its
functions are protection, transmission and
multiplication.
Encystation
 Trophozoite Cyst
Excystation

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 1.4. Epidemiology of parasite
48

 Epidimology: 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?

 A group of vertebrates maintaining the parasite

 Does it have a nidus?

 A small ecosystem that possesses all the factors to maintain
the parasite.

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 1.4.1. Geographic Distribution
49

 Global distribution
 parasite occur globally,

 the majority occur in tropical regions,

 Factors
 Favorable environmental conditions
 poverty, poor sanitation and personal hygiene

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 Factors (Endemicity):
50

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1.4.2. Transmission of parasites

51

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 1.4.2.1. Sources of Exposure to Parasitic
Infections 52

1. Contaminated soil:-
 Soils polluted with human excreta is commonly
responsible for exposure to infection with
geohelminthes
2.Contaminated water:-
Water may contain
(a) viable cysts of Amoeba, flagellates etc,
(b) cercarial stages of human blood fluke,
(c) Cyclops containing larva of Dracunculus 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 and
(f ) Water plants which are sources for Fasciolopsis buski.
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C. Raw or Insufficiently cooked meat of pork,

beef and fish
E.g., Trichinella spiralis, Taenia species,
D.latum.
D. Blood sucking arthropods:
Malaria - anopheles mosquito,
Leishmania - sand flies
Trypanosoma - tsetse fly
E. Animals (a domestic or wild animals
harboring the parasite),
e.g, 1. Dogs- the hydatid cyst caused by E.
granulosus

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 54

 F. Human beings:-
 A person his/her clothing, bedding or the immediate environment
that he/she contaminated
 Autoinfection: - e.g., S. stercoralis, E. vermicularis, and T.
solium

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 1.4.2.2. Mode of
Transmission(form) –host factor
55

Direct mode of Transmission:-

classified as:
 I. Horizontal Direct mode of transmission:
Transmission is mainly effected through:-
 Feco-oral route: most intestinal parasites transmitted in
this way.
 Sexual intercourse
 Blood transfusion
 Direct skin penetration

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 56

II. Vertical Direct Mode of Transmission:

Transmission of the parasite is from the
mother to child through:
 Congenital / transplacental
 Transmammary (breast milk)

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 57

 II- Indirect Mode of Transmission:-

 If the parasite
 has complex life cycle
 requires biological vectors and/or
 one or more intermediate hosts

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 Route of Transmission -parasitic
factor
58

I. By ingesting 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
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II. Penetration of Skin 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 sp, Plasmodium sp. etc.

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 Sexual Contact, e.g., Trichomonas vaginalis

 Transmammary, e.g., S. stercoralis
 Inhalation of contaminated air, e.g., E. vermicularis, P.
carnii
 Transplacental, e.g., T. gondii
 Kissing, e.g., Trichomonas gingivalis, T. tenax

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 1.5 General Life Cycles of
parasites
61
 Describes the cycle of development of the parasite,
 This may involve
 Passing
through a number of developmental
stages & environment
 Parasitic and non-parasitic stages.
 The life of a parasite can be divided into a number of
phases:
 Growth and maturation,
 Reproductive (sexual and asexual) and
 Transmission phases.
All vitally important for the successful survival of the
parasite.
 Can be simple or complex depending on how many different
hosts it requires to complete its cycle

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 Simple or Direct Life Cycle (monoxenous)
 only one host is required to complete
63
its cycle
 the parasite often spends most of its life, usually as an adult, and where
it reproduces
 Transmitted from one host to another through the air, by a fomite, or
in contaminated food or water.

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 Indirect or heteroxenous life cycles
 requires 2 or more hosts (a vector or intermediate host ) to
64
reproduce or grow in
 Frequently this may involve passing through a number of
developmental stages & environment.

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 1.5.3. Why study life cycles?

üControl
üTreatment
üEpidemiology
üFundamental research

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 1.6 Parasitic Infections & Disease:
 Not all parasitic infections
66 cause disease of

clinical significance.
 Both host and parasitic factors are involved for
the parasitic infection to cause disease or not

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 1.6.1. Host Factors
67

1. Genetic factors, E.g. Black population who lack Duffy

antigen resist P.vivax
2. Age,
3. Sex : e.g., T.vaginalis
4. Level of immunity: natural and acquired immunity
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

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 1.6.2. Parasite factors
68

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.

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 1.6.3. How do Parasites Cause Inquiry
to their Host?
69
 Competition for the host’s nutrients

- Eg. D.latum absorbs vitamin B-12, can cause anemia

- other tapeworms absorb large amounts of proteins and
sugars
 Use of host’s fluids
- hookworm ingests blood, can be up to 250 ml/day
 Destruction of host tissues
- some injure upon entry, some after established
- eg. Swimmers itch, cercariae penetrate and cause
inflammation
- intestinal worms, after established cause small
lesions in gut, possible secondary infection
- Entamoeba actively digest epithelial cells in large
intestine
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  Tissue changes 70

- may cause serious consequences to host

- hyperplasia,. Eg Fasciola
- hypertrophy,

- metaplasia, change of tissue cell type to another

type. Eg. Paragonims (lung fluke)
- neoplasia, growth of cell to form a new structure.
Eg. Tumors

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  Toxins and secretions 71

- some may cause pathogenic response, some may inhibit

immune function
- eg. 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

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 Host Immunity & Immuno – evasion of the
parasite
72

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1.7. Host Immunity & Immuno – evasion of the
parasite…
1.7.1.Host Responses
73

Nonspecific immunity
 Macrophage endocytosis
 Common for bacteria and small protozoa
 Inflammation
 Acute – edema and increase of leukocytes
 Subacute – monocytes and lymphocytes present, with fibrocytes
binding parasite with collagen.
 Chronic – plasma cells present and form a granuloma

 Hyperplasia – parasite causes host to produce more cells

 Liver fluke simulating enlargement of bile duct
 Neoplasia (cancer) – rare parasites have been associated
with cancer, but mechanisms are still unknown.
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 Host Responses
74

Specific Immunity
 Humeral response: Formation of antibodies or
immunoglobulin s(Ig) by B cells.
 IgE fights helminths
 IgM and IgG important against protozoans
 Cell mediated response: uses T-cells
 Cytotoxic T cells inject invading parasites
 Also release cytokines, which promote nonspecific immunity.
(interconnected)

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 1.7.2. Parasite Responses
75

 Antigenic variation
 Change surface glycoproteins regularly
 Being poorly antigenetic
 Don’t induce a response, or a most a mild one
 Hide within host cells
 Host can’t kill what it can’t find
 Camouflage
 Use bits of host cells and attach to parasite’s surface
 Depress host’s immune response
 Modulate produce of host T cell production

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 Taxonomy and nomenclature of parasites
76

 Taxonomy

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Sub kingdom
Taxonomic
Phylum
classification
Class
of helminths
Genus – examples

Metazoa Nematodes Ascaris (roundworm)

Round worms; Trichuris (whipworm)
appear round in Ancylostoma
cross section, they (hookworm)
have body cavities, a Necator (hookworm)
straight alimentary
canal and an anus Enterobius (pinworm
or threadworm)
Strongyloides

Platyhelminthes Cestodes Taenia (tapeworm)

Flat worms; Adult tapeworms are found in the
dorsoventrally intestine of their host
flattened, no body They have a head (scolex) with
cavity and, if present, sucking organs, a segmented body but
the alimentary canal no alimentary canal
is blind ending
Each body segment is hermaphrodite

Trematodes Fasciolopsis (liver fluke)

Non-segmented, usually leaf-shaped, with Schistosoma (not leaf
shaped!)
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
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 Taxonomic classification of protozoa
Sub kingdom Phylum Sub-phylum Genus- examples Species- examples

Protozoa Sarcomastig Sarcodina-- - Entamoeba E. histolytica

-ophora move by
pseudopodia
further divided into

Mastigophora Giardia G. lamblia

move by flagella

Apicomplexa Plasmodium P. falciparum,

no organelle P. vivax,
of P. malariae,
locomotion P. Ovale
p.knolesi

Ciliophora Balantidium B. coli

move by cillia

Microspora Enterocyto- E. bienusi

Spore-forming zoa
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 Nomenclature of parasites
79

 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

Para_by Shime_ArU_MW_2015 entry 5/11/2024

 NAMING AND CLASSIFYING
MICROORGANISMS

q The first letter of the genus name is always

capitalized, but never the first letter of the species
name. e.g Enterococcus faecalis (located in feces)
q The genus may be abbreviated with the first letter,
and the species is written out.

q The genus and species of an organism is always

either underlined OR italicized: E. faecalis or
E. faecalis are both acceptable.

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 HELMINTHES

PLATYHELIMINTHES
NEMATHELIMINTHES
Tape worms (Cestodes)
Intestinal Nematodes Taenia saginata
Ascaris lumbricoides Taenia solium
Hook worm species Hymenolepis nana
Strongyloides stercoralis Hymenolepis diminuta
Enterobius vermicularis Echinococcus granulosus
Trichuris trichiura Diphyllobothrim latum

Blood and Tissue nematodes The flukes (trematodes)

Wuchereria bancrofti Blood flukes
Brugia malayi/timori Schistosoma mansoni
Loa loa Schistosoma japonicum
Onchocerca volvulus Schistosoma haematobium
Trichinella spiralis Schistosoma intercalatum
Dracunculus medinensis Schistosoma mekongi
Cercarial dermatitis
Liver flukes
Intestinal flukes
Para_by Shime_ArU_MW_2015 entry 81 Lung5/11/2024
flukes
 PROTOZOA

AMOEBA FLAGELLATES
Intestinal amoeba
Intestinal flagllates
Entamoeba histolytica
Entamoeba dispar
Entamoeba coli Giardia lamblia
Entamoeba hartmanni Dientamoeba fragilis
Entamoeba polecki Chilomastix mesnili
Entamoeba gingivalis (oral) Enteromonas hominis
Endolimax nana Retortamonas intestinalis
Iodamoeba bütschlii Trichomonas hominis
Trichomonas tenax (oral)
Free living amoeba
Naeglaria species Urogenital flagellates
Acanthamoeba species
Trichomonas vaginalis (urogenital)

Blood and tissue flagellates

Leishmania species (leishmaniasis)
African trypanosomes (African sleeping sickness)
Trypanosoma cruzi (Chagas' disease)

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PROTOZOA….

CILIATE

APICOMPLEXA Balantidium coli

:
. Intestinal coccidia(Apicomplexa)
Cryptosporidium parvum Microspora
Cyclospora cayetanensis Spore-forming
Isospora belli
Blood and tissue coccidian (Apicomplexa)
Plasmodium spp
Toxoplasma gondi
Babesiasis

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