PART I

INTRODUCTION
TO
MEDICAL PARASITOLOGY

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 Specific Learning Objectives

At the end of this chapter the student 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

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 Introduction to Medical parasitology
1.1. Definition
• Medical parasitology (GK: 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 human parasites,
– the relationship between the human being and
the parasites,
– the prevention, control and treatment of the
parasitic diseases.
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 1.2. 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.

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Neglected tropical diseases

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 What are the diseases ?
• Sleeping sickness
• Leishmaniasis
• Tapeworm infection
• Guinea –worm disease
• River blindness
• Elephantiasis
• Schistosomiasis
• Echinococcosis
• Soil-transmitted helminthiasis
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 1.3. Concepts related to medical parasitology

1.3.1. Symbiosis
• Symbiosis (Greek symbioun, “to live together”)
• a close association of animals or plants of different
species that is often, but not always .
• , with each member a symbionts.
• one partner lives in or on another one’s
body.
– 3 types:
• Mutualism
• Commensalism
• Parasitism
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Mutualism
– Permanent association
– Obligatory ,physiological dependent
– Can not survive with out the other
– The mutual are metabolically dependent
on one another;
– Example ; coexistence of certain species of
algae and fungi that together compose lichens

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Commensalism
– Association of two different organisms
– Commensalism may be facultative
– One partner is benefited while the other neither
benefited nor injured
– Eg: Entamoeba dispar and human
Parasitism
– Antagonestic symbiosis
– Association of two different organisms one
partner is benefited while the other is injured.
such as human and different parasite .

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1.4 Nomenclature and classification of parasites

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
– Protozoan and helminthes
– Ectoparasite
• Stays on outside surface of the host to feed
• fleas, lice, mosquitoes, bugs, mites, ticks etc.

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• 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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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;
– Ticks, fleas, mites

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III. According to their Pathogenicity:
– Pathogenic parasites
– Non-Pathogenic (commensal)
– Opportunistic parasites

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 IV. Based on their life cycle
• Monoxenous parasites:
– Those with direct life cycles (i.e., with one host).
• Heteroxenous parasites:
– Those with indirect 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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 Other terminology
Aberrant parasite:
– Found in locations in the host where they
normally do not occur;
– e.g., Ascaris larvae may migrate to the brain
Insidental 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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• Parasites form part of the animal kingdom
which comprises about 800,000 identified
species.

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 Taxonomic classification of protozoa
kingd
phylum Sub-phylum Genus example Species
Protozoa Sub

om

Sarcomastig-ophora Sarcodina- Entamoeba E. histolytica

further divided into move by
pseudopodia
Mastigophora Giardia G. lamblia
move by flagella
Apicomplexa Plasmodium P. falciparum,
no organelle of P. vivax,
locomotion P. malariae,
P. ovale

Ciliophora Balantidium B.coli

move by cillia
Microspora Enterocyto-zoa E. bienusi
Spore-forming
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 do
Phylum class Genus example

ng
Metazoan Su

m
ki
b
Nematodes Ascaris
Round worms; appear Trichuris
round in they have Ancylostoma
alimentary canal and Enterobius
anaus
Strongyloides

Platyhelminthes Cestodes Taenia (tapeworm)

Flat worms; sucking organs, a segmented
no body cavity and, if body but no alimentary canal
present, the Each body segment is
alimentary canal is hermaphrodite
blind ending.

Trematodes Fasciolopsis (liver

Non-segmented,,with two suckers fluke)
but no distinct head Schistosoma (not
They have an alimentary canal leaf shaped!)
and are some hermaphrodite and
leaf shaped ,except shisto
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 Nomenclature of parasite

• 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

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 1.5. Hosts and types of hosts
 Host:-Hosts are organism which harbors the
parasite.
In parasitism, it is the injured partner
 Types of Hosts: -
 Definitive host:-
 Intermediate host:-

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• Definitive host:-
• harbours the adult or sexually mature stages
of the parasite. (In case of worm)
• Normally the larger of the hosts, usually a
vertebrate.
• e.g. man is DH for Schistosoma haematobium
• female Anopheles mosquito is DH for Plasmodium
species

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• Intermediate host:-
• that harbors larval or sexually immature stages of
the parasite
• then moves on to a different host before
reproducing.
– Asexual multiplication takes place
• e.g. man is IH of malaria parasites.
• Some parasites:
• Two intermediate hosts termed 1st and 2nd IH may
be needed for completion of a parasite's life cycle.

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 Other terminology
• Paratenic or Transport Host
– No development occurs but parasite remains
alive and infective to another host
– bridge gap between IH & DH
– May go dormant
– May cause damage
• e.g., dogs and pigs may carry hookworm eggs
• 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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 Types of Hosts cont….
• Reservoir Host
– Any animal that carries a parasite that can
cause infections in humans.
– It maintains the life cycle of the parasite in
nature.
• 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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 1.6. 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:

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1. Biological vectors:-
– characterized by the development of the
parasite before its transfer to another host

» E.g. Plasmodium species in

Anopheles mosquitoes.
2. Mechanical vector
– no parasitic development of
reproduction occurs

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 Other terminologies
• Infective Stage : it is a stage when a parasite can
invade human body and continue to live there.

• The infective stage of Round worm is the

embryonate egg.
• Infective Mode: means how the parasite invades
human body, penetrate the skin of a swimming man
and the infective Ascaris eggs are swallowed by man
.

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 1.7. Epidemiology of parasite
• 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

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 1.7.1. Geographic Distribution

• 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):

 Presence of a suitable host

 Habits of the host
 Escape from the host
 Favorable conditions outside of host
 Economic and social conditions

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 1.7.2. Transmission of parasites

Factors required:(Three key links of parasitic

disease transmission)
1. Source of infection
2. Mode of transmission
3. Susceptible people

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 1.7.2.1. source of exposure
1. primary Source
 infected persons
 carriers
 Animals
• 1. Sources of Exposure to Parasitic Infections
A. Contaminated soil:-
–Soils polluted with human excreta is
commonly responsible for exposure to
infection with geohelminthes
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B . 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. Taenia species, D.latum.
D. Blood sucking arthropods:
 Malaria - anopheles mosquito,
 Leishmania - sand flies
 Trypanosoma - tsetse flyb
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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• 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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 1.7.2.2 Mode of transmission
I- Indirect Mode of Transmission:-
– If the parasite
– has complex life cycle
– requires biological vectors and/or
– one or more intermediate hosts

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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 Route of Transmission
 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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 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 .
 Through Insect Bite,
– E.g., filarial worms, Trypanosoma sp, Plasmodium
 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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 1.8 General Life Cycles of parasites
• 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
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1.8.1 Simple or Direct Life Cycle (monoxenous)
• only one host is required to complete its cycle
• the parasite often spends most of its life, usually as an
adult, cycyst and tropozite where it reproduces.
• Transmitted from one host to another through the
contaminated food or water,air or a fomite.

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

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 Why study life cycles?
 Control.
 Treatment.
 Epidemiology.
 For research.

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 1.9 Parasitic Infections & Disease
• 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

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 1.9.1. 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. 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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 1.9.2. 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..

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1.9.3.How do Parasites Cause Injury to their
Host?
• 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
- may cause serious consequences to host
- metaplasia, change of tissue cell type to another type. Eg.
Paragonims (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
- 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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 1.9.4 Host Responses
Nonspecific immunity
– Macrophage endocytosis
• Common for bacteria and small protozoa
– Inflammation
– 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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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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 1.9.5 Parasite Responses
• 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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 Summary
• Symbiosis
• Types of parasite
• Types of host
• Source of infection
• Route of infection
• Host response
• Parasitic response

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