INTRODUCTION

TO
MEDICAL
PARASITOLOGY
 OUTLINE
Definition of terms

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

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
Explain briefly host immunity & immuno – evasion
mechanisms by parasites
Describe classification of medically important
parasites
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 and treatment of the
parasitic diseases.
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.
 Cont…

 Therefore, Medical parasitology consists of:-

 Protozoa (single celled animals),

 Helminths (worms)


Arthropods
 • Phylum Sarcomastigophora
• Amoeba
Medical • Flagellates
Human Parasitology

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

• Class Insecta
Medical • Class Arachnida
• Class Crustacea
Arthropodology • Class Chilopoda
The importance of parasitology
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
Why were they selected?
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
1.3. Concepts related to medical parasitology

1.3.1. Symbiosis
Any association more or less permanent is called a
symbiosis, with each member a symbiont.
Two different organisms live together and interact,
 one partner lives in or on another one’s body .
3 types:
 Mutualism
 Commensalism
 Parasitism
Mutualism
Permanent association between two different
organisms that life apart is impossible,
Two partners benefit each other,
The mutual are metabolically dependent on one
another;
One cannot survive in the absence of the other.
Commensalism
Association of two different organisms
One partner is benefited while the other neither
benefited nor injured, such as E. Coli and man.
Parasitism
Association of two different organisms
One partner is benefited while the other is injured,
such as ascaris lumbricoides and man.
1.3.1. Parasite and types of parasites

Parasite:-
In parasitism, parasite is the
benefited partner.
It is an animal organism which lives
in or on the host in order to obtain
nourishment and shelter from the host
as well as does harms to the host.
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.
Types 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
 Tapeworms, flukes, protozoans

Ectoparasite
 Stays on outside surface of the host
 leeches, ticks, fleas, brood parasites
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.
-
III. Amount of time spent
Permanent Parasite
Lives entire adult life stage on or in a host
Usually endoparasites
 One exception is eyelash mite
Temporary Parasite
Spends only a short time on a host
Usually ectoparasites
III. According to their Pathogenicity:
Pathogenic parasites
Non-Pathogenic (commensal)
Opportunistic parasites
IV. Based on their life cycle
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
V. Based on host ranges
Euryxenous parasites:
Those with a broad host range.
Stenoxenous parasites:
Those with a narrow host range;
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.
1.3.3. Hosts and types of hosts
Host:-Hosts are organism which harbors the
parasite.
Inparasitism, it is the injured
partner
Types of Hosts: -
Definitive host:-
Intermediate host:-
Definitive host:-
What characterizes the primary 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
 Intermediate host:-
 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,
Other terminology
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.
Types of Hosts
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
1.3.4. Host specificity
The number of species the 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
1.3.5. 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:-
 characterized by the development of the
parasite before its transfer to another host
 Propagative.
 E.g. Yersinia pestis in fleas

 Cyclopropagative
 E.g. Plasmodium vivax in

Anopheles mosquitoes.
  Cyclodevelopmental

 E.g.Onchocerca volvulus in black flies.

2. Mechanical vector

no parasitic development of reproduction
occurs
1.3.6. Other terminologies

*Infective Stage : it is a stage when a parasite can

invade human body and continue to live there. The
infective stage of ascarid 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 ascarid 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.
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 ascarid, 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.
Alternation of Generation:
 In life cycles of some parasites, there is the
regular alternation of sexual and asexual
reproductions, eg.Plasmodium vivax
*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
1.4. 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

 Does it have a nidus?

 A small ecosystem that possesses all the factors to

maintain the parasite..

1.4.1. Geographic Distribution
Global 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 helminths: 2 billion 200,000

 Roundworm (Ascaris) Pnemonitis, intestinal obstruction

 Whipworm (Trichuris) Bloody diarrhoea, rectal prolapse


 Hookworm (Ancylostoma Coughing, wheezing, abdominal pain
and Necator) and anaemia

Schistosoma Renal tract and intestinal disease 200 million 15,000

Filariae Lymphatic filariasis and elephantiasis 120 million Not fatal but 40 million
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 (Endemicity):

1. Presence of a suitable host

2. Habits of the host
3. Escape from the host
4. Favorable conditions outside of host
5. Economic and social conditions
1.4.2. Transmission of parasites

Factors required:(Three key links of

parasitic disease transmission)
1. Source of infection
2. Mode of transmission
3. Susceptible people
1.4.2.1. source of exposure
1. primary Source
infected persons
carriers
animals
1. Sources of Exposure to Parasitic
Infections
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.
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 flyb

E. Animals (a domestic or wild animals

harboring the parasite),
 e.g, 1. Dogs- the hydatid cyst caused
by E. granulosus
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
1.4.2.2. Mode of Transmission

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
II. Vertical Direct Mode of Transmission:
Transmission of the parasite is from the
mother to child through:
Congenital / transplacental
Transmammary (breast milk
 II- 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. 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
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.
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
 1.5 General Life Cycles of parasites
 Describes the cycle of development of the parasite,
 This may involve
 Passing through a number of
developmental stages & enviroment
 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
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,
and where it reproduces
 Transmitted from one host to another through the air, by a
fomite, or in contaminated food or water.
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 & Evt.
1.5.3. Why study life cycles?

Control.
Treatment.
Epidemiology.
Fundamental research.
 1.6 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
 1.6.1. 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
1.6.2. 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..
1.6.3. How do Parasites Cause Inquiry 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
Tissue changes
- 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
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
1.7.Host Immunity & Immune – evasion of the
parasite
1.7.1. Host Responses
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.
Host Responses
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)
1.7.2. 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
Taxonomy and nomenclature of parasites
Taxonomy
 Taxonomic classification of helminths
Sub kingdom Phylum Class Genus – examples

Metazoa Nematodes Ascaris (roundworm)

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

Platyhelminthes Cestodes Taenia (tapeworm)

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

Trematodes Fasciolopsis (liver fluke)

Non-segmented, usually leaf- Schistosoma (not leaf
shaped!)
shaped, with two suckers but
no distinct head
Taxonomic classification of protozoa
Sub kingdom Phylum Sub-phylum Genus- examples Species- examples

Entamoeba E. histolytica
Protozoa Sarcomastig Sarcodina-- -
move by
-ophora 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

Ciliophora Balantidium B. coli

move by cillia

Microspora Enterocyto- E. bienusi

Spore-forming zoa
Nomenclature of parasites

Common name vs scientific name

Parasites named by binomial nomenclature

Genus (capitalized)

Species (not capitalized)

Binimial name underlined or separately italicized

Example: Ascaris lumbricoides, Ascaris lumbricoides

Thank you so
much