parasitology Introduction.ppt

OUTLINE
 Definition of Medical Parasitology
 Scope of Medical Parasitology
 Concepts related to Medical Parasitology
 Mode of transmission & life cycle of parasites
 Parasitic diseases
 Host immunity & immune - evasion of parasites
 Classification of Parasites

Learning Objectives
At the end of this chapter the student will be able to:
 Define common terms used in Medical Parasitology
 Explain host-parasite relationship
 Explain the general life cycles of parasites
 Explain briefly host immunity & immune – evasion
mechanisms by parasites
 Describe classification of medically important parasites

1. Introduction to Medical Parasitology
1.1. Definition
 Medical Parasitology (Greece: Para = beside, Sitos = food)
 parasitos "somebody who eats from another's table”
 Medical parasitology is a science that deals with organisms
living in the human body (the host) and the medical significance
of this host-parasite relationship.
 A parasite is a living organism, which takes its nourishment and
other needs from a host
 The host is an organism which supports the parasite.

Cont….
 It is a subject that researches:
 The biological features of medically important parasites,
 The relationship between human beings and parasites,
 The diagnosis, treatment, prevention, & control of the
parasitic diseases.

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 Parasitology
Medical Protozology (single
celled animals)
Medical helminthology
(worms)
Medical Arthropodology

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

1.2. Concepts related to Parasitology
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).
 The mutual's are metabolically dependent on one another
 One classic example is the relationship between certain species of
flagellated protozoa living in the gut of termites.
 The protozoa- acquire their nutrients from termites,
 In return they synthesize and secret cellulases; for the termites

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.

1.3. 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. e.g. Plasmodium spp.
 Facultative Parasite
 A parasite that may be parasitic upon another organism
but can exist independently. E.g. Naegleria fowleri

III. Pathogenicity:
 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.

IV. 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)

V. Other types of parasites
 Accidental parasite – when a parasite attacks an
unnatural host and survives. E.g Hymenolepis
diminuta (rat tapeworm).
 Erratic parasite - is one that wanders in to an organ
in which it is not usually found E.g. Entamoeba
histolytica in the liver or lung of humans.

1.4. 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

1.5. 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
II. Cyclodevelopmental - developmental change without
multiplication.
E.g.Onchocerca volvulus in black flies.

III. 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

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.

 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.

1.6.2. Transmission of parasites
Three key links for parasitic disease transmission:
1. Source of infection
2. Mode of transmission
3. Susceptible host- a person or animal lacking sufficient resistance to a
particular pathogenic agent to prevent disease if or when exposed
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: which 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

1.6.2.1. Modes of Transmission
1. Direct mode of Transmission: is immediate transfer
of infectious agents from an infected host or reservoir to
an appropriate portal of entry
Classified as:
I. Horizontal: Transmission is mainly effected through:
 Feco-oral route: most intestinal parasites
 Sexual intercourse
 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: is when the infectious agent is
conveyed by an arthropod or contaminated inanimate objects
 It occurs, if the parasite:
 Has complex life cycle,
 Requires biological vectors and/or
 One or more intermediate hosts

1.6.2.2. Route of Transmission
 is a path in which the parasite enters to the body of a
susceptible host
I. Alimentary tract - 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 and mucous membranes
 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
 Through Insect Bite,
E.g. Filarial worms, Trypanosoma spp., Plasmodium spp.

 Sexual Contact
E.g. Trichomonas vaginalis
 Kissing
E.g. Trichomonas gingivalis, T. tenax
III. Transmammary
E.g. S. stercoralis
IV. Respiratory tract- inhalation of contaminated air
E.g. E. vermicularis,
V. Transplacental
E.g. T. gondii

1.7. 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.

1.7.1. 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.

1.7.2. 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

1.7.3. Why do we study life cycles?
 A parasite’s life cycle consists of two common
phases
I. one phase involves the route a parasite follows
inside the body. This information provides an
understanding of the-
 symptomatology and pathology
 the method of diagnosis
 selection of appropriate medication

II. The other phase, the route a parasite follows
outside of the body, provides crucial info
pertinent to
 epidemiology,
 prevention, and
 control

Medical Parasitology
Medical Protozology
Phylum
Sarcomastigophora
Phylum Apicomplexa
Phylum Microsporidia
Phylum Ciliophora
Medical helminthology
Phylum Nematoda
Phylum Trematoda
Phylum Cestoda
Phylum
Methacanthocephala
Medical Arthropodology
Phylum Insecta
Phylum Crustacea
Phylum Arachnida
Phylum Chilopoda

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