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Lec.1
Medical Parasitology
Introduction :
Human parasitology is the study of those organisms which parasitize humans . Human
parasitology, an important part of parasitology, study the medical parasites including their
morphology , life cycle, the relationship with host and environment. So it is considered that
parasitic infection /or parasitic diseases are still one of the important problems in public health in
our country. As a candidate for doctor, to learn some knowledge of parasitology is necessary.
Parasitism , Parasite ,and host :
Parasitism is a relationship between two species where member of one species (smaller ,called
parasite ) derived food and shelter from member of another species (larger ,called host ) .Host
and parasite share a dynamic relationship. Generally , parasites cannot exist independently . they
inflict injury , affecting the well-being of the host . The host on its tries to curtail the damage and
eliminate the parasite .
Types of Parasite :
A parasite is an organism which has become adapted to living on or in some other organism to
get nutrition , multiplication and protection, the host, which is usually larger than itself. or
parasite is an organism that lives at the expense of its host.
1- Microparasites : is unicellular – single cell parasites like protozoa .
2- Macroparasites :. Is multicellular parasites like helminthes .
3- endoparasite :a parasite that lives within the body of the host such as Malarial parasite
(Plamsmodium sp ) .
4- ectoparasite : a parasite that lives on the external surface of the host such as Sarcoptes Scabiei
(the mite producing scabies ) .endoparasite infect the host while ectoparasite infest the host .
parasitic infection and infestation leading to signs and symptoms are called parasitic disease .
5- Obligate parasite (permanent) : cannot exist without their host such as malaria parasite .
6- facultative parasite : can exist independently or that the organisms does not of necessity have
to be a parasite at any stage of its existence , if opportunity presents itself , may become
parasitic . such as Naegleria fowleri ..
7- Intracellular parasites (mostly microscopic) : parasite that live and multiply inside the cell
of host like amastigote stage of leishmania spp.
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Extracellular parasites (they range in size from micro- to macroscopic) : parasite that live
and multiply out of the cell of host like hydatid cyst , Ascaris lambricoides .
9- accidental parasite : a parasite that infect unusual host such as Echinococcus granulosus
which infect human being accidentally and producing hydatid diseases .
Coll.
Medicine
\3
rd
stage
Parasitology
Assist. Prof. Dr. Amal KH. KH.
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Host :
Host : Host is defined as an organism which harbors the parasite and provides the
nourishment and shelter. These hosts, in comparison to their parasites are relatively larger in size.
The hosts may be of the following types: definitive host, intermediate host, reservoir host and
paratenic host etc.
1- Definitive host : The hosts which harbor the adult parasites, most highly developed form
of the parasite or where the parasite replicates sexually.
2- Intermediate host : The hosts which harbor the larval stages of parasite development or
the asexual forms of the parasite are called intermediate host. Some times two different
hosts may be required to complete different larval stages. These are known as the first and
second intermediate hosts respectively.
3- Reservoir host: The animal which harbors the parasites and serves as an important source
of infection to other susceptible hosts are known as reservoir host.
5- Vectors : are arthropods that transmit parasite to hosts . vectors can be either biological or
mechanical . parasite multiply in the biological vectors , while they do not multiply in
mechanical vectors . for example , tsetse fly is the biological vectors of Trypanosoma bruci
. mechanical vector are arthropods that mechanically transfer infective forms of the parasite
from one host to another .house flies act as mechanical vector that transfer the cyst of
Entamoeba histolytica from infective faeces to contaminate food , human acquired this
parasite by consuming such contaminated food.
6- Accidental host – a host that is under normal circumstances not infected with the parasite.
Host selection :
Parasites may possess single or a wide range of hosts in their life cycle. They may be:
a. Monoxenous i.e if they possess a single host .
b. Oligoxenous i.e if they utilize a small range of host .
c. Polyxenous, i.e if they use many suitable hosts.
Life cycle :
Life cycle is the process of a parasite’s growth, development and reproduction, which proceeds
in one or more different hosts depending on the species of parasites . The life cycle of a parasite
may be :
1-simple life cycle (direct life cycle ): In a simple life cycle all the developmental stage of
the parasite are completed in a single host such as man .
2- complex life cycle (indirect life cycle ) : in which the parasites require two different hosts
or more to complete their various stage of development (e.g., Schistosoma japonicum
etc). In a complex life cycle many parasites require two different hosts, one definitive
host and one intermediate host to complete their life cycle(e.g., Schistosoma species
require man as definitive host and snail as intermediate hosts).
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Stages involve in the life cycle of parasite :
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Infective stage
:
is a stage when a parasite can invade human body and live in it.
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Diagnostic stage : is a stage when a parasite can exit from human body with stool
, urine or sputum.
Transmission of parasites :
Transmission of infection from one host to another, cause by a certain form of the parasite is
known as the infective stage. The infective stage of various parasites many be transmitted from
one host to another in the following ways:
1) Oral route : The infection is transmitted orally by ingestion of food, water or vegetables
contaminated by the faeces that contain the infective stages of the parasite. This mode of
transmission is referred to as faecal-oral route(e.g., cysts of Giardia and Entamoeba
histolytica； ova of Ascaris lumbricoides. Ingestion of raw or undercooked meat: The
infection is transmitted orally also by ingestion of raw or undercooked meat harboring the
infective stage of the parasite(e.g., pork containing cysticercus cellulosae the larval stage of
Taenia solium ). Ingestion of raw or under cooked water plants: Infection can be
transmitted by eating raw or under cooked water plants harboring the infective form of the
parasite(e.g., water chest nuts, etc., containing metacercariae of Fasciolopsis buski and
Fasciola hepatica).
2) Penetration of the skin and mucous membrane :The infection is transmitted by:
A) Penetration of the intact skin by filariform larvae of hookworm,
B) Piercing the skin by cercariae of Schistosoma japonicum, S. mansoni and S. haematobium
on coming in contact with infected water.
3) Inoculation by anarthropod vector : The infection also can be transmitted by
A) Inoculation into the blood by Anopheles(vector forPlasmodium).
B) Inoculation into the skin by mosquitoes(vectors for Wuchereria bancrofti, Brugia malayi
etc).
4) Sexual contact : Trichomonas vaginalis is transmitted by sexual contact. Frequently,
Entamoeba also is transmitted by sexual contact among homosexuals.
Laboratory diagnosis – depending on the nature of the parasitic infections:
1- morphological methods : the following specimens are selected for laboratory diagnosis:
a) Blood sample – in those parasitic infections where the parasite itself in any stage of its
development circulates in the blood stream, examination of blood film forms one of the
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main procedures for specific diagnosis. For example, in malaria the parasites are found
inside the red blood cells.
b) Stool sample– examination of the stool forms an important part in the diagnosis of
intestinal parasitic infections and also for those helminthic parasites that localize in the
biliary tract and discharge their eggs into the intestine. In protozoan infections, either
trophozoites or cystic forms may be detected; the former during the active phase and the
latter during the chronic phase. Example, Amoebiasis, Giardiasis, etc.
c) Urine sample – when the parasite localizes in the urinary tract, examination of the urine
will be of help in establishing the parasitological diagnosis. For example in urinary
Schistosomiasis, eggs of Schistosoma haematobium are found in the urine.
d) Sputum sample – examination of the sputum is useful in the following:
1. In cases where the habitat of the parasite is in the respiratory tract, as in the eggs of
Paragonimus westermani are found.
2. In amoebic abscess of lung or in the case of amoebic liver abscess bursting into the
lungs, the trophozoites of E. histolytica are detected in the sputum.
e) Biopsy material - varies with different parasitic infections. For example spleen punctures
in cases of kala-azar, muscle biopsy in cases of Cysticercosis, Trichinelliasis, and Chagas’
disease, Skin snip for Onchocerciasis.
f) Urethral or vaginal discharge – for Trichomonas vaginalis.
g) Skin scraping : for leishmania tropica , Scabeis .
2- Immunodiagnostic methods : this is rely on demonstrating either specific parasitic
antigen or antibody against parasite . when tests that demonstrate antibodies against
parasitic antigen are used , it used to remember that IgG antibody persist for a long in the
serum of person after infection .
3- Polymerase chain reaction (PCR) : it is newer method and refer to amplification specific
gene of parasite DNA for million copies in a short time , it is different from each above
methods because its ability to detect each of viable and non viable organism as well as
require one cell for diagnosis at least . PCR also used to recognize between parasite
species such as Entameoba histolytica and Entameoba dispare .
Classification of medical Parasitology :
Parasites of medical importance come under the kingdom called protista and animalia.
Protista includes the microscopic single-celled eukaroytes known as protozoa. In contrast,
helminthes are macroscopic, multicellular worms possessing well differentiated tissues and
complex organs belonging to the kingdom animalia. Each parasite of these group have scientific
name based on the binomial nomenclature system in which the name of parasite consist of two -
part scientific name , first part is the generic name and the second part is the name of species . the
generic name begins with capital letter while the name of species begins with small letter . for
example , the Latinized name of amoeba infecting human body is Entamoeba histolytica . in the
case of Entamoeba is the generic name and histolytica is the species name . Medical Parasitology
is generally classified into:
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a. Medical Protozoology - Deals with the study of medically important protozoa.
b. Medical Helminthology - Deals with the study of helminthes (worms) that affect man.
c. Medical Entomology - Deals with the study of arthropods which cause or transmit disease
to man.
Chapter One : Medical Protozoology
Introduction to protozoa :
1- Protozoan parasites consist of a single "cell-like unit" which is morphologically and
functionally complete and can perform all functions of life.
2- They are made up of a mass of protoplasm differentiated into cytoplasm and nucleoplasm.
The cytoplasm consists of an outer layer of hyaline ectoplasm and an inner voluminous
granular endoplasm. The ectoplasm functions in protection, locomotion, and ingestion of
food, excretion, and respiration. In the cytoplasm there are different vacuoles responsible for
storage of food, digestion and excretion of waste products. The nucleus also functions in
reproduction and maintaining life.
Reproduction – the methods of reproduction or multiplication among the parasitic protozoa are of
the following types:
1. Asexual multiplication:
(b) Simple binary fission – in this process, after division of all the structures, the individual
parasite divides either longitudinally or transversely into two more or less equal parts.
(c) Multiple fission or schizogony – in this process more than two individuals are produced,
e.g. asexual reproduction in Plasmodia.
2. Sexual reproduction:
(a) Conjugation – in this process, a temporary union of two individuals occurs during which
time interchange of nuclear material takes place. Later on, the two individuals separate.
(b) Syngamy – in this process, sexually differentiated cells, called gametes, unite
permanently and a complete fusion of the nuclear material takes place. The resulting
product is then known as a zygote.
Medical classes of protozoa:
Four classes of protozoa are medically important which are infects the human being and arranged in
groups based on the organelles of locomotion that are :
1- Lobosea (sarcodina ) : movement by pseudopodia , pseudopodia are temporary projections
of the cytoplasm used for movement and ingestion of food.
2- Flagellata (zoomastigophora): flagella are long , delicate , thread like extensions of the
cytoplasm . depending on the species , the number of flagella varies from one to eight .
flagella are longer than cilia .
3- Ciliata : mean of movement by cilia cilia are hair- like organelles that cover the surface of the
cell .
4- Sporozoa.there are not mean of movement .
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Occurrence (location of protozoa ) :
1- class :lobosea ( sarcodina) : This class include the amoeba species of human which can be
divide into two group based on the location of infection :
Intestinal species
Entamoeba histolytica
Entamoeba dispar
Entamoeba hartmanni
Entamoeba coli
Endolimax nana
Iodamoeba butschlii
Extra - Intestinal species
Entamoeba gingivalis (mouth)
Naegleria fowleri (brain)
2- Flagellata (zoomastigophora) : which is include three basic group and classified according to
their location in the body of the host :
Intestinal species
- Giardia lamblia
- Trichomonas hominis
- Chilomastix mesnili
- Enteromonas hominis
- Retortamonas intestinalis
Oral and vaginal species .
- Trichomonas tenax
- Trichomonas vaginalis
Blood species
- Leishmania donovani
- Leishmania tropica
- Leishmania brazielinses
- Trypanosoma cruzi
- Trypanosoma gambiense
- Trypanosoma rhodesience
3- Class : Sporozoa: include three major group occupying various location of host body :
Blood species :
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- Plasmodium falciparium
- Plasmodium vivax
- Plasmodium ovale
Plasmodium malariae
tissue species
- Toxoplasma gondii
- Sarcocystis hominis
Intestinal species :
- Isospora belli
- Cryptosporidium parvum
4- Ciliata : only one species is lie under this class which cause infection to human , that is :
- Balantidium coli (intestine )