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.
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, 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 (permenant) : 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.
Coll. Medicine \ third stage
Parasitology
.Prof. Dr. Amal KH. KH
8.
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 .
10. proliferous parasite : Proliferous parasites are those that proliferate in
the human body so that the parasite originally introduced multiplies many
fold to cause high intensity of infection. Protozoan parasites are
proliferous.
11.
nonproliferous parasite : like the most adult helminths do not multiply
in the human body. They are nonproliferous. High intensity of infection
results from repeated infection as in roundworm, or from high
multiplicity of initial infection as in trichinosis. A few helminths, such as
Strongyloides stercoralis and Hymenolepis nana multiply in the human
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).
Stages involve in the life cycle of parasite :
- Infective stage
:
is a stage when a parasite can invade human body and live in it.
- Diagnostic stage : is a stage when a parasite can exit from human body with
stool , urine or sputum.
Sources of Infection:
Parasitic infections originate from various sources and are transmitted by various
routes. The major sources of infection are listed below:
1- Soil:
a. Embryonated eggs which are present in soil may be ingested, e.g.
roundworm, whipworm.
b. Infective larvae present in soil may enter by penetrating exposed skin,
e.g. hookworm, strongyloides.
2- Water:
a. Infective forms present in water may be swallowed, e.g. cysts of
amoeba and giardia.
b. Water containing the intermediate host may be swallowed, e.g.
infection with guinea worm occurs when the water that is drunk
contains its intermediate host
c. cyclops.
d. Infective larvae in water may enter by penetrating exposed skin, e.g.
cercariae of schistosomes.
e. Free-living parasites in water may enter through vulnerable sites, e.g.
N a e g l e r i a m a y e n t e r t h r o u g h n a s o p h a r y n x a n d c a u s e
meningoencephalitis.
3- Food:
a. Contamination with human or animal feces, e.g. amoebic cysts.
pinworm eggs,
b. echinococcus eggs. toxoplasma oocysts.
c. Meat containing infective larvae, e.g. measly pork. Trichinella spiralis.
4- Insect Vectors:
1. Biological vectors
a. Mosquito—malaria, filariasis
b. Sandflies—kala-azar
c. Tsetseflies—sleeping sickness
d. Reduviid bugs—Chagas’ disease
e. Ticks—Babesiosis.
2. Mechanical vectors
a. Housefly—amoebiasis.
5- Animals:
1. Domestic
a. Cow, e.g. beef tapeworm, sarcocystis.
b. Pig, e.g. pork tapeworm, Trichinella spiralis
c. Dog, e.g. hydatid disease, leishmaniasis
d. Cat, e.g. toxoplasmosis, opisthorchis.
2. Wild
a. Wild game animals, e.g. trypanosomiasis.
b. Wild felines, e.g. Paragonimus westermani
3. Fish, e.g. fish tapeworm
4. Molluscs, e.g. liver flukes
5. Copepods, e.g. guinea worm.
6- Other Persons:
- Carriers and patients, e.g. all anthroponotic infections, vertical transmission of
congenital infections.
- Self (autoinfection)
a. Finger to mouth transmission, e.g. pinworm.
b. Internal reinfection, e.g. strongyloides.
Modes of Infection
The major modes of transmission are the following:
1. Oral Transmission :
The most common method of transmission is oral, through contaminated food, water,
soiled fingers or fomites. Many intestinal parasites enter the body in this manner, the
infective stages being cysts, embryonated eggs or larval forms. Infection with
Entamoeba histolytica and other intestinal protozoa occurs when the infective cysts
are swallowed. In most intestinal nematodes, such as the roundworm. Whipworm or
pinworm, the embryonated egg which is the infective form is swallowed. In
trichinellosis and in beef, pork and fish tapeworm, infection occurs by ingestion of
flesh containing the mature larval stages. Infection with the tissue nematode guinea
worm follows consumption of water containing its arthropod host cyclops carrying
infective larvae.
2. Skin Transmission
Entry through skin is another important mode of transmission. Hookworm infection
is acquired when the larvae enter the skin of persons walking barefooted on
contaminated soil. Schistosomiasis is acquired when the cercarial larvae in water
penetrate the skin. Many parasitic diseases, including malaria and filariasis are
transmitted by blood sucking arthropods. Arthropods which transmit infection are
called vectors.
3. Vector Transmission:
Parasites undergo development or multiplication in the body of true vectors, which
are called biological vectors. Some arthropods may transmit infective parasites
mechanically or passively without the parasites multiplying or undergoing
development in them. For example, the housefly may passively carry amoebic cysts
from faeces to food. Such vectors which act only as passive transmitters are called
mechanical vectors. In the case of a mechanical vector there need be no delay
between picking up a parasite and transferring it to a host. A housefly picking up
amoebic cysts from feces can within seconds transfer the cysts by landing on food
being eaten by a person, who may thereby get infected. But in the case of biological
vectors. A certain period has to elapse after the parasite enters the vector before it
becomes infective. This is necessary because the vector can transmit the infection
only after the parasite multiplies to a certain level or undergoes a developmental
process in its body. This interval between the entry of the parasite into the vector
arthropod and the time it becomes capable of transmitting the infection is called the
extrinsic incubation period. For example, an Anopheles mosquito picking up
Plasmodium vivax gametocytes from a person in its blood meal becomes capable of
transmitting the infective stage of the malaria parasite only some ten days later, i.e.
the extrinsic incubation period is ten days.
4. Direct Transmission :
Parasitic infection may be transmitted by person-to-person contact in some cases by :
a- kissing in the case of gingival amoebae.
b- sexual intercourse in trichomoniasis.
c- Inhalation of air-borne eggs may be one of the methods of transmission of
pinworm infection.
d- Congenital infection (vertical transmission) may take place in malaria and
toxoplasmosis.
e- Iatrogenic infection may occur as in transfusion malaria and toxoplasmosis after
organ transplantation.
Pathogenesis :
Some parasites may lead to completely asymptomatic infection even though they
live inside tissues. Many persons with filarial infection may not develop any clinical
illness though microfilariae are demonstrable in their blood. Clinical infection
produced by parasites may take many forms—acute, subacute, chronic, latent or
recurrent. Some of the pathogenic mechanisms in parasitic infections are as follows:
1. Intracellular protozoa can damage and destroy the cells in which they
multiply.Malarial parasites rupture the infected erythrocytes causing
anaemia as a long-term effect and fever as the immediate response.
2. Enzymes produced by some parasites can induce lytic necrosis. E.
histolytica lyses intestinal cells, enabling it to penetrate the gut wall
and produce abscesses and ulcers.
3. Damage may be due to physical obstruction. Masses of roundworms
cause intestinal obstruction. Even a single worm can cause damage
when it blocks the appendix or bile duct.
4. damage by pressure.Hydatid cysts cause illness due to pressure on
surrounding tissues. Parasites in vulnerable sites such as brain and eyes
may produce serious damage by pressure.
5. Physical obstruction may sometimes cause severe secondary effects.
Falciparum malaria may produce blockage of brain capillaries leading
to fatal cerebral malaria.
6. Clinical disease may sometimes be due to trauma inflicted by parasites.
7. Hookworms feeding on jejunal mucosa leave numerous bleeding
points which ultimately lead to anaemia.
8. Schistosome eggs with their hooks tear vesical blood vessels and
produce haematuria.
9. Roundworms may perforate the intestine and cause peritonitis.
10. Clinical illness may be caused by host response to parasitic infection.
This may be due to inflammatory changes and consequent fibrosis, as
in the case of filariasis in which it leads ultimately to lymphatic
obstruction and oedema.
11. Host response may also be hypersensitive or allergic. Fatal
anaphylactic shock may occasionally be caused by escape of hydatid
fluid from the cyst.
12. A few parasitic infections have been shown to lead to malignancy. The
liver flukes Clonorchis may induce bile duct carcinoma and
Schistosoma haematobium may pave the way for bladder cancer.
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 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:
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 are hair- like organelles that cover the surface
of the cell .
4. Sporozoa.there are not mean of movement .
Beeeeeeeeeeeeeeest Wishes ………….. Prof. Dr. Amal KH. Khalaf