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MEDICAL PARASITOLOGY
Bushehr University of Medical Sciences
Department: Microbiology and Parasitology
Module: Medical Parasitology (Introduction)
Instructor: Mohammad Rayani, Ph.D
Protozoa
Cyst Trophozoite
Helminths
Egg Larvae Worm
2
Nematodes
Cestodes
Trematodes 3
Introduction to parasitology
Parasitology is a type of SYMBIOSIS (living together)
(Any plant, animal, or protist that is intimately
associated with another organism of a different species;
each member is termed a SYMBIONT).
Commensalism, Mutualism and Parasitism are
various type of symbiosis.
Dr. M. Rayani (Bushehr University of Medical Sciences) 4
COMMENSALISM
(when one symbiont, the COMMENSAL, benefits and the
other animal is neither helped nor harmed.
An association in which the commensal takes the benefit
without causing injury to the host.
E.g. most of the normal floras of the humans’ body can be
considered as commensals.
Dr. M. Rayani (Bushehr University of Medical Sciences) 5
(each member, a MUTUALIST, depends upon the other;
oblilgatory or facultative).
An association in which both partners are metabolically
dependent upon each other and one cannot live without the
help of the other; however, none of the partners suffers any
harm from the association.
For instance, flagellates produce cellulase in gut of
termites; ciliates in ruminants.
MUTUALISM
Dr. M. Rayani (Bushehr University of Medical Sciences) 6
PARASITISM
(where one member, the PARASITE, lives in or on another
organism, the HOST, at the expense of that organism).
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.
An association where one of the partners is harmed and the
other lives at the expense of the other.
E.g. worms like Ascaris lumbricoides reside in the
gastrointestinal tract of man, and feed on important items
of intestinal food causing various illnesses.
Dr. M. Rayani (Bushehr University of Medical Sciences) 7
Parasitology The study of the relationship between a
parasite and its host.
Medical parasitology is the science that deals with
organisms living in the human body (the host) and the
medical significance of this host-parasite relationship.
In medical parasitology we will focus on most of the disease
causing (pathogenic) parasites.
The parasites included in medical parasitology are
protozoa, helminths, and some arthropods.
Dr. M. Rayani (Bushehr University of Medical Sciences) 8
(Parasites) :ه انگل
ف انواع كه ائ زنده موجودات داشته مختي درجه نظر از م و بوده پست تك
در مين به ق ت ت ج نيستند (حرارت درجه و ،رطوبت غذا،مسكن) خود زيستي احتين)ديگر موجودات به وابسته منظور اين براي و و هستند (ميزب
هی نند صدمه آن به گ .ميرس
Dr. M. Rayani (Bushehr University of Medical Sciences) 9
انواع انگل ه خته ه (Protozoa)تک ی
ک ه( (Helminths
د ي گ (Nematoda) ك هن ي پ (Platyhelminths) ك ه
ي نواري شكل (Cestoda) ك ه گي شكل ي ب (Trematoda) ك ه
ی دپ (Arthropoda) ب
Dr. M. Rayani (Bushehr University of Medical Sciences) 10
DIFFERENT KINDS OF PARASITES
Ectoparasite – a parasitic organism that lives on the outer surface of its
host, e.g. lice, ticks, mites etc. Appropriate terminology includes the terms
"infected" and "infested").
Endoparasites – parasites that live inside the body of their host, e.g.
Entamoeba histolytica. appropriate terminology is "infected;
roundworms in gut.
Obligate Parasite - This parasite is completely dependent on the host
during a segment or all of its life cycle, e.g. Plasmodium spp.
Facultative parasite – an organism that exhibits both parasitic and non-
parasitic modes of living and hence does not absolutely depend on the
parasitic way of life, but is capable of adapting to it if placed on a host;
e.g. Naegleria fowleri.
Dr. M. Rayani (Bushehr University of Medical Sciences) 11
DIFFERENT KINDS OF HOSTS
Definitive or final host – a host that harbors a parasite in the adult stage
or where the parasite undergoes a sexual method of reproduction.
(parasite reaches sexual maturity and reproduces).
Intermediate host - harbors the larval stages of the parasite or an asexual
cycle of development takes place. (some development in host, but does not
reach sexual maturity; often asexual stages).
Reservoir host – a host that makes the parasite available for the
transmission to another host and is usually not affected by the infection.
(non-human animals that serve as sources of infection to humans).
Dr. M. Rayani (Bushehr University of Medical Sciences) 12
EFFECT OF PARASITES ON THE HOST
The damage which pathogenic parasites produce in the tissues of the
host may be described in the following two ways;
Mechanical injury - may be inflicted by a parasite by means of pressure
as it grows larger, e.g. Hydatid cyst causes blockage of ducts such as
blood vessels producing infraction.
Deleterious effect of toxic substances- in Plasmodium falciparum
production of toxic substances may cause rigors and other symptoms.
Deprivation of nutrients, fluids and metabolites -parasite may produce
disease by competing with the host for nutrients.
Immunological reaction: Tissue damage may be caused by
immunological response of the host, e.g. nephritic syndrome following
Plasmodium infections.
Dr. M. Rayani (Bushehr University of Medical Sciences) 13
EFFECT OF PARASITES ON THE HOST
Invasion by some parasites can also cause tissue damage in man, e.g.
fibrosis of liver after deposition of the ova of Schistosoma.
Ulceration of intestinal wall and liver by cysteine proteases of
Entamoeba histolytica;
Ulceration due to insertion of hooks, spines, etc. into intestinal wall
Eosinophilia; occurs with helminths, results from tissue migration
Anaphylaxis of excretory products of some trematodes and cestodes
Fibrosis and inflammation around schistosome eggs
Blood loss, hookworms and anaemia
Nutritional diversion; giardiasis results in diarrhea and
malabsorption; Diphyllobothrium absorbs vitamin B12.
Dr. M. Rayani (Bushehr University of Medical Sciences) 14
Mechanisms for evading the host response
Parasites often have evolved methods of evading host
immune system:
Antigenic variation of trypanosomes
Intracellular habitat of coccidia and Trichinella larvae
Camouflage schistosomes
Suppression eosinophil or neutrophil migration to the site of
the parasite
Encystment, encystation amoebae
Ability to cleave antibodies
Dr. M. Rayani (Bushehr University of Medical Sciences) 15
Some Terminology
Epidemic (disease that affects a large number of humans and spreads
rapidly)
Heteroxenous (alternation of generations of a parasite; altrnative host life cycle)
Infection (parasitic invasion resulting in injury and reaction to injury)
Monoxenous (single host life cycle)
Parasitemia (parasites in blood)
Pathogenic (results in disease or morbid symptoms)
Prevalence (number of organisms in a population infected with a parasite
at any one time)
Virulence (relative infectiousness of a parasite)
Zoonosis (animal diseases that may be transmitted from animals to
humans
Dr. M. Rayani (Bushehr University of Medical Sciences) 16
Typical ways parasites transmitted:
INGESTION from food or water / inhalation
DIRECT PENETRATION of skin from environment
VECTORS (transmits parasites from host to host)
BIOLOGICAL
(essential in life-cycle of parasite)
MECHANICAL
(unessential in life-cycle of parasite(
Dr. M. Rayani (Bushehr University of Medical Sciences) 17
BASIC CONCEPTS IN MEDICAL PARASITOLOGY
In medical parasitology, each of the medically important
parasites are discussed under the standard subheadings of
morphology, life cycle, means of infection, pathology, clinical
manifestations of infection, laboratory diagnosis, treatment,
geographical distribution, preventive/control measures of
parasites.
Dr. M. Rayani (Bushehr University of Medical Sciences) 18
MORPHOLOGY
Includes size, shape, color and position of different
organelles in different parasites at various stages of their
development.
This is especially important in laboratory diagnosis which
helps to identify the different stages of development and
differentiate between pathogenic and commensal
organisms. For example, Entamoeba histolytica and
Entamoeba coli.
Dr. M. Rayani (Bushehr University of Medical Sciences) 19
LIFE CYCLE of PARASITES
The route followed by a parasite from the time of entry to the host to
exit, including outside the host life.
It can either be simple, when only one host is involved, or complex,
involving one or more intermediate hosts.
A parasite’s life cycle consists of two common phases:
One phase involves the route a parasite follows inside the body. This
information provides an understanding of the symptomatology and
pathology of the parasite. In addition the method of diagnosis and
selection of appropriate medication may also be determined.
The other phase, the route a parasite follows outside of the body,
provides crucial information pertinent to epidemiology, prevention, and
control.
Dr. M. Rayani (Bushehr University of Medical Sciences) 20
LABORATORY DIAGNOSIS
Depending on the nature of the parasitic infections, the
following specimens are selected for laboratory diagnosis:
a) Blood – 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.
in malaria the parasites are found inside the red blood cells.
in Bancroftian and filariasis, microfilariae are found in the
blood plasma.
Dr. M. Rayani (Bushehr University of Medical Sciences) 21
B) Stool – examination of the stool forms an important part
in the diagnosis of intestinal parasitic infections; 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.
In the case of helmithic infections, the adult worms, their
eggs, or larvae are found in the stool.
LABORATORY DIAGNOSIS
Dr. M. Rayani (Bushehr University of Medical Sciences) 22
c) Urine – when the parasite localizes in the urinary tract, examination of
the urine will be of help in establishing the parasitological diagnosis.
in urinary Schistosomiasis, eggs of Schistosoma haematobium are found
in the urine.
d) Sputum – examination of the sputum is useful in the following:
In cases where the habitat of the parasite is in the respiratory tract, as in
Paragonimiasis, the eggs of Paragonimus westermani are found.
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.
spleen punctures in cases of kala-azar, muscle biopsy in cases of
Cysticercosis, Trichinelliasis, and Chagas’ disease,
f) Urethral or vaginal discharge – for Trichomonas vaginalis
LABORATORY DIAGNOSIS
Dr. M. Rayani (Bushehr University of Medical Sciences) 23
Indirect evidences; changes indicative of intestinal parasitic
a. Cytological changes in the blood
Eosiniphilia often gives an indication of tissue invasion by
helminthes, a reduction in white blood cell count is an
indication of kala-azar, and anemia is a feature of
hookworm infestation and malaria.
b. Serological tests – are carried out only in laboratories
where special antigens are available.
LABORATORY DIAGNOSIS
Dr. M. Rayani (Bushehr University of Medical Sciences) 24
Treatment
Many parasitic infections can be cured by specific
chemotherapy.
To obtain maximum parasiticidal effect, it is desirable that
the drugs administered should not be absorbed and the
drugs should also have minimum toxic effect on the host.
Dr. M. Rayani (Bushehr University of Medical Sciences) 25
GEOGRAPHICAL DISTRIBUTION
Although there is a much stronger association with the level of housing,
nutrition, sanitation and general public health than climate; many of
parasitic diseases are still found in abundance in the tropics.
Once we are clear about the geographical distribution, effective
preventive and control measures can more easily be devised and
implemented.
Distribution of parasites depends upon:
a. Environmental conditions- favoring survival outside the body of the
host, i.e. temperature, the presence of water, humidity etc.
Dr. M. Rayani (Bushehr University of Medical Sciences) 26
a. The presence of a suitable host and food habits
• Host specificity, for example, Ancylostoma duodenale requires man as
a host where Ancylostoma caninum requires a dog.
• Food habits, e.g. consumption of raw or undercooked meat or
vegetables predisposes to Taeniasis.
c. Easy escape of the parasite from the host- the different
developmental stages of a parasite which are released from the body
along with faeces and urine are widely distributed in many parts of the
world as compared to those parasites which require a vector.
d. The presence of an appropriate vector or intermediate host –
parasites that do not require an intermediate host (vector) for
transmission are more widely distributed than those that do require
vectors.
GEOGRAPHICAL DISTRIBUTION
Dr. M. Rayani (Bushehr University of Medical Sciences) 27
PREVENTION
Preventive measures designed against every parasite
infectiving humans to break the transmission cycle.
Such measures include:
Reduction of the source of infection-
A prompt diagnosis and treatment of parasitic diseases is
an important component in the prevention of
dissemination. (The parasite is attacked within the host, thereby
preventing the dissemination of the infecting agent).
Dr. M. Rayani (Bushehr University of Medical Sciences) 28
Sanitary control of drinking water and food.
Proper waste disposal – through establishing safe sewage
systems, use of screened latrines, and treatment of night
soil.
The use of insecticides and other chemicals used to control
the vector population.
Protective clothing that would prevent vectors from
resting in the surface of the body and inoculate pathogens
during their blood meal.
Good personal hygiene.
Avoidance of unprotected sexual practices.
PREVENTION
Dr. M. Rayani (Bushehr University of Medical Sciences) 29
NOMENCLATURE
Describing animal parasites follow certain rules of
zoological nomenclature and each phylum may be further
subdivided as follows:
Phylum Subphylum Class Order
Family Genus Species
Dr. M. Rayani (Bushehr University of Medical Sciences) 30
Medical Protozoology
Deals with the study of medically important protozoa.
Medical Helminthology
Deals with the study of helminths (worms) that affect
man.
Medical Entomology
Deals with the study of arthropods which cause or
transmit disease to man.
CLASSIFICATION OF MEDICAL PARASITOLOGY
Dr. M. Rayani (Bushehr University of Medical Sciences) 31
PROTOZOAN parasites consist of a single "cell-like unit"
which is morphologically and functionally complete and
can perform all functions of life.
They are made up of a mass of protoplasm differentiated
into cytoplasm and nucleoplasm.
The protozoal parasite possesses the property of being
transformed from an active (trophozoite) to an inactive
stage (cyst). The cyst is the resistant stage of the parasite
and is also infective to the human host.
The HELIMINTHIC parasites are multicellular and well
developed organ systems.
32 Dr. M. Rayani (Bushehr University of Medical Sciences)
SUMMARY
A parasite is an organism which lives in/on the body of a
host.
A host is that which harbors the parasite.
There is usually some association such as mutualism,
commensalisms, or parasitism between the parasite and the
host.
This association may produce a variety of effects and the
host usually tends to react to it.
Dr. M. Rayani (Bushehr University of Medical Sciences) 33
Symbiosis: “Any two organisms living in close association, commonly one living in or on the body of the other, are symbiotic, as contrasted with free living.”
Commensalism: One partner benefits but the other is not hurt.
Mutualism: Both partners benefit.
Parasitism: One partner (the parasite) harms or lives on the expense of the other (host).
SUMMARY
Dr. M. Rayani (Bushehr University of Medical Sciences) 34
Medical Parasitology focuses on parasites which cause disease in humans.
Parasitic diseases are in their majority the diseases of the poor around the globe. Poverty is a major risk factor for disease but disease is also a major contributor to poverty.
In general, the protozoa, helminthes and arthropods are the most commonly studied and the most important parasites in medical parasitology.
SUMMARY
Dr. M. Rayani (Bushehr University of Medical Sciences) 35
1) Brown & Neva. Basic Clinical Parasitology
2) Markell & Voge. Medical Parasitology
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http://dpd.cdc.gov/dpdx/Default.htm
http://www.who.int/tdr/diseases-topics/en/
References:
Dr. M. Rayani (Bushehr University of Medical Sciences) 36