Parasitology Dr_Mohammed OmarAkoo
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1
st
Lecture
Introduction
Parasitolagy: is a branch of science that deals with the parasites, their biology, and
pathogenic effect and control measures against them.
Parasites can be defined as an organism that lives at the expense of another, which is called
host, derives food and shelter from it and usually causing certain harm or injury to it.
There are associations between different organisms as commensalisms, symbiosis,
enquilines, mutualism, predation and saprophytism.
Commensalisms
(com = together, mensal = table). This means an association in which one
organism gains benefit while the other does not, as well as the fact there is no harm in this
association, e.g. the presence of Entamoeba coli in the colon of human beings.
Symbiosis
(sym = together, bios 3.life) this term means living together and each gains
benefit from this association. Both are so dependent upon each other that one cannot live
without the help of the other e.g. flagellates living in the gut of termites.The termites feed on
wood and the flagellates digest the cellulose in wood; thus both gain benefit.
Enquilines:
These are organisms living in the same habitat or nest of other species
causing harm or benefit to neither of them.
Mutualism:
it is an association between two organisms without which each can live alone
without causing any harm to other, any such as some protozoan colonies that live on the
outer shell of some snails or on the external covering of some crustaceans.
Predation:
The organism may obtain food at the expense of other ones, where larger animal
prey on smaller ones which they kill and eat. The attacker is the predator, and the victim is
the prey.
Saprophytism:
Is a process in which organisms feed on dead and decaying bodies of
animals, plants and other organic matter and help to decompose them.
Classification of parasites
The parasites are divided according to:
A-Habitat:
a) External parasites or ectoparasites are those living on the outer surface of the host they
either suck the blood and lymph or feed on the hairs, feathers and epidermis.
b) Internal parasites or endoparasites are those living inside the body of the host .They
may live in the digestive system, known as enteric parasites; in blood and–haemobiotic
system are haemoparasites and in the reproductive organs are called venereal parasites.
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B- Specialization:
a) Obligatory parasites: are those which are completely adapted to certain host and their
life cycle is completed in them, when such parasites occur in other hosts they never develop
to the adult stage. The obligatory parasites may be:
1-Permanent parasites:
those are associated with their hosts in most of their life span, e.g
Taenia spp., Ascaris …etc.
2-Temporary parasites
: are associated with their hosts for certain periods during which
time they obtain a blood meal, e.g. mosquitoes, bugs and blood sucking flies.
3- Incidental parasites:
are those which are occasionally found in other hosts rather than in
their normal ones e.g. Dipylidium caninum in man.
4. Erratic parasites:
are those which wander into the organs and tissues of the host and
reach unusual organs, i.e. they have lost their way and gone to abnormal places where they
may be unable to complete their life cycle e.g. the migration of Enterobius vermicularis
into the vagina.
5-Periodic or seasonal parasites:
are those found on or in the body of the host during a
certain period of the year e.g. Dermatobia hominis, Oestrus ovis, mosquitoes and others.
6-Specific parasites:
are those which have special hosts and live in certain parts of the body
e.g. Taenia saginata live in the intestines of man only and its larval stages Cysticercus bovis
are found only between muscle fibers of cattle.
B) Facultative parasites: are parasites which can live freely without the need of a host and
under certain conditions they enter the body of the host and become parasitic e.g.
Strongyloides spp and flies as Lucilia, Calliphora etc.
C) Accidental parasites: are those that are living a free life on the food of man or animals
and when they have access to the body of a host, they may either complete their
development or die e.g. the fruit fly, Drosophila.
Mode of transmission of parasites:
1. The transmission of the parasites differs according to their habitat in the body of the host:
A-Transmission of external parasites may be.
1-Direct contact e.g. mange parasites.
2-Indirectly during blood suction e.g. mosquitoes.
B-Transmission of internal parasites may be:
1-Through the mouth, ingestion of the infective stages with food or water e.g.
Entamoeba, Ascaris and others.
2-Through penetration of the skin or mucous membranes e.g. Schistosoma and
Ancylostoma.
3-Through blood sucking insects as Plasmodium spp. by mosquitoes.
4-Through inhalation of infective stages, e.g. Toxoplasma, Enterobius and others.
5-Through sexual organs during sexual contact e.g. Trichomonas vaginalis.
6-Congenital transmission, by the migration of the larval stages of the parasites through
the foetal circulation or membranes to the foetus e.g. Toxoplasma gondii.
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Hosts:
The host is the bigger organism which offers shelter and food to the smaller organism, and
suffers from the harm done by the parasite. Some parasites occur during their whole life cycle
in one host or in the several hosts. The hosts may be classified as:
1-Definitive or final host: is the host in which the parasite reaches sexual maturity i.e. the
hosts which harbors the adult parasites. The definitive hosts may be human or non human
living things.
2-Intermediate hosts: are those harboring the larval stages of the parasites. Some parasites
need more than one intermediate host during their life cycle and therefore divided into:
a-First intermediate host., in which the early larval stages occur.
b-Second intermediate host, in which the later larval stages, usually the infective stages
occur. An example to that is Heterophyes heterophyes which is a parasite of the small
intestine of man, dog and cat. It needs for the early stages of the life cycle, a snail Pirenella
conica which is the first intermediate host and another host is the fish of the genus Tilapia or
Mugil, the second intermediate host in which the infective stage (encysted metacercaria) is
found.
3-Carrier: It is the definitive host which carries the parasite without the appearance of any
pathological lesions and at the same time this host spreads the infection to others e.g.
Trichomonas vaginalis in male.
4-Reservoir host: Is an animal species on which the parasite depends for its survival in
nature. The host never suffers from the infection but it serves as a source of infection for other
susceptible host, including man e.g. Antelopes in Central Africa infection with Trypanosoma
brucei.
5-Vector: When the definitive host or intermediate host is an arthropod, it is described as
vector e.g. mosquitoes are the vectors of Plasmodium species. The vectors may transfer the
infection either mechanically or biologically.
Epidemiology:
It is the study of all factors that help in the induction and spread of the parasitic diseases
among hosts in certain areas or a country. This include the study of different methods of
diagnosis of diseases, statistical studies on the incidence and seasonal occurrence of the
infection as well as the study of the different factors that affect the life cycle of a parasite as
temperature, relative humidity, sunlight, heat, the alkalinity of the soil, and the intermediate
hosts. It also includes the study of cultivation and growing vegetables.
The epidemiological data collected are employed, for understanding how a host acquires a
parasitic disease and also provide a guide for the control of parasitic diseases.
Control of a disease is almost the result of the practical application of epidemiological data
which have previously accumulated with the efforts of research workers. The success of the
control depends mainly on the accuracy of the information collected when applied to the
problem, the simplicity of methods of application as well as the degree of intelligence and
cooperation of the population who will be engaged with such control measures e.g. if the
spread of the disease is discovered in definite environment such as swamps, or insects of
certain type or specific agriculture or environment, it would be possible to draw a plan
suitable for controlling that disease as the successful plan for controlling of malaria,
bilharziasis and ancylostomiasis.
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Parasitic and Clinical Periods:
The periods of relations between a parasite and its host during an infection are:
A. Periods concerning the parasite:
1- Pre-patent period: It begins from the time of infection with the infective stage till the
appearance of diagnostic stages such as eggs, cysts and others.
2- Patent period: It is the period during which the parasite could be diagnosed with
laboratory methods till the disappearance of the parasite.
B-Periods concerning the host:
1- Incubation period: Is the period which begins from the time of infection till the
appearance of clinical symptoms and it is usually longer than the prepatent period.
2- Period of symptoms: Is the period during which time a disease condition appears as
fever, diarrhea, vomiting …etc.
3- Period of convalescence: Begins from the time of clinical disappearance of symptoms
till the complete cure from the infection with the parasite.
4- Period of relapse: Is the period during which the clinical symptoms reappear again after
the convalescent period.
Laboratory diagnosis:
Laboratory diagnosis of parasitic diseases could be attained by examining the patients or
their excretions, blood and body exudates looking for the adult parasites or their larval or
cystic stages. It includes:
1-Direct method: To see the adult parasite or its developmental stages by:
a-Direct examination of the sample obtained from the individual either macroscopically or
microscopically.
b-Cultivation of samples on different media or in laboratory animals.
c-Xenodiagnosis: This method is used when the parasite is scanty, increasing its number
will help in the diagnosis by the use of the arthropod vector e.g. in Chagas disease, bugs are
allowed to suck blood from the patient and 2 weeks later they are examined for the presence
of the parasite.
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2-Indirect methods: This method is used to diagnose the presence of the parasite through
the effect produced on the biochemical and cytological components of the hosts tissues,
serum, blood, secretions and excretions. This may be divided into the following:
a-Blood changes: Red blood cells and white blood cells counts and differential counts of
leucocytes.
b-Body exudates and fluids: This includes:
1- Physical reactions e.g. colour of faeces and urine, surface tension of blood serum,
precipitation.
2- Chemical reactions: This includes the changes occurring in the components of the blood
serum, cerebrospinal fluids and body fluids.
3- Serological techniques e.g. agglutination and precipitation techniques.
4- Anaphylactic reaction.
5- Polymerase chain reaction (PCR).
Treatments:
The treatments of parasitic infections can be attained by solitary treatment or in groups by:
1-Symptomatically: By giving medicaments that decrease the pain produced by the parasite
or its toxins or bad effect such as colic or skin irritation without interfering with the parasite
itself.
2-Therapeutically: By giving suitable medicaments that kills or expels the parasite from the
body of the host. The drug used should be safe, should have no-ill effect or toxic action on
the general health of the host.
3-Surgically: Some parasites are able to produce abnormal growth tumours or abscesses or
grow in certain organs or tissues to large sizes where surgical treatment or interference may
be found necessary.
Control:
Control of parasitic disease includes two methods:
1-Control measures to protect the host from an invading parasite by:
A- Following all hygienic regulations such as protection the host from the insect vectors, use
of clean and well balanced diet and water, avoiding over crowding……etc and this can be
attained by public health education and explaining the importance of a parasite, its life cycle,
mode of infection and methods of control.
b-Protection of the host from infection by providing the host with clean food and water,
keeping them in good hygienic houses, screening systems of doors and windows and
periodic examination of people.
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2-Therapeutic measures to kill or eradicate the parasite itself, its development stages,
intermediate hosts and vectors and this could be attained by studying the parasites, their
habitats, biology and best method of eradication.
Chemoprophylaxis provides effective protection for travelers visiting endemic areas.
Prophylaxis should begin on the day of arrival and be continued for several weeksafter
departure as in prophylaxis against malaria infection.
Scientific Nomenclature:
The main divisions of the living organisms are the kingdom, phylum, class, order, family,
genus and species. Each of these may be subdivided or grouped to form intermediate groups
such as subclasses, suborders and subfamilies. There are no accepted ending for the names
of phyla but the others may be recognized as follows:
Example
ُEnding
Nematoda
Rhabditida or
Rhabditidea
Strongylata
Strongyloidea
Strongylidae
Strongylinae
-a
-ida or
-Idea
-ata
-oidea
-idae
-inae
Class
Order
Suborder
Super family
Family
Subfamily
The name of the genus and species must always be in italic; in writing the names they are
customarily underlined. The genus always begins with a capital and the species with a small
letter (e.g. Ascaris lumbricodes).
In addition to that the common name may be refered to a parasite such as pin worm for
Enterobius vermicularis or Medina worm for Dracunculus medinensis but the scientific
name remains the same all over the world.
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Classification
:
Classification started with Linnaeus's publication of the 10 th edition of Systema Nature in 1758.
Linnaeus himself is credited with the description of the sheep liver fluke, Fasciola hepatica, and
over the next 100 years many common parasites, as well as their developmental stages were
described, the systematization relying on published species descriptions, as well as on studies of
DNA, protein, ecological niches and geographical distribution to develop phylogenesis or
evolutionary of the parasites (Roberts and Janovy, 2005). Nothing remains as it is. This sentence is
true in particular with respect to the recent situation of taxonomy. The earlier classification was
based on morphological and developmental similarities, and individuals resembling each other were
grouped as a species, similar species as genera and so on.
Nowadays, the use of molecular biological methods delivers new and deeper insights into the
genetic repertoire of organisms (Mehlhorn, 2001).
The medical important animal parasites are divided into two kingdoms: Protista and Animalis. The
microscopic, single –celled, eukaryotic organisms are designated as Protozoa, belong to the
kingdom protista. The macroscopic multicellular helminthes possessing well differentiated tissues
and complex organ systems belong to the kingdom Animalia.
The classification of medical parasites in to the following phyla:
1-Phylum: Sarcomastigophora: Divided into two subphyla
a. Mastigophora or flagellates
b. Sarcodina or amoebae
2-Phylum: Apicomplexa, previously referred to as sporozoa.
3-Phylum: Microsporidia, formerly classified with sporozoa.
4-Phylum: Ciliophora.
5-Phylum: Platyhelminthes (flat worms).
a. Class: Trematoda
b. Class: Cestoda
6-Phylum: Nematoda (roundworms).
7-Phylum: Acanthocephala (thorny-headed worms).
8-Phylum: Arthropoda
a. Class: Insecta
b. Class: Arachnida
c. Class: Chilopoda
d. Class: Crustacea
9-Phylum: Pentastomida (Tongue worm and linguatulids).