Introductory Parasitology
Session 36: Introductory Parasitology
Total Session Time: 60 minutes
Pre-requisites
Students Learning Tasks
By the end of this session students are expected to be able to:
Resources Needed:
SESSION OVERVIEW
|Step |Time |Activity/ |Content |
| | |Method | |
|1 |5 minutes |Presentation |Introduction, Learning Tasks |
|2 | 15 minutes|Presentation |Common Terms used in Parasitology |
|3 |30 minutes |Presentation |Classification of and General |
| | | |Characteristics of Medical |
| | | |Parasites |
|4 |5 minutes |Presentation |Key Points |
| 5|5 minutes |Presentation |Evaluation |
SESSION CONTENTS
STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)
READ or ASK students to read the learning tasks and clarify
ASK students if they have any questions before continuing
STEP 2: Common Terms used in Parasitology (15 minutes)
o Parasitology is the study of parasites that infect humans.
Parasitology deals with characteristics, transmission, clinical
features and diagnosis, treatment and prevention of diseases caused
by the parasites.
o A parasite is an organism that depends on another organism (a host)
for food, shelter or other benefits and which receives these
benefits is such a way that the host experience detrimental effects
as a consequence. It is simply an organism that depends on other
organisms and at the same time causing harm to that organism.
o Examples of parasites included in parasitology are protozoa,
helminthes, and some arthopods.
o A host is an organism that harbours the parasite
o Is a host that harbours the larval or asexual stages of the
parasite. Sometimes larval development is completed in two different
intermediate hosts, referred to as first and second intermediate
hosts
o Is a host that either harbors the adult stage of the parasite or
where the parasite utilizes the sexual method of reproduction. In
the majority of human parasitic infections, man is the definitive
host except for example in malaria however, man acts as the
intermediate host.
o A parasite that lives on the outer surface of its host e.g. lice,
ticks.
o A parasite that lives inside the body of its host e.g. Entamoeba
histolytica
o Is a parasite that completely depends on the host during a segment
or all of its life cycle e.g. Plasmodium species.
o Is an organism that exhibits both parasitic and non-parasitic modes
of living and therefore does not absolutely depend on the parasitic
way of life but is capable of adopting it if placed in a host
o Is a parasite that wonders in to an organism in which it is not
usually found e.g. Entamoeba histolytica in the liver or lungs of
humans
o Is an association in which both the parasite and the host are so
dependent upon each other that one cannot live without the other and
none suffers any harm from the association
o This is an association in which only the parasite derives benefit
from the association but without causing injury to its host.
o This is an association in which the parasite derives benefit and the
host gets nothing in return but always suffers some injury
o This is the transformation of a parasite from an active
(trophozoite) to an inactive stage through losing its power of
motility and enclosing itself within a tough wall (cyst).
o The cyst is the resistant stage of the parasite and is also
infective to its human host.
o This is the interval between the acquision of an infectious agent by
a vector and the vector’s ability to transmit the agent to other
susceptible vertebrate host
o A perfect host-parasite relationship where tissue destruction by a
parasite is balanced with host’s tissue repair
STEP 3: Classification and General Characteristics of Medical Parasites (30
minutes)
General Characteristics
o Because of this they exhibit different immune response, makes it
difficult to formulate vaccines and are difficult to control
period it is within or on the host
multicellular (helminths and arthropods)
Classification of Medical Parasites
ectoparasites.
o Protozoa are microscopic, unicellular organisms that can be free-
living or parasitic in nature
o Protozoans are eukaryotic
o They are made up of cytoplasm and nucleus
o They are able to multiply in humans, which contributes to their
survival and also permits serious infections to develop from just a
single organism
o Transmission of protozoa that live in a human’s intestine to another
human typically occurs through a fecal-oral route (for example,
contaminated food or water or person-to-person contact).
o Protozoa that live in the blood or tissue of humans are transmitted
to other humans by an arthropod vector (for example, through the
bite of a mosquito or sand fly)
o The protozoa that are infectious to humans can be classified into
four groups based on their mode of movement;
histolytica
o Tropozoite (feeding stage)
o Cyst (non-feeding stage)
Leishmania
bloodstream
and cyst stages
o Trypanosoma have trypomastigote (in human) and
epimastigote (in vector) stages in their life cycle
o Leishmania have amastigote (in human) and promastigote
(in vector) stages in their life cycle
Balantidium
cycle
Plasmodium, Cryptosporidium
schizonts, merozoites, gametocytes and oocyst
o The helminths are invertebrates characterized by elongated, flat or
round bodies
o They are large, multicellular organisms that are generally visible
to the naked eye in their adult stages
o Like protozoa, helminths can be either free-living or parasitic in
nature.
o In their adult form, helminths cannot multiply in humans.
o There are three main groups of helminths that are human parasites:
(flukes) and cestodes (tapeworms)
reside in the gastrointestinal tract, blood, lymphatic system
or subcutaneous tissues.
o Although the term ectoparasites can broadly include blood-sucking
arthropods such as mosquitoes (because they are dependent on a blood
meal from a human host for their survival), this term is generally
used more narrowly to refer to organisms such as ticks, fleas, lice,
and mites that attach or burrow into the skin and remain there for
relatively long periods of time (e.g., weeks to months).
o Arthropods are important in causing diseases in their own right, but
are even more important as vectors, or transmitters, of many
different pathogens that in turn cause tremendous morbidity and
mortality from the diseases they cause.
o The Neglected Tropical Diseases (NTDs), which have suffered from a
lack of attention by the public health community, include parasitic
diseases such as lymphatic filariasis, onchocerciasis, and Guinea
worm disease.
STEP 4: Key Points (5 minutes)
ectoparasites (arthropods)
cyst
are resistant to harsh environmental conditions
trophozoites to cyst, a process known as encystation. And when the cysts
return to conducive environment they open to form trophozoiets
(excystation).
STEP 5: Evaluation (5 minutes)
References
Hugo and Russell (2011), Pharmaceutical Microbiology 8th Edition, Willey-
Blackwel publications
Karen C. Carroll et al (2013); Jawetz, Melnick and Adelberg’s Medical
Microbiology 26th Ed. McGraw Hill Co. Inc.
Greenwood et al (2012); Medical Microbiology, 18th edition Churchill
Livingstone
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