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Ruel C. Maddawin, RMT
MedicalParasitology
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*A parasite is a living organism, which takes
its nourishment and other needs from a
host; the hostis an organism which supportsthe parasite.
*The parasites included in medical
parasitology are protozoa, helminthes, andsomearthropods.
*The hosts vary depending on whether they
harbor the various stages in parasiticdevelopment.
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Kinds of Parasites
*Ectoparasite a parasitic organism that lives on
the outer surface of its host, e.g. lice, ticks, mites
etc.*Endoparasites parasites that live inside the
body of their host, e.g. Entamoeba histolytica.
*Obligate Parasite This parasite is completely
dependent on the host during a segment or all ofits life cycle, e.g. !lasmodium spp.
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ectoparasitism
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Kinds of Parasites
*Facultative parasite an organism that e"hibits
both parasitic and nonparasitic modes of living and
hence does not absolutely depend on the parasiticway of life, but is capable of adapting to it if
placed on a host. #.g. Naegleria fowleri
*Accidental parasite when a parasite attacks an
unnatural host and survives. #.g. Hymenolepisdiminuta $rat tapeworm%.
*Erratic parasite is one that wanders in to an
organ in which it is not usually found. #.g.
Entamoeba histolytica in the liver or lung ofhumans.
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!ypes of "osts
*#efinitive host a host that harbors a parasite in
the adult stage or where the parasite undergoes a
se"ual method of reproduction.*$ntermediate host harbors the larval stages of
the parasite or an ase"ual cycle of development
takes place. &n some cases, larval development is
completed in two different intermediate hosts,referred to as first and second intermediate hosts.
*Paratenic host a host that serves as a temporary
refuge and vehicle for reaching an obligatory host,
usually the definitive host, i.e. it is not necessary
for the completion of the parasites life cycle.
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*%eservoir host a host that makes the parasite
available for the transmission to another host and
is usually not affected by the infection.
*&atural host a host that is naturally infected
with certain species of parasite.
*Accidental host a host that is under normal
circumstances not infected with the parasite.
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"elminth eggs
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*There is a dynamic e'uilibrium which e"ists in the
interaction of organisms.
*(ny organism that spends a portion or all of its life
cycle intimately associated with another organism
of a different species is considered as 'ymbiont
$symbiote% and this relationship is called symbiosis$symbiotic relationships%.
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)*Mutualism an association in which bothpartners are metabolically dependent upon each
other and one cannot live without the help of theother; however, none of the partners suffers any
harm from the association. E*g* cellulase
producing proto)oans inside termites.
+*ommensalism an association in which thecommensal takes the benefit without causing
in*ury to the host. E*g* Most of the normal floras
of the humans+ body can be considered as
commensals.
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-arnacles on .halesommensalism
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*Parasitism an association where one of the
partners is harmed and the other lives at the
e"pense of the other. E*g* orms likeAscarislumbricoides reside in the gastrointestinal tract of
man, and feed on important items of intestinal
food causing various illnesses.
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Parasitism
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*The damage which pathogenic parasites produce in
the tissues of the host may be described in the
following two ways-)* #irect effects of the parasite on the host+* $ndirect effects of the parasite on the host
7/25/2019 Parasitology Lec Intro
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*Mechanical in/ury may be inflicted by a parasite
by means of pressure as it grows larger, e.g.
ydatid cyst causes blockage of ducts such asblood vessels producing infraction.
*#eleterious effect of to0ic substances in
Plasmodium falciparum production of to"icsubstances may cause rigors and other symptoms.
*#eprivation of nutrients, fluids and metabolitesparasite may produce disease by competing with
the host for nutrients.
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*$mmunological reaction1 Tissue damage may be
caused by immunological response of the host, e.g.
nephritic syndrome following !lasmodiuminfections.
*E0cessive proliferation of certain tissues due to
invasion by some parasites can also cause tissuedamage in man, e.g. fibrosis of liver after
deposition of the ova of /chistosoma.
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'chistosomiasis
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*&n medical parasitology, each of the medically
important parasites are discussed under the
standard subheadings of morphology, geographical
distribution, means of infection, life cycle,
host0parasite relationship, pathology and clinical
manifestations of infection, laboratory diagnosis,
treatment and preventive0control measures ofparasites.
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A* Morphology includes si)e, 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.
*1or e"ample, Entamoeba histolytica and
Entamoeba coli.
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2. 2eographical distribution #ven though
revolutionary advances in transportation has made
geographical isolation no longer a protection against
many of the parasitic diseases, many of them are
still found in abundance in the tropics. 3istribution
of parasites depends upon-
a% The presence and food habits of a suitable hostb% #asy escape of the parasite from the host
c% #nvironmental conditions favoring survivaloutside the body of the hostd% The presence of an appropriate vector or
intermediate host
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* 3ife cycle of parasites the route followed by a
parasite from the time of entry to the host to e"it,
including the e"tracorporeal $outside the host% life.
&t can either be simple, when only one host is
involved, or comple", involving one or more
intermediate hosts.
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#* "ost parasite relationship infection is the
result of entry and development within the body of
any in*urious organism regardless of its si)e. 4nce
the infecting organism is introduced into the body
of the host, it reacts in different ways and this
could result in-
a* arrier state a perfect host parasiterelationship where tissue destruction by a parasite
is balanced with the host+s tissue repair. (t this
point the parasite and the host live harmoniously,
i.e. they are at e'uilibrium.
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b* #isease state this is due to an imperfect host
parasite relationship where the parasite dominates
the upper hand. &t can result either from lower
resistance of the host or a higher pathogenecity of
the parasite.
c* Parasite destruction occurs when the hosttakes the upper hand.
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*3aboratory diagnosis depending on the nature of
the parasitic infections, the following specimens
are selected for laboratory diagnosis-
a% -lood in those parasitic infections where the
parasite itself in any stage of its development
circulates in the blood stream, e"amination ofblood film forms one of the main procedures for
specific diagnosis.
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b% 'tool e"amination of the stool forms an
important part in the diagnosis of intestinal
parasitic infections and also for those helminthic
parasites that locali)e in the biliary tract and
discharge their eggs into the intestine.
c% 4rine when the parasite locali)es in the urinarytract, e"amination of the urine will be of help in
establishing the parasitological diagnosis.
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d% 'putum e"amination of the sputum is useful in
the following-
5 &n cases where the habitat of the parasite isin the respiratory tract, as in !aragonimiasis, the
eggs of Paragonimus westermani are found.
5 &n amoebic abscess of lung or in the case of
amoebic liver abscess bursting into the lungs, thetropho)oites of E. histolytica are detected in
the sputum.
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e% -iopsy material varies with different parasitic
infections. 1or e"ample spleen punctures in cases of
kalaa)ar, muscle biopsy in cases of Cysticercosis,
Trichinelliasis, and Chagas+ disease, /kin snip for
4nchocerciasis.
f% 4rethral or vaginal discharge for Trichomonasvaginalis
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!* vaginalis
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*&ndirect evidences changes indicative of
intestinal parasitic infections are-
a* Cytological changes in the blood eosiniphiliaoften gives an indication of tissue invasion by
helminthes, a reduction in white blood cell count is
an indication of kalaa)ar, and anemia is a featureof hookworm infestation and malaria.
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b* Serological tests are carried out only inlaboratories where special antigens are available.
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*Many parasitic infections can be cured by specific
chemotherapy.
*The greatest advances have been made in the
treatment of proto)oal diseases.
*1or the treatment of intestinal helminthiasis,
drugs are given orally for direct action on the
helminthes
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*Measures may be taken against every parasite
infectiving humans. !reventive measures designed
to break the transmission cycle are crucial to
successful parasitic eradication.
/uch measures include-
a* %eduction of the source of infection theparasite is attacked within the host, thereby
preventing the dissemination of the infecting agent.
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b* 'anitary control of drin5ing .ater and food*
c* Proper .aste disposal through establishing safesewage systems, use of screened latrines, and
treatment of night soil.
d* !he use of insecticides and other chemicals
used to control the vector population*
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e* Protective clothing that would prevent vectors
from resting in the surface of the body and
inoculate pathogens during their blood meal.
f* 2ood personal hygiene*
g* Avoidance of unprotected se0ual practices*
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3A''$F$A!$O& OF ME#$A3
PA%A'$!O3O261
a* Medical Protozoology 3eals with the study ofmedically important proto)oa.
b* Medical "elminthology 3eals with the study ofhelminthes $worms% that affect man.c* Medical Entomology 3eals with the study of
arthropods which cause or transmit disease to
man.
6 (n organism which supports a parasite is called a
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6. (n organism which supports a parasite is called a7777777777777.
8 9 the : ma*or classifications of parasitic organisms
that we study in medical parasitology are
. ( parasite that wanders in to an organ in which it is not
usually found is called
?. ( type of host that harbors a parasite in the adult stage
or where the parasite undergoes a se"ual method
of reproduction.
@. a host that is naturally infected with certain species of
parasite.
6A. an association where one of the partners is harmed
and the other lives at the e"pense of the other.
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6. ost8. !roto)oans:. elminths
9. (rthropods. #rratic
?. 3efinitive@. Batural6A.parasitism