Introduction to parasites
Dr Nazia Khan
Define parasitism
Describe general properties of parasites
Classify parasites
Discuss concept of life cycle of a parasite
and host parasite relationship
• List common parasitic diseases
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INTRODUCTION
PARASITE
A parasite is an organism that
lives on or inside another
organism to the detriment of
the host organism
Traditionally parasite referred
to organisms with life stages
that needed more than one host
PARASITISM
A form of symbiosis in which
one organism (called parasite)
benefits at the expense of
another organism usually of
different species(called host).
INTRODUCTION
Parasites that live on the
surface of host are called
Ectoparasites (e.g. lice, mite)
Parasites that live inside the
host are called Endoparasites
(e.g. Giardia lamblia, Ascaris
lumbricoides etc.)
Parasites usually require more
than one host for completion of
life cycle e.g. Plasmodium
falciparum
GENERAL PROPERTIES OF PARASITES
Eukaryotes-closely related to
human
Slow infections
Asymptomatic infections
Usually associated with poor
socio-economic status
Usually confined to specific
area
Minimum tissue reaction
Difficult to diagnose
Difficult to treat
Host immune status-imp in
warding off infection
INTRODUCTION TO BASIC PARASITOLOGY
Parasites are Eukaryotes
Two major groups
 Protozoa (unicellular)
 Metozoa/Helminth (Multicellular)- worms
 Trophozoite -trophē=nourishment,
zōon=animal (active feeding stage of a
protozoal parasite)
 Cyst/ oocyst -inactive and infective form
 Protozoa-motility via pseudopods, flagella,
cilia etc.
INTRODUCTION TO BASIC PARASITOLOGY
Helminths-worms
Definitive host-which harbor sexual
phase of parasite
Intermediate host-which harbor
asexual phase of parasite
CLASSIFICATION
PROTOZOA
a. Flagellates have one or more whip-like
flagella e.g. Giardia lamblia, Trichomonas
vaginalis
b. Amebae use pseudopodia or protoplasmic
flow to move e.g. Entamoeba histolytica,
Naegleria fowleri
c. Sporozoa undergo a complex life cycle,
alternating sexual & asexual phases e.g.
Plasmodium spp, Cryptosporidium spp
d. Ciliates are complex protozoa bearing cilia
e.g. Balantidium coli
CLASSIFICATION
HELMINTHS/metazoa
A. Nematoda (roundworms)
 Elongated , round and unsegmented
 Complete digestive system ,
highly developed separatesexes
 Eggs & larva- suited for
external environment.
 Most human infections-ingestion
of egg or larva
 Examples Ascaris lumbricoides,
Ankylostoma duodenale,
Enterobius vermiculoaris etc
CLASSIFICATION
B. Platyhelminthes
 Flatworms are flattened,
hermaphroditic, with a few
exceptions
 Two classes, Trematoda
(flukes) & Cestoda
(tapeworms).
i. Trematoda (flukes)
Fascioloa hepatica
Fasciola hepatica, Clonorchis
sinensis, Schistosoma spp
ii. Cestodes, or tapeworms
Taenia solium, Echinococcus
granulosus,
Taenia saginata
LIFE CYCLE
Direct Life cycle
Only humans are host
Infective stage like ovum, cyst,
larva passed out of body that
infect healthy person
Example E histolytica, Giardia,
Ascaris lumbricoides.
LIFE CYCLE
Indirect Life cycle
 Multiple hosts or
involvement of vector
 Definitive host,
Intermediate host
 Example Taenia saginata
spp, Schistosoma spp etc
Life cycle of Taenia saginata
HOST PARASITE RELATION
Parasites utilize nutrition from host
resulting in damage
Loss of nutrition e.g. Iron def in
hookworm infestation, malabsorption by
Giardia
Morbidity-due to tissue injury e.g. E
histolytica dysentery, severe itch due to
Enterobius vermicularis
Mortality- fulminant diarrhea due to
Cryptosporidium parvum inf &
hyperinfection due to Strongyloides
stercoralis in HIV
Strongyloides stercoralis hyper-infection in HIV
ROLE OF VECTOR
Vector, a Latin word meaning "carrier“
Imp in transmission of parasite
No direct damage by vector
The Anopheles mosquito transmit Malaria,
Filaria
Sandfly is vectors for Leishmaniasis
Domestic cats-vector of Toxoplasma gondii,
Echinococcus granulosis
COMMON PARASITIC DISEASES
Amoebiasis: Entamoeba histolitica
Giardiasis: Giardia lamblia
Leishmaniasis: Leishmania donovani
Malaria: Plasmodium falciparum
Hook worm: Ancylostoma duodenale
Strongyloides stercoralis
Round worm: Ascaris lumbricoides
Echinococcosis: Echinococcus granulosa
(tape worm)
Pin worm: Enterobious vermicularis
Scabies: Sarcoptes scabiei
Entamoeba histolytica trophozoites
SUMMARY
Parasites are eukaryotes
Chronic infections due to host evasion
Live on or inside a host for living
Simple & Complex life cycle
Protozoa(unicellular) & Metozoa(Multicellular)
Vector carry parasite to host
• A parasite: “a living organism that acquires some of its basic
nutritional requirements through its intimate contact with
another living organism”.
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Parasites may be simple unicellular protozoa or complex
multicellular metazoan
• Protozoa: unicellular organisms, e.g. Plasmodium (malaria)
• Metazoa: multicellular organisms, e.g. helminths (worms) and
arthropods (ticks, lice)
• An endoparasite: “a parasite that lives within another living
organism” – e.g. malaria, Giardia
• An ectoparasite: “a parasite that lives on the external surface of
another living organism” – e.g. lice, ticks