2. LECTURE OUTCOME
• To define relevant terms
• To classify and distinguish
parasites
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3. BIOTIC RELATIONSHPS
• Mutualism
– Long term relationship or interaction between
organisms of different species where both benefit
• Commensalism
– Relationship between organisms where one benefits
while the other neither gets harmed nor benefits
• Parasitism
– Relationship between organisms where one benefits
(parasite) while the other is harmed (host).
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4. TYPES OF HOSTS
• Host
– An organism that acts as a refuge for another organism
(usually parasite) to dwell
• Intermediate Host
– An organism that harbours the larval stage or asexual
forms of a parasite
• Definitive/Final/Primary Host
– An organism that harbours the adult parasite and where
sexual reproduction usually takes place
• Reservoir Host
– An organism that serves a source of infection and sustains
a parasite in the absence of its preferred hosts
• Vector
– An organism that facilitates the spread of disease
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5. PARASITES
• Parasitology is divided into two (2) basic
groups
–Protozoology
• Study of protozoa
–Helminthology
• Study of helminthes
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6. Protozoa
• These are single-celled eukaryotes
– Free-living
– Parasitic
– Symbiotic
• Ubiquitous
• Heterotrophic
• Life cycle
– Two-phased (proliferative and dormant)
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7. Protozoa
Amoebae
– The simplest protozoa
– Some are ‘true parasites’ (obligate intracellular
parasites) Others occur naturally in soil and water,
'free-living' and infect humans as opportunistic
pathogens.
– Characterized by a feeding and dividing
trophozoite stage
• It moves by temporary extensions of the cell called
pseudopodia
– Trophozoite may form a resistant cyst stage able
to survive in the environment
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9. Protozoa
Flagellates
–These organisms have a trophozoite form
and possess flagella for locomotion and
food gathering
–All pathogenic species are true parasites
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11. Protozoa
Ciliates
– These possess rows of hair-like cilia around the
outside of the body
• for motility and to direct food into a primitive mouth termed
a cytostome
• All possess two nuclei: a large polyploid micronucleus and a
small micronucleus active only during sexual reproduction
• Some species form cysts
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13. Protozoa
Apicomplexa
– A unique group lacking any visible means of
locomotion
– They are ALL parasitic and most are intracellular
– Have a life cycle involving both sexual and asexual
reproduction
– The common feature of all members is the
presence of an apical complex (visible only by
electron microscopy) at the anterior pole in one or
more stages of the life cycle.
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15. Helminths
• They are complex, multicellular organisms
– Range in size from the microscopic filarial
parasites to the giant tapeworms
– Sexual reproduction occurs in all cases
• Mating between male and female larvae
• Parthenogenesis where the worm possesses both male
and female reproductive organs
– Grouped as nematodes, cestodes and trematodes
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16. Helminths
• Nematodes (Round worms)
– They possess a mouth, digestive tract, anus and
sexual organs
– They occur as male or female forms and
reproduce by mating or through parthenogenesis
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17. Helminths
• Nematodes (Round worms)
– The intestinal pathogenic nematodes
1. Development in soil - larvae are infective.
The larvae are shed in the faeces and mature in the
soil
Infect humans by burrowing into the skin (usually
through the soles of the feet) and enter the blood
stream to the heart and lungs
They force their way into the alveolus and trachea
and on reaching the epiglottis, are swallowed. The
life cycle then continues in the small intestine
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18. Helminths
• Nematodes (Round worms)
–The intestinal pathogenic nematodes
2. Survival in soil - eggs being infective
The eggs are the infectious form in which the
larvae develop. When ingested the larvae hatch
in the small intestine, penetrate the mucosa and
are carried through the blood stream to the
heart and lungs.GSP2620_NURSING 2022
19. Helminths
Filaria
– Microscopic nematodes transmitted by biting
insect vectors in which part of the organism's life
cycle is completed.
– On infecting humans the larvae mate and the
females produce microfilariae which develop in
the blood, lymphatic system, skin and eye.
Resulting in gross swelling of infected tissues
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20. Helminths
• Cestodes (Tapeworms)
– Are flat, ribbon-like worms (grow up to 10m in
length)
– They produce eggs which can infect a variety of
hosts in which the life cycle continues. Humans
become infected from consuming
contaminated meat
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21. Helminths
• Cestodes (Tapeworms)
– Special feature is a scolex (head) that attaches
to the intestinal wall by suckers
– Eggs hatch in the gut releasing motile larvae
that migrate through the gut wall and blood
vessels to encyst in muscle
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22. Helminths
• Trematodes (flukes)
– Are flat, leaf-like organisms
– Have complicated life cycles, alternating
between a sexual reproductive cycle in the
final host (man) and an asexual multiplicative
cycle in a snail host
– They cause infection of the liver, bladder and
rectum
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23. Helminths
• Trematodes (flukes)
–They are hermaphroditic, except for the
schistosomes which have a boat-shaped
male and a cylindrical female form
• Part of their life cycle is completed in
an aquatic snail host
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