Eukaryotic microorganisms &
parasites
Chap 11
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Parasite: is an organism that lives at the expense of another organism
(host).
•
They cause mild to severe damage. Pathogen: If they cause disease
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Parasitology: study of parasites
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Most parasites have miocroscopic stage during their life cycle
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Parasites include protozoa, helminths, arthropods
• Significance of parasitism
1)Parasitic infections are more than living humans
2) Cultivable land is under cultivation due to parasites
3) Diseases & deaths in wild & domestic animals resulting in : human
infections & reduction in food
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• PARASITES IN RELATION TO THEIR HOSTS
• Parasites are divided into:
– ectoparasites, e.g ticks and lice, which live on the surface of
other organisms
– endoparasites, such as some protozoa and worms, which livewithin the bodies of other organisms.
• Most parasites are obligate parasites: They must spend
at least some of their life cycle in or on a host. E.g the
protozoan that causes malaria invades red blood cells.
• Few parasites are facultative parasites: They normally
are free-living, such as some soil fungi, but they can
obtain nutrients from a host, as many fungi do when they
cause skin infections.
• Hosts that are invaded by parasites usually lack effective
defenses against them, So such diseases can be serious
& fatal.
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Parasites are also categorized according to the duration of their
association with their hosts:
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Permanent parasites, e.g tapeworms, remain in or on a host once they
have invaded it.
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Temporary parasites, e.g some biting insects, feed on and then leave
their hosts.
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Accidental parasites invade an organism other than their normal host, e.g
Ticks ordinarily attach to dogs or to wild animals sometimes attach to
humans
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Hyperparasitism: a parasite itself having parasites. Some mosquitoes,
which are temporary parasites, harbor the malaria parasite or other
parasites. These insects serve as vectors (an organism that transfers a
parasite to a new host is a vector; agents of transmission) of many
human parasitic diseases.
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– Biological Vector: A vector in which the parasite goes through part of its life
cycle. E.g malaria mosquito is both a host and a biological vector
Malaria life cycle here •
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-Mechanical vector: a vector in which the parasite does not go through any
part of its life cycle during transit. Flies that carry parasite eggs, bacteria
viruses etc from faeces to food.
• Hosts are classified to
– Definitive hosts if they harbor a parasite while it reproduces sexually,
e.g The mosquito & the malaria parasite because the parasite
reproduces sexually in the mosquito
– Intermediate host if they harbor the parasite during some other
developmental stages e.g the human harboring malaria parasite is an
intermediate host, even though humans suffer greater damage
– Reservoir hosts are infected organisms that make parasites available
for transmission to other hosts. Reservoir hosts for human parasitic
diseases are wild or domestic animals.
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– Host specificity refers to the range of different hosts in which a
parasite can mature. Some parasites are quite host specific-they
mature in only one host

Mechanisms for evading host defense mechanisms:
1.
Encystment: the formation of an outer covering that protects against
unfavorable environmental conditions. Also they may help attach a
parasite to a host, or serve to transmit a parasite from one host to
another.
2.
Changing the parasite's surface antigens (molecules that elicit
immunity) faster than the host can make new antibodies (molecules
that recognize and attack antigens).
3.
Causing the host's immune system to make antibodies that cannot
react with the parasite's antigens.
4.
Invading host cells, where the parasites are out of reach of host
defense mechanisms
Parasites take large quantities of nutrients from the host (malnutrition), or
cause significant trauma to the host tissues, open sores in skin or
damage tissues, clog & damage blood vessels. Some trigger severe
inflammatory & immunological rxns
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 Protists
• Members of kingdom protista are unicellular, eukaryotes
(have nucleus & membrane enclosed organelles).
• Classification:
– Plant like protists
– Fungus like protists
– Animal-like protists
• Animal like protists (protozoa):
– most are free living, some are commensals (live in or on organisms
without harming them), others are parasites
– Include: mastigophorans, amebozoa, apicomplexans, ciliates
• Mastigophorans: have flagella.e.g of human pathogens
include Trypanosoma (African sleeping sick), Leishmania
(skin lesion or systemic disease), Giardia (diarrhea),
Trichomonas (vaginal inflammation)
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Amebozoa (formerly: sarcodines): move by pseudopodia. numerous
species of amoebas are capable of inhabiting human intestinal tract, e.g
Entamoeba (diarrhea).
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Apicomplexans (or sporozoans): immobile parasites. Have enzymes in
groups or complexes of organelles at the tips (apices) of their cells to
digest their way into host cells.
– e.g Plasmodium. It has 3 forms in its hosts (sporozoite in the saliva of
mosquito, merozoite in the human liver, trophozoites in RBC-reproduce
asexually-rupture the RBCs), some merozoites inter sexual cycle & become
gametocytes (male & female cells).
– Several species of plasmodium cause malaria with some variation in life
cycle.
– Toxoplasma:transmitted by contacting infected cats & their feces or urine
(cats definitive host), or eating raw meat, it is present in birds, rodents &
cows): lymphatic infections & blindness in adults, severe neurological
damage to the fetus of infected pregnant women. Recently causes
schizophrenia
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Ciliates: the largest gp of protozoans, have cilia over most of their
surfaces. Balantidium coli is the only ciliate that parasitizes humans
(dysentery).
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Fungi
 CHARACTERISTICS OF FUNGI
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Fungi are heterotrophs. Many are saprophytes (digest dead organic
matter and organic wastes). Some are parasites that obtain nutrients
from the tissues of other organisms.
•
Most fungi, such as molds and mushrooms, are multicellular, yeasts are
unicellular.
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The body of a fungus is called a thallus. The thallus of most multicellular
fungi consists of a mycelium a loosely organized mass of threadlike
structures (hyphae)
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The mycelium is embedded in decaying organic matter, soil, or the tissue
of a living organism.
•
Mycelial cells release enzymes to digest the surfaces of invaded matter
and absorb small nutrient molecules.
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The cell walls of few fungi contain cellulose, but most of fungi contain
chitin (a polysaccharide).
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All fungi have lysosomal enzymes that digest damaged cells and help
parasitic fungi to invade hosts.
•
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Hyphal cells are separated by cross walls called septa
Most fungi reproduce both sexually & asexually
 THE IMPORTANCE OF FUNGI
• Important decomposers, contribute significantly to the recycling of
substances in ecosystems (ecology)
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Important in health sciences, important as facultative parasites-they can
obtain nutrients from non living organic matter or from living organisms.
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Fungi are never obligate parasites because they can obtain nutrients
from dead organisms. When fungi parasitize living organisms, they kill
cells and obtain nutrients as saprophytes.
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Nearly every form of life is parasitized by some type of fungus.
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Some fungi excrete metabolic wastes that are toxic to other organisms,
especially soil microorganisms. In the soil, the production of such toxins
(antibiotics), is called antibiosis. These toxins help the fungus compete
and survive.
•
Parasitic fungi can be destructive when they invade other organisms.
Many fungi reach their hosts by producing spores that are carried by wind
or water. Human fungal diseases are called mycoses which are often
caused by more than one organism
• Classification of Fungi
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They are classified according to the nature of the sexual stage in their life
cycle (some times for each stage the fungus has different name)
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Many fungi look different when growing in tissues (yeastlike) & when
growing in their natural hibitats (filamentous).
The ability of an organism to alter its structure when it changes its habitat
is called dimorphism
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 Helminths
• Have head & tail end. The ones that parasitize humans
include flatworms & roundworms
• Flatworms (platyhelminthes) have small thickness (not more
than 1mm) but some are very long (10m). Have suckers or
hooks to attach to their hosts, e.g tapeworm (Taenia
saginata) Flukes (Schistosoma). They have complicated life
cycle that involves human & another host (Cow- Taenia
saginata, snail-Schistosoma)
• Roundworms (nematodes): have cylindrical bodies. They
vary in length from 1mm to 1m. e.g Ascaris lumbricoides
(intestines)
 16Arthropods: act as vectors
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