introduction to parasitology
TRANSCRIPT
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Parasitology• Parasites include things that live internal
(endoparasites) or external (ectoparasites) to the infected (or infested) host
• Broadly speaking, all pathogens may be classified as parasites
• Narrowly speaking, Parasitology is the science that studies the relatively large parasites including parasitic Protists, Worms, and, to a lesser degree, pathogenic Fungi and Arthropod ectoparasites
• Today we’ll consider Protists, Helminths (worms), Fungi, and the Arthropod Vectors of infectious disease www.freelivedoctor.com
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Vectors and Hosts
• A Vector is a living organism that carries a disease-causing organism to new Hosts
• This distinction can be arbitrary, however, depending on which organism’s infection we are most concerned with
• Thus, the Anopheles mosquito is a vector for the parasitic disease malaria because we care more about the health of the human host than that of the mosquito
• However, in addition, the malaria parasite has a much greater impact on the health of the human host than it does on the health of the mosquito vectorwww.freelivedoctor.com
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Biological vs. Mechanical Vector• The real confusion between host and vector comes
from the concept of Biological Vector
• Within (or upon) both a host and a Biological Vector the parasite undergoes some aspect of its life cycle
• Thus, in one sense, all hosts that can pass a parasite on, particularly to another species of host, is also a Biological Vector
• However, if the parasite does not undergo some aspect of its life cycle as it is transported by one organisms to a second, then the first organism is described as a Mechanical Vector
• E.g., flies can be mechanical vectos of feces-born pathogens such as Salmonella
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Types of Hosts
• Definitive Host: The host in which the parasite goes through its sexual cycle (I.e., fertilization & meiosis)
• E.g., mosquitoes serve as definitive hosts of the malaria parasite (Plasmodium spp.)
• E.g., snakes (~30 species) serve as definitive hosts of Sarcocystis singaporensis, a disease of mammals
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Types of Hosts
• Intermediate Host: Host in which the parasite replicates but does not go through its sexual cycle
• E.g., mammals, including humans, serve as intermediate hosts of the malaria parasite (Plasmodium spp.)
• E.g., mammals (~30 species, including humans) serve as intermediate hosts of Sarcocystis singaporensis
S. singaporensis has been proposed as biological control of rodents pests—apparently it can selectively kill them!
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Types of Hosts• Reservior Hosts: The reservoir host is the population
in which a parasite resides when it isn’t affecting a population that we care more about
• E.g., the rabies virus normally is passed back and forth among wild mammals (these serve as the reservoir hosts—the populations that we don’t much care about)
• Occasionally, however, rabies can infect pets, and thereby us (the population we do care about)
• To prevent the latter (i) we vaccinate pets, (ii) avoid handling wild animals, and (iii) otherwise attempt to reduced the incidence of rabies in reservoir pops.www.freelivedoctor.com
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Types of Hosts
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Par
asiti
c P
rotis
ts
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Ent
amoe
ba h
isto
lytic
a
Trophozoites engulfing detached epithelial cells
ingested epithelial cell
trophozoite
second epithelial cell Parasitology Today
Vol. 3, 117www.freelivedoctor.com
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Gia
rdia
lam
blia
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Mic
orsp
orid
ia
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Pla
smod
ium
spp
.
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Sar
cocy
stis
spp
.
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Tetr
ahym
ena
spp.
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Toxo
plas
ma
gond
ii
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Tric
hom
onas
vag
inal
is
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Tryp
anos
oma
bruc
ei
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Tryp
anos
oma
cruz
i
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Fun
gal A
nato
my
• Heterotrophic, exoenzyme-producing absorbers
• Chitinaceous cells walls
• Yeasts or, more typically, Hyphae
• Mycelia (tangled masses of hyphae, typically that has grown into a substrate)
• Thallus, the word used to describe the “body” of a fungus (same word as used to describe the body of an algae
• Fungi live by growing mycelia into (or onto) substrate (e.g., a dead plant), secreting exoenzymes from these cells, and then absorbing digested nutrient into the mycelia www.freelivedoctor.com
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Fun
gus
Life
Cyc
le
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Germination from Spore
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Septa (sing. Septum)
aseptate
septate
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Myc
elia
(si
ng.
Myc
eliu
m)
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Fungal Ecology• Fungi and bacteria are principle decomposers in biosphere
(they and bacteria)
• The secrete exoenzymes to digest nutrients extracellularly, which are then brought into cells directly across plasma membranes
• Fungi are virtually the only organism capable of breaking down lignin (the stuff that makes wood woody)
• Fungi act as disease-causing organisms of animals and, especially, of plants
• Fungi produce antibiotics (e.g., Penicillin) which they use to limit ecological competition from bacteria
• We take advantage of fungus-mediated decomposition in the production of bread, beer, wine, cheeses, and soy sauce!www.freelivedoctor.com
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Myc
oses
• Mycoses are diseases caused by fungi
• Mycoses are particularly a problem given weakened immunity or habitual exposure to moist conditions (e.g., wet socks)
• We can distinguish mycoses into:
• Superficial (meaning affecting only surface, keratinized tissues)
• Subcutaneous (meaning invading below the skin or into the lymph)
• Systemic (meaning invasion throughout the the body)
• Greater penetration generally results in greater body damagewww.freelivedoctor.com
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Sup
erfic
ial M
ycos
es
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Sub
cuta
neou
s M
ycos
es
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Sys
tem
ic M
ycos
esExamples
• Coccidioidomycosis
• Histoplasmosis (disease of Ohio Valley)
• Blastomycosis
• Paracoccidioidomycosis
These are thermally dimorphic fungi that exist in nature, soil
Inhalation pulmonary inf. dissemination
No evidence of transmission among humans or animalswww.freelivedoctor.com
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Fungal Classification
We can also distinguish fungi into:
• Yeasts (single-celled fungi)
• Molds (filamentous, asexual fungi)
• Macrofungi (macroscopic fruiting bodies supported by mycelia and hyphae)
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Yeasts
Candida albicans
Budding yeast
Schizosaccharomyces pombe
Fission yeast
• Generic name given to unicellular fungi
• About 250 named species
• Most yeast reproduction is asexual and takes place by cell fission or budding
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Mol
ds
• Molds are rapidly, asexually reproducing filamentous fungi
• Note that many molds can go on to reproduce sexually, producing fruiting bodies, though at that point they are no longer considered “molds”
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Mac
rofu
ngi
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Dim
orph
ism
• Some fungi can grow as yeasts or as hyphae, depending on growth conditions
• This can lead to a single fungal species being classified as more than one species
• Fungi also can display sexual versus asexual growth, further confusing fungal classification
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Fungal Spores
• Spores are a means of fungal reproduction
• (fungi also can reproduce by fragmentation)
• All fungal spores are haploid and non-motile
• Asexual spores (produced by mitosis)
• Sexual spores (produced by meiosis)
• Zygospores, Ascospores, Basidiospores
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Zygomycete Life Cycle
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Ascomycete Life Cycle
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Basidiomycete Life Cycle
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Sac
char
omyc
es c
erev
isia
e
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Sor
daria
mac
rosp
ora
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Parasitic Helminths
• Flat worms (Phylum Platyhelminthes)
• Flukes (Schistosoma—schistosomiasis)
• Tape worms (Taenia—beef tape worm)
• Round worms (Phylum Nematoda)
• Ascaris, Dracunculus (Guinea Worm),Trichinella, Wuchereria
• Hookworms (Ancylostoma)
• Pinworms (Enterobius)www.freelivedoctor.com
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Anc
ylos
tom
a du
oden
ale
• Flat worms (Phylum Platyhelminthes)
• Flukes (Schistosoma—schistosomiasis)
• Tape worms (Taenia—beef tape worm)
• Round worms (Phylum Nematoda)
• Ascaris, Dracunculus (Guinea Worm),Trichinella, Wuchereria
• Hookworms (Ancylostoma)
• Pinworms (Enterobius)
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Dra
cunc
ulus
med
inen
sis
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Ent
erob
ios
verm
icul
aris
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Sch
isto
som
a ja
poni
cum
• Flat worms (Phylum Platyhelminthes)
• Flukes (Schistosoma—schistosomiasis)
• Tape worms (Taenia—beef tape worm)
• Round worms (Phylum Nematoda)
• Ascaris, Dracunculus (Guinea Worm),Trichinella, Wuchereria
• Hookworms (Ancylostoma)
• Pinworms (Enterobius)
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Taen
ia s
agin
ata • Flat worms (Phylum Platyhelminthes)
• Flukes (Schistosoma—schistosomiasis)
• Tape worms (Taenia—beef tape worm)
• Round worms (Phylum Nematoda)
• Ascaris, Dracunculus (Guinea Worm),Trichinella, Wuchereria
• Hookworms (Ancylostoma)
• Pinworms (Enterobius)
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Tric
hine
lla s
pira
lis • Flat worms (Phylum Platyhelminthes)
• Flukes (Schistosoma—schistosomiasis)
• Tape worms (Taenia—beef tape worm)
• Round worms (Phylum Nematoda)
• Ascaris, Dracunculus (Guinea Worm),Trichinella, Wuchereria
• Hookworms (Ancylostoma)
• Pinworms (Enterobius)
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Wuc
here
ria b
ancr
ofti
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