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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
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
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
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
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
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
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
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
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
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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