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

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 vector

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

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

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!

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.

Types of Hosts

Par

asiti

c P

rotis

ts

Am

oeba

pro

teus

Bal

antid

ium

col

i

Ent

amoe

ba h

isto

lytic

a

Trophozoites engulfing detached epithelial cells

ingested epithelial cell

trophozoite

second epithelial cell Parasitology Today

Vol. 3, 117

Gia

rdia

lam

blia

Leis

hman

ia s

pp.

Mic

orsp

orid

ia

Pla

smod

ium

spp

.

Sar

cocy

stis

spp

.

Tet

rahy

men

a sp

p.

Tox

opla

sma

gond

ii

Tric

hom

onas

vag

inal

is

Try

pano

som

a br

ucei

Try

pano

som

a cr

uzi

Kingdom Fungi

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

Fun

gus

Life

Cyc

le

Germination from Spore

Septa (sing. Septum)

aseptate

septate

Myc

elia

(si

ng.

Myc

eliu

m)

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!

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 damage

Sup

erfic

ial M

ycos

es

Sub

cuta

neou

s M

ycos

es

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 animals

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)

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

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”

Mac

rofu

ngi

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

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

Zygomycete Life Cycle

Ascomycete Life Cycle

Basidiomycete Life Cycle

Sac

char

omyc

es c

erev

isia

e

Sor

daria

mac

rosp

ora

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)

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)

Asc

aris

lum

bric

oide

s

Dra

cunc

ulus

med

inen

sis

Ent

erob

ios

verm

icul

aris

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)

Tae

nia

sagi

nata

• 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)

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)

Wuc

here

ria b

ancr

ofti