Lecture 1 and 2: The Parasite
I. The importance of parasites
A. parasites are everywhere
1. Parasites are on us, in us, and all around us.
a. Approximately 27% of described eukaryotes are parasites (477,780 of
1,797,431 species)
1. 40% of ecdysozoa
a. 60% of nematodes
2. 20% of lophotrochozoa
a. 80% of Platyhelminthes
2. Pneumonia example
a. caused by any number of parasites including different species of bacteria,
viruses, fungi, protists that are ever-present in our environment
b. parasite quickly becomes established when host immune defenses are
compromised in some capacity. This is why those with advanced stage
HIV-AIDS often succumb to pneumonia.
B. Yellow Fever and Humans
1. What is Yellow fever?
a. Yellow fever is an acute hemorrhagic disease caused by an enveloped RNA
virus. Originated in West-Africa
b. Virus transmitted via the mosquito Aedes aegypti
c. Pathogenesis – destroy liver and heart cells. Mortality rate is 15%
2. Impact of yellow fever
a. Panama canal (ca. 1880) – caused 80,000 deaths; Delayed project by 25 years;
caused catastrophic financial collapses of several companies
b. Solidified Haitian revolution of 1801
1. Napoleon sent army to subdue including brother-in-law. Most died
while the Haitians, who were of west-African heritage where the
disease originated, were resistant.
c. Facilitated Louisiana Purchase of 1803– after Haitian defeat, Napoleon
abandons plans for a new world empire, selling Louisiana to US for a
significantly reduced price.
d. Promoted west-African slave trade – due to their resistance, west-Africans
were highly desired (compared to Native Americans) in the new world to
work the agricultural fields due to their resistance.
C. Bottom line - Parasites have profound impact on host physiology, behavior, populations, and
communities. This is the point of the course. Parasites are everywhere, and they have
had a tremendous impact on how the natural world is shaped.
II. What are parasites?
A. Parasitism – symbiotic relationship between two organisms in where one organism benefits
from the relationship (the parasite) while the other incurs a cost (the host).
B. Pathogen – any parasite that induces disease (i.e. abnormal condition) in the host; from the
Greek “Pathos” meaning suffering. However, pathology means the study of disease,
not mean the study of pathogens, as many diseases are induced by non-parasites
(e.g. some cancers, autoimmune diseases, obesity, etc).
NOTE: if we define any reduction in host function as a disease, then nearly all
parasites would be pathogens. For our purposes, we will define a pathogen as a
parasite that has a significant cost to host fitness (i.e. ability to reproduce).
C. Parasitism is one of many types of organismal interactions
1. Competition (-/-)
2. Predation (+/-)
Lecture 1 and 2: The Parasite
3. Herbivory (+/-)
4. Symbiosis (+/?)
a. mutualism (+/+)
b. commensalism (+/0)
c. parasitism (+/-)
D. Parasites are diverse
1. All Viruses
a. viral origins
1. Progressive hypothesis - Viruses are mobile genetic elements that have
evolved the added ability to escape from the cellular genome
(e.g. retro-transposons).
2. Regressive hypothesis – viruses are degenerate intra-cellular parasites
that have eliminated all but essential features (e.g. membrane,
replication machinery, etc.)
3. Virus first - originated independent of cells but parallel with the
earliest forms of life.
2. Some bacteria (note, archea are non-pathogenic)
3. Some protozoa (i.e. animal-like heterotrophic protists)
4. Some fungi
5. Some plants (e.g. mistletoe)
6. Some animals (e.g. ticks, mites, worms, etc.)
7. Some proteins (e.g. prions)
E. Parasite diversity
1. Microparasites – viruses, bacteria, fungia, protozoa. Short generation times, multiply
quickly
2. Macroparasites – typically helminthes (worm-like parasite: nematodes, cestodes,
trematodes); relatively long generation times
3.Parasitoids - parasites that have free-living stage; typically parasitic insects (e.g.
ichneumonid wasps). Larvae infect hosts. Generally kill host upon adult eclosion.
4.Endoparasites – in the host
5. Ectoparasites – on the host
6.Social parasites – parasites exploit social structure of a host group, but don’t live on or
in a host. These include brood parasites (E.g. cuckoo birds), cleptoparasites
(robing food from other species, like in spiders)
III. Parasite origins
A. All parasites evolved from non-parasitic, free-living ancestors (except maybe viruses)
B. Parasitism has evolved independently in numerous lineages
C. How do parasites evolve?
1. Frequent interaction btw two organisms
2. Large size difference btw two organisms (generally)
3. pre-adaptation (aka exaptation)
a. Persistence - able to stay in or on the host for extended periods of time (for
viruses, bacteria, fungi, and protozoa, persistence is a minor issue, but
for eukaryotes, this is more difficult)
1. Phoresy - using larger organisms for dispersal. This is an excellent
path towards parasitism. Phoresy tends to be important in
terrestrial parasites (water is an excellent transport medium).
Example – parasitic nematodes have phoretic / nonparasitic
relatives.
Lecture 1 and 2: The Parasite
2. Environmental buffering – avoid variance in environmental conditions
(e.g. periods of low resource availability and diapause). Hosts
provide a steady environment!
Example – as copepods invade insland water ways from marine
systems, they encounter greater environmental variability. To
buffer variation, copepods evolve either a diapause or parasitism
strategy. In short, parasitism reduces temporal variation in food
availability.
3. Refuge – using a species to escape predation
Example: Pearlfish live inside various invertebrate hosts (clams,
starfish, sea squirts, etc). Some species have evolved to be
parasitic on sea cucumbers.
4. Predator exploitation – prey able to survive within predator and
exploit it. This is how we get eukaryotes!
Example –Mosquito larvae prey upon free swimming ciliate.
Ciliate develops into parasitic cell in presence of mosquito larvae
and able to attach / penetrate mosquito cuticle, parasitizing its
predator (Washburn et al. 1988. Science 240: 1193-95)
b. Exploitation - Some manner of extracting resources from the host
1. Opportunistic foragers in eukaryotes
Examples: some small crabs occasionally will enter gills of fish
and feed on gill tissues. Cymothoidae isopods are obligate
parasites, but their free-living relatives (aegidae and
corallanidae) can obtain occasional blood meals on fish.
END LECTURE 1
IV. Life history
A. Life cycle
1. Find host - Size differences btw many parasites and hosts make finding a new host
difficult. For E. coli (1um) to travel 1 meter, it must travel 1,000,000 times its
body length. The same relative distance you (1m) would travel to reach the
Nashville Tennessee (1,000 km or 625 miles).
a. Dispersal strategies
1. Passive transport – needs lots of offspring, environmentally resistant
outside of host by forming spores, cysts, etc.
Examples:
- host defecation (sit and wait parasites; e.g. cholora)
- host sneezing (e.g. common cold)
2. Active transport – seek host; tend to produce few offspring who are
vulnerable outside the host.
Examples:
- parasitiods
- ecotparasites (ticks, mites)
2. Infect / establish within host
a. Ectoparasite must attach. Endoparasites must overcome external or internal
barriers
b. for endoparasites, infective form is often different from the form during
differentiation (e.g. malaria or alien (face sucker to larvae)
3. Growth, differentiation, multiplication (asexual) – extract resources from host.
Lecture 1 and 2: The Parasite
4. Reproduce (sexual) - optional
5. Transmit – parasite (or its offspring) transfer from one host to the next. See step 1.
B. transmission mode
1. Direct – intermediate host is absent
a. vertical – individual  offspring
b. horizontal individual  individual (including sexual)
2. Indirect – intermediate host present
a. paratenic – transport via an optional substitute host. Typically paratenic hosts
gain parasite by ingesting original host. Parasite does not undergo any
development within paratenic host.
Example- broad fish tapeworm that infects humans (Diphyllobothrium
latum). Normally cycles between plankton-feeding fish and copepod. If
predatory fish eats plaktonic-feeding fish after parasite rearchs infective
stage, then it can also suffer from infection. If humans eat undercooked
infected predatory fish, they can also suffer from infection.
b. vector – vectors differ from paratenic hosts in that vectors are often a
permanent stage in the parasites life cycle, and the relationship
between parasite and vector is evolutionary distant and complex
Example – malaria
3. Complex – multiple hosts and vectors
a. Digenic parasite (multiple hosts); versus monogenic
1. Digenic systems favor asexual or hermaphroditic organisms, as
finding a mating partner already established in host 2 is rare. Trematodes
are often digenic, as well as hermaphroditic.
b. evolution of crazy cycles
1. Upward incorporation –Predator eats prey (host 1). If host 1 is a
routine prey item, then the parasite may evolve to also exploit
the predator (host 2). Considering that predators of usually
larger than their prey, predator may offer greater parasite
resources, causing a shift in parasite reproductive output to
host 2. The original host is kept, because the parasite has
already evolved a way to penetrate host
2. Downward incorporation –host 2 added from a lower trophic level.
Host 2 is a paratenic host (parasite does not develop or
reproduce in host 2. In this scenario, original host (host 1) is the
final host. Downward incorporation is favored if the new host
increases the probability of encountering the final host.
V. Host Specificity (i.e. resource specialization in free living organisms)
A. Organisms evolve from free-living  facultative parasite  obligate parasite in a gradual
fashion. parasites improve their ability to exploit hosts and become more specialized.
B. Obligate parasitism may be coupled with the loss of other methods for exploiting the
environment (example: viruses lose the ability to self-replicate in the degenerative
hypothesis), locking them into a parasite lifestyle. Reversals are rare (think Dollo’s
law)
1. Some cases of parasitism reversal
a. leeches (Light and Siddall 1999.Journal of Parasitology 85:815-823)
NOTE: reversals more common in ecotparasites than endoparasites
C. Host specificity - number of hosts used at a given life stage. one of the most important
characteristics of a parasite
1. Parasites may be specialists or generalists.
Lecture 1 and 2: The Parasite
a. Generalist parasites have multiple parallel hosts (i.e. not a series hosts as with
complex cycles)
2. Measuring host specificity
a. Host number – how many hosts exist
1.host lists – suffers from geographical/temporal/host sampling bias.
Ignores severity of infection.
b. Rohde’s index (s) – examines parasite abundance within each host
and number of hosts
1. values of 1 = specificity, values < 1 tends to generalist
c. Specificity index(STD)- measures the average taxonomic ‘distinctness’ of all
host species
1. Inverse to S. A high value means hosts taxonomically diverse
(maximum is 5, which means that parasite can infect multiple
classes within a phylum). Although some parasites infect across
phyla (think flukes in snails and humans), they tend to use one
as an intermediary, causing very little harm. That is because they
are specialized on the final host.
2. specificity affects the likelihood of a parasite to ‘jump’ to a new host,
which may lead to ‘emerging diseases’ and pandemics.
a. Ability to jump phylogenetically constrained, especially for
specialists. This is why we shouldn’t fear space herpes.
D. Evolution of host specificity
1. Ecological constraints – encountered versus non-encountered
2. Physiological constraints – can exploit versus cannot exploit.
a. Phylogenetic constraints - As stated above, congeners should be
physiologically / phylogenetically similar to current host, making host
expansion or shifts easier.
c. trade-offs –highly reproductive specialist (can manipulate one host well)
versus moderately reproductive generalist (manipulates manipulate
many hosts poorly; i.e. multitasking is bad)
3. Host constraints (non-physiological) - abundance, lifespan, size
a. parasites specialize on abundant, large, long lived hosts with few
sympatric congeners
b. parasites generalize on low density, small, short lived hosts with
many sympatric congeners