1. Parasitism
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Wikipedia “Cordyceps” page; accessed 02-X-2014
Photo of Cordyceps fungus growing out of the insect it parasitized from Wikimedia Commons

2. Exploitation (+/- or antagonistic interaction)
Parasites consume tissues or fluids of their host organisms;
typically infecting just 1 individual, generally without killing it
(at least not immediately)
Ectoparasite (external) vs. Endoparasite (internal)
Macroparasite vs. Microparasite
Free-living vs. Symbiont
Complex Life Cycle (generally involving multiple host species) vs. Simple Life Cycle
Horizontally Transmitted vs. Vertically Transmitted
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Wikipedia “Parasitism” page; accessed 09-X-2014
A louse is an ectoparasitic, symbiotic, macroparasite.
Photo of human head louse from Wikimedia Commons

3. Exploitation (+/- or antagonistic interaction)
Pathogens
Parasites that cause disease
(which manifests as pain, dysfunction or death)
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Wikipedia “Ebola virus disease” page; accessed 09-X-2014
This example illustrates that parasites can be living organisms or infectious non-living particles (such as viruses).
Photomicrograph of an Ebola virion (a complete virus particle) from Wikimedia Commons

4. Rely on other organisms to raise their young
Brood Parasites
Rely on other organisms to raise their young
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Photo from Wikimedia Commons: "Reed warbler cuckoo" by Per Harald Olsen - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons -
Photo of Reed Warbler & Cuckoo chick from Wikimedia Commons

5. Thieves (many are therefore competitors)
Kleptoparasites
Thieves (many are therefore competitors)
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Photo from

6. Symbionts Live in close association with their hosts;
amensal, commensal, parasitic, or mutualistic
Some mutualistic defensive symbionts protect their
hosts against parasitic symbionts
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Photomicrograph of endophytes in a plant from

7. Co-cladogenesis & co-speciation
Two associated lineages (especially symbionts)
diverge together, and potentially co-speciate
Dipodomys
merriami
Fahrenholzia
pinnata
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Wikipedia “Merriam’s kangaroo rat” page; accessed 16-X-2014
Wikipedia “Fahrenholzia pinnata” page; accessed 16-X-2014
Light, Jessica E. & Mark S. Hafner Codivergence in heteromyid rodents (Rodentia: Heteromyidae) and their sucking lice of the genus Fahrenholzia (Phthiraptera: Anoplura). Systematic Biology 57:
“FIGURE 4. Results of reconciliation analysis (TreeMap 2.0β) for heteromyid rodents and their ectoparasitic lice of the genus Fahrenholzia. Gray lines between taxa indicate host-parasite associations. Shaded circles at nodes indicate instances of perfect cophylogeny (i.e., putative codivergence events). The number of reconstructed codivergence events (Table 2) was greater than expected by chance (P < 0.001).”
Photos from Wikimedia Commons; figure from Light & Hafner (2008) Systematic Biology

8. Horizontal vs. Vertical Transmission
Horizontal – between individuals that are not
linked by a parent-child relationship
Vertical – from mother to offspring
Mother
Horizontal
Other non-offspring member of
the population
Vertical
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Daughter
Son
Other non-parent
member of
the population
Horizontal or

9. Host switch Cospeciation Duplication Host Parasite Failure to speciate
Missing the boat
Extinction
Coexistence
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Think of these as macro-evolutionary trajectories…
From Jason Weckstein, who completed a Ph.D. at LSU in 2003
Which are most likely under strictly vertical transmission?
From J. Weckstein (2003)

10. *When is it Coevolution?
Reciprocal adaptive evolution in each of 2 interacting species
in response to adaptations in the other species
Tribolium castaneum is infected by microsporidian Nosema whitei;
parasite virulence and host recombination frequencies co-evolve
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Wikipedia “Host-parasite coevolution” page; accessed 16-X-2014
Recombination – in eukaryotes, genetic exchange between DNA molecules during meiosis.
Virulence – host’s parasite-induced loss of fitness.
Your textbook provides many examples of adaptations, counter-adaptations, and co-evolutionary adaptations of parasites and hosts.
Bérénos, C., P. Schmid-Hempel & K. M. Wegner Evolution of host resistance and trade-offs between virulence and transmission potential in an obligately killing parasite. Journal of Evolutionary Biology 10:2049–2056.
Fischer, O. & P. Schmid-Hempel Selection by parasites may increase host recombination frequency. Biology Letters 22:
Janzen, D. H When is it coevolution? Evolution 34:
Photo of T. castaneum flour beetle from Wikimedia Commons; *original idea from Janzen (1980) Evolution

11. Macroparasite vs. Microparasite
Catagories based on function rather than taxonomy or phylogenetics
Macroparasites – parasites grow, but have no direct reproduction within the host (they produce infective stages that must colonize new hosts); typically much larger and have longer generation times than microparasites; immune response in host is typically absent or very short-lived; infections are often chronic as hosts are continually reinfected;
e.g., helminthes, arthropods, etc.
Microparasites – parasites that reproduce within the host, often within the host’s cells, and are generally small and have short lifespans relative to their hosts; hosts that recover often have an immune period after infection (sometimes for life); infections are often transient; e.g., bacterial, viral, fungal infectious agents, as well as many protozoans, etc.
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12. Parasites can influence individuals, populations, interactions between species, communities & ecosystems
Tribolium castaneum outcompetes T. confusum when both are healthy;
T. confusum outcompetes T. castaneum in the presence of
protist parasite Adelina tribolii (which is especially virulent towards T. castaneum)
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Parasites can influence individuals, populations, communities & ecosystems (and, of course, we could say this about predators, herbivores, competitors, etc.).
Park, T Experimental studies of interspecies competition. I. Competition between populations of flour beetles, Tribolium confusum Duvall and Tribolium castaneum Herbst. Ecological Monographs 18:
Cain, Bowman & Hacker (2014), Fig ; after Park (1948)

13. “Proportion of grouse treated…
Parasites can influence individuals, populations, interactions between species, communities & ecosystems
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Peter J. Hudson, Andy P. Dobson & Dave Newborn Prevention of Population Cycles by Parasite Removal. Science 282:
Fig. 3. The influence of treatment on the cycling of grouse populations. Changes in the number of grouse are shown in relation to the proportion of grouse treated. No treatment, dashed line; 5%, dotted line; 10%, thick solid Line; and 20%, thin solid line.
Red Grouse are naturally infected by a nematode parasite; antihelminthics cure the infected hosts
“Proportion of grouse treated…
No treatment, dashed line; 5%, dotted line; 10%, thick solid Line; and 20%, thin solid line.”
Photo of from fig. from Hudson et al. (1998) Science

14. Modeling Microparasite Disease Dynamics
The SIR Model – a compartmental model
(population is sub-divided into compartments) for epidemiology
Birth a a a
Susceptible
hosts (S) β
Infected
hosts (I) v
Recovered &
Immune
hosts (R)
transmission
recovery
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Epidemiology – the science that studies the patterns, causes, and effects of disease in populations.
Anderson, Roy M. & Robert M. May Population biology of infectious diseases: Part I. Nature 280:
Anderson, Roy M. & Robert M. May Population biology of infectious diseases: Part II. Nature 280:
May, Robert M Parasitic infections as regulators of animal populations. American Scientist 71:36-45. b
α + b b
Death 
After Anderson & May (1979ab) & May (1983)

15. Modeling Microparasite Disease Dynamics
Coupled differential equations; one for each host compartment: dS/dt = fxn; dI/dt = fxn; dR/dt = fxn
S, I & R are numbers of individuals; the other variables are rates
Birth a a a
Susceptible
hosts (S) β
Infected
hosts (I) v
Recovered &
Immune
hosts (R)
transmission
recovery
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We are only going to look in detail at dI/dt…
Anderson, Roy M. & Robert M. May Population biology of infectious diseases: Part I. Nature 280:
Anderson, Roy M. & Robert M. May Population biology of infectious diseases: Part II. Nature 280:
May, Robert M Parasitic infections as regulators of animal populations. American Scientist 71:36-45. b
α + b b
Death 
After Anderson & May (1979ab) & May (1983)

16. Modeling Microparasite Disease Dynamics
dI/dt = βSI – (α + b + v)I
dI/dt = βSI – mI
βSI =
Disease transmission rate
m =
combined death & recovery rate
Birth a a a
Susceptible
hosts (S) β
Infected
hosts (I) v
Recovered &
Immune
hosts (R)
transmission
recovery
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Your textbook uses m in place of (α + b + v).
SI = encounter rate between a susceptible individual and an infected individual.
β = transmission coefficient.
Anderson, Roy M. & Robert M. May Population biology of infectious diseases: Part I. Nature 280:
Anderson, Roy M. & Robert M. May Population biology of infectious diseases: Part II. Nature 280:
May, Robert M Parasitic infections as regulators of animal populations. American Scientist 71:36-45. b
α + b b
Death 
After Anderson & May (1979ab) & May (1983)

17. Modeling Microparasite Disease Dynamics
If dI/dt > 0, disease will establish & spread
βSI – mI > 0
ST > m / β
We refer to this as the threshold density for disease progression
What are the public policy or management implications?
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Reduce susceptible population (ST) by culling or vaccinations, increase m by improving early detection and clinical treatment, or reduce β by quarantining infected individuals, etc.
βSI – mI > 0
βSI > mI
βS > m
S > m/β

18. Transmission & Virulence
Transmission – passing a parasite or pathogen
from an infected host to another individual
Virulence – the host’s parasite-induced loss of fitness
Host Fitness
(Inversely related to
Pathogen Virulence)
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Production & spread of disease organisms from a host
(Transmission)

19. Darwinian (Evolutionary) Agriculture &
Darwinian (Evolutionary) Medicine
The application of modern evolutionary theory to
understand crop & livestock production (Darwinian Agriculture)
and human health & disease (Darwinian Medicine)
E.g., Why do new diseases continue to appear
in human, crop & livestock populations?
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20. Mind-Controlling Parasites
“Cocoon web” of spider
parasitized by specialist wasp
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Eberhard, William G Spider manipulation by a wasp larva. Nature 406:
Eberhard, William G Under the influence: webs and building behavior of Plesiometa argyra (Araneae, Tetragnathidae) when parasitized by Hymenoepimecis argyraphaga (Hymenoptera, Ichneumonidae). Journal of Arachnology 29:
Normal web
Bill Eberhard
Cain, Bowman & Hacker (2014), Fig ,
from Eberhard (2000) Nature & (2001) Journal
of Arachnology; photo of Eberhard from
headline_news/news/article.php?id=1015