Parasitism

Photo of Cordyceps fungus growing out of the insect it parasitized from Wikimedia Commons

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)

Photo of human head louse from Wikimedia Commons

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

Exploitation (+/- or antagonistic interaction)

PathogensParasites that cause disease

(which manifests as pain, dysfunction or death)

Photomicrograph of an Ebola virion (a complete virus particle) from Wikimedia Commons

Brood Parasites

Rely on other organisms to raise their young

Photo of Reed Warbler & Cuckoo chick from Wikimedia Commons

Kleptoparasites

Thieves (many are therefore competitors)

Photo from http://antediluviansalad.blogspot.com/2012_09_02_archive.html

Symbionts

Live in close association with their hosts; amensal, commensal, parasitic, or mutualistic

Photomicrograph of endophytes in a plant from http://www.entm.purdue.edu/turfgrass/research.html

Some mutualistic defensive symbionts protect their hosts against parasitic symbionts

Co-cladogenesis & co-speciation

Photos from Wikimedia Commons; figure from Light & Hafner (2008) Systematic Biology

Two associated lineages (especially symbionts) diverge together, and potentially co-speciate

Dipodomys merriami

Fahrenholzia pinnata

Horizontal – between individuals that are not linked by a parent-child relationship

Vertical – from mother to offspring

Horizontal vs. Vertical Transmission

Mother

Son

Other non-offspring member of

the population

Daughter

or

Other non-parentmember of

the population

Vertical

Horizontal

Horizontal

Cospeciation Host switch Duplication

Missing the boat Extinction

Host

Failure to speciate

Parasite

Coexistence

Which are most likely under strictly vertical transmission?From J. Weckstein (2003)

*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

Photo of T. castaneum flour beetle from Wikimedia Commons; *original idea from Janzen (1980) Evolution

Macroparasite vs. Microparasite

Catagories based on function rather than taxonomy or phylogenetics

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.

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.

Parasites can influence individuals, populations, interactions between species, communities &

ecosystems

Cain, Bowman & Hacker (2014), Fig. 14.17; after Park (1948)

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)

Photo of from http://www.pbase.com/wildbirdimages/image/129932709; fig. from Hudson et al. (1998) Science

“Proportion of grouse treated… No treatment, dashed line; 5%, dotted line;

10%, thick solid Line; and 20%, thin solid line.”

Parasites can influence individuals, populations, interactions between species, communities &

ecosystems

Red Grouse are naturally infected by a nematode

parasite; antihelminthics cure the infected hosts

Modeling Microparasite Disease Dynamics

The SIR Model – a compartmental model (population is sub-divided into compartments) for epidemiology

Susceptible hosts (S)

Infected hosts (I)

Recovered & Immune hosts (R)

Birth

Death

a a a

b α + b b

β v

After Anderson & May (1979ab) & May (1983)

transmission recovery

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

Susceptible hosts (S)

Infected hosts (I)

Recovered & Immune hosts (R)

Birth

Death

a a a

b α + b b

β v

After Anderson & May (1979ab) & May (1983)

transmission recovery

Modeling Microparasite Disease Dynamics

dI/dt = βSI – (α + b + v)I dI/dt = βSI – mI

Susceptible hosts (S)

Infected hosts (I)

Recovered & Immune hosts (R)

Birth

Death

a a a

b α + b b

β v

After Anderson & May (1979ab) & May (1983)

βSI =Disease transmission rate

m =combined death & recovery rate

transmission recovery

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?

Transmission & Virulence

Production & spread of disease organisms from a host

(Transmission)

Host Fitness

(Inversely related to Pathogen Virulence)

Transmission – passing a parasite or pathogen from an infected host to another individual

Virulence – the host’s parasite-induced loss of fitness

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?

Mind-Controlling Parasites

Cain, Bowman & Hacker (2014), Fig. 14.22, from Eberhard (2000) Nature & (2001) Journal of Arachnology; photo of Eberhard from http://www.stri.si.edu/english/about_stri/headline_news/news/article.php?id=1015

Bill Eberhard

Normal web

“Cocoon web” of spider parasitized by specialist wasp