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Parasitism and Disease
Lyme Disease Cycle in the UK
Evolution of Host-Parasite
Interactions
European rabbits as pests in Australia - 1938
Introduced pests in Australia –
red fox, rabbit, cat, pig, & goat
Parasite effect on host population
Parasite can cause direct mortality but then can only persist in
a large host population
Usually parasite lowers host reproduction, growth or survival
- often this effect is indirect by way of
1) lowers host stamina - more subject to predation,
competition
2) increases conspicuousness - predation risk increases
3) disorient host via neurological damage
4) alters host response to environmental stimuli
Fungal parasites alter insect behavior
Giant ant w/o and with fungus
Moose and White-tailed Deer
Deer – Moose brain worm interaction
Avian malaria occurs in areas below white
line on Island of Hawaii – highest incidence
between yellow and white lines
Hawaiian Crow – Extinct in Wild
I’iwi Honeycreeper – highly
susceptible to avian malaria
Akiapolaau Honeycreeper –
restricted to high elevation today
Amakihi Honeycreeper – shows
evidence of evolving resistance
Lord Robert May
Sir Roy Anderson
Spread of HIV in Russia
Dynamics of parasite populations
Most important parameter is basic reproductive rate of the
parasite, symbolized Rp
Rp will:
1) increase with increasing density of susceptible hosts - N
2) increase with increasing transmission rate beta β
3) increase with increasing fraction of infected hosts that
survive long enough to be infectious to other hosts symbolized by f
4) increase with increasing average time that host remains
infectious - symbolized by L
Dynamics of parasite populations
• We can also examine the reproductive rate of
infection (Ri) = average number of secondary
cases of infection generated by one primary case
in a population where almost everyone is
susceptible to infection
• Ri > 1 each infection has more than one
"offspring" - chain reaction of epidemic
• Ri < 1 infection cannot sustain itself
Incidence of HIV in Africa
You’re never too old to need
protection