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

Sir Robert May 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