Parasitism and Disease Lyme Disease Cycle in the UK

Evolution of Host-Parasite Interactions

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

Terms used in box model equations x - # of susceptible uninfected hosts y - # of infectious hosts (infected individuals) H = x + y - total number of hosts a - per capita host birth rate - assumed to be = for x and y b - natural (non-disease) host mortality rate r = a - b (intrinsic rate of increase of host) alpha  - disease induced mortality rate (pathenogenicity) beta  - transmission coefficient gamma  - rate of host recovery from infection

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

Global HIV data - 2014

Incidence of HIV in Africa

You’re never too old to need protection