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Parasites feed on living hosts and do not always kill them
Endemic parasites cause chronic impacts on a host – low-level, persistent, nonlethal debilities or
diseases
Other parasites cause epizootic disease (animals) or epidemic disease (humans)
Death from parasites is unusual and occurs only if:
o Serious illness facilitates transmission (rabies)
o The parasitedoes not depend on the infected host for survival and complete its life cycle
after the host dies
o The pathogen moves through host populations over a wide geographic area and over a
long period of time
Disease can be drastic, but is more often subtle with impacts to affect natality or normal
movement
Parasites can have important indirect effects on their hosts by
o Responding to the nutritional state of the host and becoming pathogenic or otherwise
increasing vulnerability to predation
o Altering the behavior of the host
Pathogenictity is influenced by the nutritional statues of the host (Keymer and Dobson 1987) –
low protein increased susceptibility to infection
Murray et al 1997 – snowshoe hare supplemented with high-quality food during winter had
better survival than those not supplemented although both popn’s were treated with
antihelminthics - there was a synergistic effect on survivorship:
o 56% survival for control group
o 60% survival for dewormed but not fed group
o 73% survival for fed but not dewormed group
o 90% survival for fed and treated group
Soay sheep – interaction between nutritional condition and parasites (Gulland 1992)

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Interactions of parasites with predators
o Parasites affect a host’s vulnerability to predation by changing its escape ability (Murray
et al
...
1992a)

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Parasites have three types of impact on host communities
o Competition
 Affected when parasites have a greater effect on one of the competitors
(northward spread of white-tailed deer and brainworm wiping out moose and
caribou)
o Reducing predation
 Can reduce efficiency of predators so that the prey increase at the expense of
their competitors (alter the effect of apparent competition)
o Increasing prey susceptibility
 Can increase the availability of prey for a predator and so alter competitive
relationships between predators



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