interaction of food and parasites. Overwinter survival was 56% in control animals
(unfed and normal worm burdens). In unfed but parasite-reduced animals survival
was 60%, while survival was 73% in untreated but fed animals. However, in fed and
parasite-treated animals survival reached 90%.
Field experimental studies of the effects of parasites are rare, and most information
comes from descriptive studies where animals dying in poor nutritional condition
also have high parasite burdens. Studies of the periodic mortality of Soay sheep on
St Kilda indicate that animals were emaciated and malnutrition was the cause of death.
However, dead animals also had high nematode counts, indicating an interaction
between food and parasites (Gulland 1992).Parasites and pathogens can increase a host’s vulnerability to predation by changing
its ability to escape the predator. Snowshoe hares with high nematode burdens in
spring were more likely to be caught in live traps than those with lower worm
burdens (Murray et al. 1997). Wood bison (Bison bison) populations may be held at
low densities by predators only when there is a high prevalence of diseases such as
tuberculosis and brucellosis ( Joly and Messier 2004).
In the red grouse (Lagopus lagopus) there is a complex interaction between the
nematode Trichostrongylus tenuisand predators such as red fox (Vulpes vulpes). These
game birds, being ground nesters, are vulnerable to predation while incubating eggs.
Normally grouse emit scent in the feces that can be detected by trained dogs (and
presumably by foxes) up to 50 m away. However, during incubation female grouse
stop producing cecal feces and dogs cannot locate the birds more than 0.5 m away.
The parasite T.tenuisburrows into the cecal mucosa and disrupts its function so that
the bird cannot control its scent (Dobson and Hudson 1994). Hudson et al. (1992)
demonstrated experimentally the effect of these worms on the detectability of incu-
bating red grouse by dogs. They treated some birds with anthelmintic drugs to reduce
their worm burdens. Trained dogs found many fewer treated birds than untreated
birds with naturally high worm burdens (Table 11.1). Thus, parasites increased the
susceptibility of grouse to predation.
Parasites may also increase predation on hosts by altering the behavior of the host
either as an incidental consequence of debility or as a specific adaptation to enhance
transmission; the latter occurs when the predator is the final host in the life cycle
of the parasite. In the former case a disease that causes debility of the host makes
it more conspicuous to predators through abnormal behavior and especially flightPARASITES AND PATHOGENS 185
11.6.2Interaction of
parasites with
predators
Number foundYear Treatment Dog scenting Human search1983
Treated Low worm burden 6 7
Untreated High worm burden 37 10
1984
Treated Low worm burden 9 7
Untreated High worm burden 29 7From Hudson et al. (1992).Table 11.1Red grouse
nests found by dogs
(scent) and random
search (researchers)
with respect to
treatment of the female
with an anthelmintic to
reduce burdens.