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In three-host life cycles, trematodes make use of food-chains or food

webs (Fig. 1.4). In doing so, trematodes invade a prey–predator system

where the relationship has undergone selection, i.e. the predator is very

likely to eat the species prey that is infected by the parasite. Qualitatively,

this gives the metacercariae the maximum probability of reaching the

correct host. However, quantitatively, everything will depend on the

number of prey items that are actually consumed by the target host, the

proportion of the prey population infected, the diversity of the predator’s

diet and the number of different species that feed upon the prey. For

instance, a metacercaria of an amphibian trematode that is carried by an

insect will certainly end in voles (microtine rodents), lizards and spiders

as well. Thus it is in the interest of the parasite that natural selection max-

imizes the probability of metacercariae being ingested by an acceptable

host. This is the case when favourization processes (see definition in

Combes, 1991, 2001) modify the morphology, colour or behaviour of the

vector (see Moore and Gotelli, 1990; Poulin, Chapter 12, this volume).

Little is known as to how ‘risky’ such a strategy is, because, when a selec-

tion process makes a vector more conspicuous in its environment, this

may increase its consumption by many predators and not only by suitable

hosts. For instance, the metacercariae ofClinostomum golvanicause the

formation of yellow spots on the backside of small fishes in tropical

ponds, which render infected individuals more obvious. This may

increase ingestion by fish-eating birds. However, nothing indicates that

all these birds are suitable hosts. One may only suppose that, if natural

Trematode Transmission Strategies 7

Foraging for the definitive host

USH DSH

Fig. 1.4. Transmission to the definitive host makes systematic use of food-chains.

Modification of morphology, colour or behaviour of infected USHs may occur, significantly

increasing their predation by the DSHs. USH, upstream host; DSH, downstream host.