Trematode Transmission Strategies

Trematode Transmission 1

Strategies

Claude Combes,

1

Pierre Bartoli

2

and André Théron

1

(^1) Laboratoire de Biologie Animale, UMR CNRS-UP 5555, Centre de Biologie
et d’Ecologie Tropicale et Méditerranéenne, Université Perpignan,
52, Av. de Villeneuve, 66860 Perpignan cedex, France;^2 Centre
d’Océanologie de Marseille, UMR CNRS 6540, Campus
Universitaire de Luminy, 13288 Marseille cedex 9, France

Introduction

Trematodes all have a heteroxenous life cycle. Typically, they exploit

three hosts in succession; the first host is always a mollusc and the third

one is always a vertebrate, whereas the second (herein called the vector)

can belong to nearly any group of metazoans. In some particular cases, the

life cycle comprises only two hosts (the mollusc and the vertebrate) and

exceptionally only one (the mollusc). One of us (Combes, 1991, 2001) pro-

posed using the term upstream host (USH) for the host where the parasite

originates and downstream host (DSH) for the one that the parasite goes

to, at any step of the life cycle.

Although life cycles of trematodes seem extremely diverse at first

glimpse, with few exceptions they exhibit three fundamental strategies:

1. When they leave the vertebrate host that harbours the adults, trema-
   todes follow a complicated route through the ecosystem(s), with at least
   one intermediate host and usually two, with two different free-living
   stages transferring between hosts.


2. Along with classical sexual reproduction, which takes place in the
   vertebrate host, there is an additional phase of multiplication that occurs
   in the mollusc host.


3. In all three-host life cycles, the transmission between the second
   intermediate host (in fact, a vector) and the definitive host involves a link
   in a food-chain: the third host eats the second.

CABInternational2002.The Behavioural Ecology of Parasites
(eds E.E. Lewis, J.F. Campbell and M.V.K. Sukhdeo) 1