Evolution of complex life cycles in helminth parasites

Abstract

The fundamental question of how complex life cycles--where there is typically more than one host-evolve in host--parasite systems remains largely unexplored. We suggest that complex cycles in helminths without penetrative infective stages evolve by two essentially different processes, depending on where in the cycle a new host is inserted. In 'upward incorporation', a new definitive host, typically higher up a food web and which preys on the original definitive host, is added. Advantages to the parasite are avoidance of mortality due to the predator, greater body size at maturity and higher fecundity. The original host typically becomes an intermediate host, in which reproduction is suppressed. In 'downward incorporation', a new intermediate host is added at a lower trophic level; this reduces mortality and facilitates transmission to the original definitive host. These two processes should also apply in helminths with penetrative infective stages, although the mathematical conditions differ.

Gastrointestinal Helminth Parasites Community of Fish Species in a Niger Delta Tidal Creek, Nigeria

“…Ingestion of larval helminth parasites by fishes is a frequent event due to the abundance and diversity of these tropically transmitted parasites in aquatic ecosystems [25][26][27]. Comparison of the results of this study with earlier studies is limited due to the paucity of the literature on brackish water/marine fish helminth parasites, particularly on helminth community studies of fish species of an entire ecosystem.…”

Evolution of complex life cycles in trophically transmitted helminths. I. Host incorporation and trophic ascent

“…Hyman's dilemma initially besets anyone considering helminth life histories, but the problem is not why there is more than one host -if the costs of generalism are sufficiently small (Parker et al, 2003), survival on ingestion by new potential hosts is always favourable (e.g. Smith Trail, 1980;Brown et al, 2001).…”

Alternative Life Cycle Strategies and Colonization of Young Anurans by Gorgoderina attenuata in Nebraska

“…Recently there has been an increased interest in attempts to understand adaptations involved in the evolution of complex life cycles of parasitic organisms (see Poulin and Cribb, 2002;Parker et al, 2003). These studies have derived mathematical models of how complex life cycles might have evolved from simple life cycles or conducted meta-analysis from a phylogenetic perspective on life cycle variation among different genera and families of parasites.…”