When should the worm turn? The evolution of host manipulation strategies in parasites

Many experimental studies have shown how parasites manipulate their intermediate hosts in order to increase their likelihood of transmission, through making them more obvious to their predatory definitive hosts. However, little attention has been paid to how manipulation strategies have evolved in so many different groups of parasites, and a unifying theoretical framework of parasite manipulation behaviour is lacking. Here, we present a game-theoretic model that considers when and how parasites should manipulate their host. We show that many mixed parasite strategies are possible, particularly where there is a low cost to manipulation, or if manipulation increases the likelihood of a parasite reaching its correct definitive host. Therefore it is possible for cheats or "hitch-hikers" to evolve, which do not manipulate the host themselves, but instead rely on another co-infecting parasite to pay the cost of manipulation. This may mean that the detection and quantification of manipulation in the field may be made difficult by the wide diversity of strategies that are possible. Furthermore, the consequences of such manipulation behaviour on the dynamics of the intermediate and definitive host populations and the prevalence of infection may be unpredictable. This framework reveals that many parasite manipulation strategies are possible, allowing the exploration of new facets of parasite behaviour.