Abstract
Parasites that alter their intermediate host's behavior to favor its predation by the definitive host are known from a wide range of host-parasite associations. Recently, we found a new category of parasites, so-called 'hitchhikers,' unable to modify the behavior of their intermediate host but exploiting the same host spectrum, that gain benefits in transmission success from the infection of these behaviorally manipulated hosts. Because the probability of successful transmission by 'hitchhiking' depends on 1) the efficiency of the 'favorization' process of the debilitating parasite and 2) the effects of infection by the hitchhiker parasite on the host survival, we investigated these two aspects. In the laboratory, we showed that, in the absence of a predator (definitive host), there is no significant difference between the mortality rates of Gammarus insensibilis (second intermediate host) uninfected and infected by the debilitating trematode Microphallus papillorobustus. In the field, we showed that the hitchhiker trematode Maritrema subdolum does not significantly reduce host survival. These results suggest that the higher mortality rate of manipulated hosts in the field could be explained by the predation by the definitive hosts and that M. subdolum does not alter, through survival reduction, the efficiency of the favorization process.