Analogously as to how the highly complex vertebrate nervous system evolved from much simpler signaling systems, the immune system evolved from simpler defense mechanisms. Multicellular animals achieve defense against other, disease-causing organisms by means of a range of complex immune responses. Introduction: Parasites and the Evolving Immune System This means that not only will host avoidance of infection not develop, but the molecular arms race for increased immunological defense will also likely be halted in such cases. Here we advocate another alternative: If or when the benefit of increased prey acquisition outweighs pathogenicity or resource drain, there should be little or no selection on final hosts to minimize parasite infections. Current theories for the evolution of reduced pathogenicity in predatory final hosts primarily focus on parasites minimizing pathogenicity to increase their own reproductive output and/or avoid selection on host preference for non-infected prey types. There are exceptions to this pattern also among cases of parasite mediated trophic transmission, but major pathology in definitive hosts seems for the most part restricted to cases where manipulated intermediate hosts are of minor energetic importance. The birds get a free meal, but the presence of adult trematodes in the gut would seem to incur few if any negative consequences for the birds. In another well-documented case of parasite-mediated trophic transmission, trematode ( Euhaplorchis californiensis) infected killifish ( Fundulus parvipinnis) abandon normal caution and exhibit a range of behaviors which makes them more conspicuous to predatory birds. gondii are known, but infection in adult feline definitive hosts are most commonly asymptomatic and apparently innocuous. ![]() One example is provided by the protozoan Toxoplasma gondii, which conspicuously impairs predator avoidance in rodents. We will review some well-established host-manipulation models, where such a scenario seems likely. We explore here the proposition that an evolutionary driver in such cases is the energetic benefit to the final host, in addition to increased parasite fitness. This includes parasites that render intermediate hosts more susceptible to predation by final hosts. Traditionally the “extended phenotype” concept refers to parasites that manipulate host phenotype to increase parasite fitness. ![]() Department of Food Safety and Infection Biology, Faculty of Veterinary Medicine, Norwegian University of Life Sciences, Oslo, Norway.
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