ENDOCRINE AND NEUROENDOCRINE HOST-PARASITE RELATIONSHIPS

The relationships between parasites and hosts are complex, with many o f these interactions involving an amazing degree of biochemical coevol ution and communication. Hormones, neurohormones, and growth factors f igure prominently in these relationships. In vertebrate hosts, many pa rasites secrete hormones, neuropeptides, or cytokine-like molecules th at influence the host's physiological and immunological responses. Alt ernatively, the parasites secrete factors that alter the host's hormon e levels. Simultaneously, molecules emanating from the host strongly i nfluence the parasites' success. In some cases the host's hormones dir ectly influence the parasites; in others, effects are mediated indirec tly via the host's immune system. In invertebrates, the presence of pa rasites likewise has a major influence on the host's endocrine status and the normal suite of processes governed by hormones, including host development, metamorphosis, and reproduction. In insects, interaction s involving juvenile hormone and ecdysteroids are especially well-docu mented, and recent evidence suggests that neuropeptides may also be af fected by parasitism. Moreover, recent data suggest that in some speci es, such as snails, the host's nervous, neuroendocrine, and immune sys tems are functionally linked, similar to the complex interactions seen in vertebrates.