Host-Switching: How It Starts

Abstract

A parasite depends, during its entire life or at least part of it, on other organisms, but parasites often "jump" from one host species to another and may be able to colonize new host species. The chances of parasite spillover, the first step in such a host switch, may be influenced by factors such as the local ecosystem, community composition, and modes of transmission, among others. In Galapagos, for example, seabirds show a spatially clustered community, with several species that are related and/or nest in close proximity, a seemingly perfect scenario for host switching. However, only one instance of a straggling ischnoceran louse and larva (indicating successful reproduction on the new host) was found on a different host species, suggesting that the specifics of ectoparasite body size and host feather interbarbular space may prevent lice from readily switching hosts. On the other hand, the haemosporidian parasite, Haemoproteus multipigmentatus, of the Columbiform-specific sub-genus Haemoproteus, was found in significant numbers of Galapagos passerines. The spillover events occur where Galapagos doves (Zenaida galapagoensis), a widespread endemic, are present or abundant enough; however, there is no evidence of parasite development in the passerine birds. Thus, the Galapagos archipelago provides an exceptional host-parasite system to investigate details of parasite spillover and its implications for host health and survivorship.