Prevalence and Transmission Cycle of Avian Pathogens in the Isolated Oceanic Islands of Japan

Avian haemosporidian parasites and avian pox virus (APV) are well-known pathogens for their impact on avian populations, especially in oceanic islands where introduced pathogens show strong virulence for endemic and naïve birds. The Bonin Islands are a group of oceanic islands 1000 km south of Tokyo. Like the Hawaiian Islands, there are many endemic and endangered species as well as introduced species, which have greatly affected the native avian fauna. However, pathogens in wild birds of this archipelago had not been investigated. In this study, we investigated the prevalence of avian haemosporidian parasites and APV among birds and mosquitoes in this unique ecosystem of the Bonin Islands. From 2014 to 2020, 524 birds of 39 species either rescued, deceased, or caught by mist-netting were sampled. APV-like lesions were sampled from nine birds. 262 mosquitoes were collected by sweeping nets or CDC traps. All samples were tested via PCR for haemosporidian infection, and lesions were tested for APV.209 birds (39.9%) of 11 species were positive for haemosporidian parasite DNA, and all three parasite genera were detected. Prevalence was particularly high for Plasmodium elongatum (pGRW06) and Prelictum relictum (pGRW04). The former was detected from both resident birds and mosquitoes, suggesting local transmission. An introduced species, the warbling white-eye (Zosterops japonicus), had a particularly high prevalence of pGRW06 (68.3%) and may be a reservoir of this lineage. Both APV and Plasmodium spp. were detected from all APV-tested birds, suggesting that these two pathogens may be transmitted simultaneously via mosquitoes. The presence of avian haemosporidian parasites and APV was confirmed in the Bonin Islands for the first time. However, the virulence and origin of these pathogens remain unknown, and many bird species are still understudied. Further investigations are required to contribute to the conservation of this unique avifauna.