Qualitative risk assessment of Japanese encephalitis virus introduction and transmission in Pacific Island countries and territories

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BACKGROUND: The emergence of Japanese encephalitis virus (JEV) in Australia raises questions about transmission potential across neighbouring Pacific Island countries and territories (PICs), where surveillance infrastructure and diagnostic capacity are limited. METHODOLOGY: We conducted a qualitative risk assessment of 22 PICs structured around three components: (1) 'release assessment', evaluating JEV introduction pathways based on geographic proximity, migratory waterbird flyway connectivity, and windborne mosquito dispersal potential; (2) 'exposure assessment', evaluating ecological transmission suitability based on vector and host competence and occurrence; and (3) 'risk characterisation', integrating release and exposure components to stratify countries into transmission risk profiles (endemic, epidemic, and low potential). Introduction potential varied across the region, with northern and western PICs having greatest connectivity to endemic regions. Ecological suitability for transmission was sufficient to support JEV transmission in 13 of the 22 PICs if introduced. Eight PICs were characterised as having endemic transmission potential (Papua New Guinea, which already has confirmed circulation, Solomon Islands, New Caledonia, Vanuatu, Fiji, Palau, Guam, and Northern Mariana Islands), based on diverse competent vector communities, high ardeid waterbird diversity, and moderate-to-high introduction potential. Five PICs (Federated States of Micronesia, Tonga, Samoa, Tuvalu, and Nauru) were characterised as having epidemic transmission potential, supported by pig densities capable of amplifying viral titres and sufficient vector diversity, but lacking ardeid populations necessary to sustain endemic cycles. The remaining nine PICs had low transmission potential due to limited vector and host communities. These risk stratifications are relative; the absolute probability of JEV establishment in any PIC remains uncertain. CONCLUSIONS: Distinct transmission profiles require differentiated public health responses, from sustained surveillance in endemic-potential settings to early detection and rapid outbreak response in epidemic-potential settings. Regional coordination through shared diagnostic networks and vaccine access pathways is critical to support preparedness before JEV becomes established across the Pacific.

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