SHIC-Funded Assessment Identifies US South as Highest Risk Region for JEV Introduction and Transmission

March 27, 2024 — Megan Niederwerder

Global swine disease monitoring identified Japanese encephalitis virus, a transboundary disease transmitted by Culex mosquitos, as an emerging threat to the US swine industry. The 2022 JEV outbreak in Australia heightened the need to define the risk this virus poses to US pork production, informing prevention, preparedness, and response efforts. In 2022, the Swine Health Information Center funded a study led by Dr. Natalia Cernicchiaro, Kansas State University, in collaboration with researchers at the USDA ARS Foreign Arthropod-borne Animal Diseases Research Unit, to update her group's 2018 qualitative assessment estimating the risk of JEV emergence and subsequent transmission in the continental US. Incorporating the latest scientific information and elements into the new study, the updated semi-quantitative assessment evaluated the risk of JEV introduction into seven US regions, its subsequent spread, and economic impact. Study results found the overall risk, reflecting the rate of introduction and economic impact of a JEV incursion, was non-negligible for the south, west, midwest, and northeast regions.

Although currently free from JEV, the US is considered a susceptible region for incursion due to multiple factors. Specifically, the availability of competent insect vectors, susceptible maintenance (avian) hosts, large populations of susceptible and amplifying hosts (domestic and feral pigs), extensive travel and trade to and from JEV-affected countries, as well as similar climatic and environmental conditions as endemic countries fuel susceptibility. To reassess the current risk of JEV emergence into the US, the study sought to update the 2018 assessment and add information regarding transmission post-emergence by incorporating the latest scientific information, updating assumptions, adding data sources, including the recent outbreak data from Australia, and considering regional elements contributing to the risk. Find the industry summary of the JEV risk assessment here.

To perform the updated semi-quantitative JEV risk assessment, the 48 contiguous US states were grouped into five regions based on climate, domestic swine production, and presence of feral swine. Due to their geographic isolation, Hawaii and Alaska were identified as their own respective regions. Using the risk framework, introduction of JEV into the seven US regions was evaluated from the current international regions of JEV distribution (i.e., southeast Asia, Australia, western Pacific rim). Many pathways of entry were considered, including infected vector eggs/larvae via imported goods, infected adult vectors via aircraft, ships, or shipping containers, and infected ardeid birds migrating from the current region of distribution.

Information utilized to update model parameters included data from an updated systematic review of the literature on vector and host competence for JEV. The updated review, assessing literature published from 2016-2022, reported a slightly higher number of articles that focused on host competence and susceptibility to JEV compared to vector competence. This shift in focus could be attributed to, in part, the realization of certain host species' (e.g., pigs, migratory and domestic birds) high susceptibility to JEV. Additional sources of information for the risk assessment models included global databases, scientific literature, and expert opinion obtained from the JEV advisory group and project collaborators. The JEV advisory group consisted of swine producers, veterinarians, entomologists, researchers and stakeholders from the US and Australia with knowledge on commercial swine production, JEV, and the 2022 Australian outbreak.

Summary output parameters calculated by this risk assessment included the rate of introduction, estimated epidemic size, and the overall risk estimate. The highest risk scores for the rate of introduction were deemed moderate and associated with the entry of adult mosquitoes in ships into the US south (AL, AK, DE, Fl, GA, KY, LA, MD, MS, NC, OK, SC, TN, TX, VA) and west (CA, ID, OR, WA) regions, and via mosquito eggs/larvae in imported tires into the northeast (ME, MA, NH, NY, VT) and west regions. The overall risk, which reflects the rate of introduction and economic impact of a JEV incursion into the region at risk, was very high for the south, moderate for the west, and very low for the northeast and the midwest regions.

In summary, based on the results from this risk assessment, the US south region had the highest risk of introduction and impact and should be prioritized for preparedness, in addition, the west and northeast regions should promote surveillance to facilitate early detection. This qualitative risk assessment is crucial to prepare for the threat of an emerging disease and protect the swine industry from suffering production and economic losses. Defining the US regions at greatest risk for JEV is an important step towards US prevention and preparedness as well as allocation of efforts for surveillance.

The Swine Health Information Center, launched in 2015 with Pork Checkoff funding, protects and enhances the health of the US swine herd by minimizing the impact of emerging disease threats through preparedness, coordinated communications, global disease monitoring, analysis of swine health data, and targeted research investments. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. For more information, visit http://www.swinehealth.org or contact Dr. Megan Niederwerder at mniederwerder@swinehealth.org or Dr. Lisa Becton at lbecton@swinehealth.org. 

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Megan Niederwerder, Executive Director, Swine Health Information Center