Following the outbreak of Japanese encephalitis virus in Australia in early 2022, the Swine Health Information Center funded a study to reassess the risk of JEV introduction and establishment in the United States. Natalia Cernicchiaro, Kansas State University, is leading the efforts to build on a 2018 qualitative risk assessment to estimate the risk of emergence of JEV into the United States. The updated risk assessment will add information regarding transmission, establishment and spread by incorporating the latest scientific information, and elements contributing to the risk, to improve upon the previous work.
As part of these efforts to update the risk assessment, three objectives are in progress, including:
- Updating the systematic review of the literature on JEV (Oliveira et al., 2018) to inform risk assessment model parameters.
- Reassessing the risk assessment models to estimate the risk of JEV emergence into the United States by incorporating the latest scientific information.
- Soliciting expert opinion via questionnaire to members of an advisory board on JEV and swine production.
To support the updated risk assessment and provide guidance on model parameters, an advisory group has been formed consisting of U.S. and Australian swine producers, veterinarians, researchers and other stakeholders with knowledge on commercial swine production and JEV.
For the systematic literature review, a total of 282 abstracts (from 2016 to 2022) were retrieved from the initial search, with 99 abstracts being deemed relevant. Of the relevant abstracts, 37 were excluded due to no host and vector competence data or not being related to JEV at the level of the full text review. Data was extracted from a total of 62 relevant articles of which 20 (32.3%) were experimental, 41 (66.1%) observational and one (1.6%) displaying data from both experimental and observational study designs.
With regards to vector and host competence, 22 (35.5%) articles reported on vector competence, 28 (45.2%) articles reported on host competence, and 12 (19.3%) articles reported on both vector and host competence. Results for vector competence across all observational studies show the proportion of JEV infection across all 43-mosquito species ranges from 0 to 63.6%. Culex species were the most common species showing competence to JEV, with an 5.9% positive proportion of samples tested (337/5,744). Among the Culex species, Culex tritaeniorhynchus (319/3577, 8.9%) was identified as the most common species with positive samples. The host species in which mosquitoes mostly fed consisted of cattle, pigs (mostly wild boars) and birds (mostly chickens).
With regards to the host competence for JEV infection, various host species were identified to exhibit JEV infection and/or antibodies across all observational studies. This included birds (171/1,462, 11.7%), dogs (65/188, 34.6%), feral horses (177/242, 73.1%), monkeys (62/167, 37.1%), rats (44/96, 45.8%) and pigs (3,349/11,335, 29.5%), which include both feral pigs (101/183, 55.2%) and domestic pigs (3,248/11,152, 29.1%). Overall, the proportion of JEV infected hosts varied between 0 and 96.6%.
In pigs, the duration of viremia and duration of infectivity in host species across all experimental studies varied between 1 to 14 days and 4 to 10 days, respectively. New data is being gathered on other potential competent hosts, which may have a role on JEV spillover into amplifying hosts and/or mosquito vectors. All information on vector and host competence for JEV transmission is currently being used to inform parameters for the risk assessment models and insight for development of JEV models for transmission and spread.
In meeting with the advisory group, researchers redefined some of the original study questions and elicited expert opinion on certain parameters pertaining to JEV introduction, transmission and establishment. Based on the advisory group recommendation, Alaska and Hawaii are being incorporated as additional regions considered in the risk assessment models.
Advisory group members have also completed a questionnaire designed by Cernicchiaro and her team to facilitate successful achievement of the projects' objectives. The questionnaire was divided into three sections, including
- Assessing the different JEV introduction pathways to the United States.
- Assessing the different paths or mechanisms of JEV transmission after a potential incursion while considering regional differences.
- Assessing the prioritization of factors for evaluating the role of pigs (domestic and feral) in the transmission of JEV in United States.
For each of the factors evaluated, advisory group members were asked to assess the level of risk (evaluated in risk categories from very low to very high) and the level of uncertainty about the risk scoring (low to high).
As the reassessment proceeds, updates will be provided for preparedness and prevention efforts for the health of the U.S. swine herd regarding JEV.