On March 29, the Swine Health Information Center and American Association of Swine Veterinarians hosted a webinar focused on the recent outbreak of Japanese encephalitis virus in pigs and people in Australia. Veterinarians from Australia along with U.S.-based experts shared field experience, epidemiology and potential risks for transboundary introduction of JEV into the United States.
The webinar offered a snapshot of the current situation and provided information to improve the identification and management of an unexpected outbreak. SHIC remains attentive to emerging swine disease issues around the globe to inform the U.S. pork industry for better preparedness and response activities; 183 participants from 21 countries joined the webinar live.
Kirsty Richards, a veterinarian with the SunPork Group in Australia, shared background information on JEV in their country during the SHIC/AASV webinar. A flavivirus, JEV is in the same family as West Nile virus, St. Louis encephalitis virus and Murray Valley encephalitis virus. The virus is maintained in a cycle between mosquitoes and vertebrate hosts, mainly ardeid birds such as herons, egrets and bitterns.
There is spillover of JEV to other species including pigs, humans, horses and other domestic animals. Humans and horses are considered dead-end hosts because they usually do not develop high levels of viremia, while pigs are an amplifying host. Richards noted human cases have been diagnosed, with some mortalities, in Australia as well as cases of reproductive and neurological disease in pig herds.
In February 2022, JEV genotype 4 was identified in New South Wales, Victoria, Queensland and South Australia. As of March 29, more than 50 pig farms across the eastern seaboard of Australia and in South Australia were diagnosed with JEV. As of March 31, the Australian Government/Department of Health is reporting 34 human cases, 24 confirmed and 10 probable.
Richards said Australia is addressing the outbreak with a one health approach, combining human and animal response protocols. Current activities at infected premises include mosquito trapping and control, vaccination of people working at and residing on those premises, and risk-assessed movement of pigs and semen.
Her colleague at the SunPork Group, Bernie Gleeson, shared clinical and pathological findings reported at the JEV-affected farms. They have observed apparently healthy sows with abortions and delayed farrowing, greater than 118 days. Aborted litters had fetuses with domed heads, subcutaneous oedema, ascites and arthrogryposis. Necropsy of aborted piglets revealed the absence of forebrain, hindbrain and cerebellum. Some affected piglets born alive were shaky with poor survival.
Leela Noronha, with the Foreign Arthropod-Borne Animal Disease Research Unit at USDA's National Bio and Agro-Defense Facility, in Manhattan, Kansas, said the United States has the necessary components for JEV transmission – competent insect vectors and susceptible vertebrate maintenance and amplifying hosts. Concern over JEV is based in part on the precedent set by West Nile virus which grew exponentially since being introduced in 1999. However, there is no active surveillance in the United States for JEV at this time. JE is a reportable disease in the United States.
Insect vectors of JEV are numerous though it is primarily transmitted by Culex spp. mosquitoes. In experimental conditions, a few North American Culex species and others have been shown to be competent insect vectors. Avian hosts of JEV can serve as mobile virus reservoirs which are typically asymptomatic. Noronha said wild and domestic North American species have demonstrated susceptibility experimentally.
JEV-affected swine play an important role in virus amplification. Wild pigs are wildcards that can impact transmission given they represent a rapidly expanding, free-range population of vertebrate hosts.
Experience shows that clinical presentation in swine varies by age and prior exposures. In adult swine, JE is primarily a reproductive disease, though affected adults may present with nonspecific transient febrile illness. Sows can exhibit abortion, stillborn, fetal mummification or subsequent production of weak piglets. Boars exhibit orchitis and infertility. In piglets, non-specific signs and wasting are evident as are neurologic disease and high mortality.
Natalia Cernicchiaro, Kansas State University, presented information on the risk of introduction of JEV in the continental United States. Maps illustrated the global expansion of JEV from Asia to northern Europe to Africa where competent vectors and hosts exist. The fact that the United States shares similar climate and environmental conditions with countries where JEV is epidemic and has competent vectors and hosts as well as increased travel and trade from and to JEV-affected regions since World War II, along with the fact that there is no active JEV surveillance program in the United States, make this region susceptible to a JEV incursion.
Cernicchiaro and a team of experts have conducted a qualitative and quantitative risk assessment addressing the likelihood of JEV reaching the United States. This risk assessment addressed:
- Probability of entry
- Probability of transmission
- Probability of establishment
- Extent of spread
- Likelihood of persistence
- Impact of disease
In the study, aircraft and cargo ships emerged as the most likely pathways of introduction for JEV via infected adult mosquitoes. The probability of introduction of JEV through infected adult mosquitoes via aircrafts was deemed very high whereas the probability of entry via ships/containers was considered of low to moderate risk. Although the probability of transmission was deemed of variable risk, from low to high, depending on the U.S. area of introduction and the season, under current conditions, the probability of JEV establishment in the United States is considered negligible because of the low availability of amplifying hosts (pigs) and the limited contact rates between infected mosquitoes and hosts in airports and seaports, areas at high risk of introduction of infected vectors.
Given the study findings, prevention and control should be directed towards the aircraft pathway and include aircraft disinfection in the countries of origin, or when arriving at the destination. Cernicchiaro also said the study results point to active surveillance programs on the east and west coasts of the United States as being warranted.
SHIC, launched by the National Pork Board in 2015 solely with Pork Checkoff funding, continues to focus efforts on prevention, preparedness and response to novel and emerging swine disease for the benefit of U.S. swine health. SHIC is funded by America's pork producers to fulfill its mission to protect and enhance the health of the U.S. swine herd.
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