JEV: Japanese Encephalitis Virus

Virology was one of my favorite classes in vet school. I’ve always found it fascinating how such unimaginably small strands of DNA or RNA can impact humans and animals. One of the things that drew me to focus on swine medicine was the vast number and diversity of viruses affecting swine. I was also drawn to the data, knowledge, and information being shared in the industry to minimize, reduce, and eliminate those viruses of importance. From a practical control and elimination standpoint, the swine industry is leaps and bounds ahead of human healthcare when dealing with some common viral respiratory disease challenges. I will concede that medical doctors lack access to the “Guinea pigs” like we do to conduct challenge studies. In the U.S., we are dealing with several viruses, including PRRSv, PCV2, IAV, PED, PDCoV, TGE, PPV1&2, and SVV. Then there are those foreign animal diseases (FADs) that are of top concern, including FMD, ASF, CSF, pseudorabies, etc. Another FAD that has popped up recently in the news is Japanese Encephalitis Virus (JEV). There has been some interesting information in the spotlight recently after the detection of the virus in Australia last year. Concerning that experience, the following information was gleaned from numerous sources. There are a few websites with more details about FADs and JEV, including the USDA and WHO.

Top ten key points for JEV:
1. Japanese Encephalitis Virus is related to West Nile Virus, Dengue, and yellow fever. It is a single-stranded RNA Flavivirus, making it unstable in the environment outside of a host and easy to “kill” or inactivate. It is endemic in Southeast Asia and the Western Pacific.

2. JEV plays a significant role in human health, and where endemic, it is the primary cause of encephalitis in humans. Most infections in humans are asymptomatic. It also infects horses similarly, causing encephalitis and resulting in death.

3. Where JEV is endemic, mosquitos are the primary transmission vector to water birds (like Heron), who serve as the reservoir host. Pigs are the primary amplifying host, as once infected,
they can infect other mosquitoes that feed on them. Horses and humans are the main dead-end hosts, meaning after horses or humans are infected, they tend not to re-infect mosquitos to continue the cycle. Cattle, bison, and goats are also dead-end hosts.

4. Clinical signs of infection in pigs include increased reproductive failure or issues such as stillborns, mummies, weakly born piglets, and increased central nervous system (CNS) disease. Young
pigs may show CNS disease. Older pigs tend not to display many symptoms, yet as an amplifying host, they can re-infect mosquitos that feed on them. This results in increased levels of the virus in the environment. In the Australian outbreak, four affected herds found up to a 9% reduction in pigs weaned.

5. Swine vaccination has historically not been commercially effective in endemic areas, partly due to maternal antibodies interfering with vaccination. Therefore, most vaccine development has been focused on humans. Australia doesn’t currently have a commercially available vaccine but is working on developing one specific to the current strain associated with this outbreak. In 2022, Australia reported that 84 sites had been diagnosed with JEV. It was estimated that 60% of the industry had been impacted, resulting in lower supply in the fall. Through epidemiological
surveillance, the outbreaks were associated geographically with a river basin that flooded multiple times before the outbreak.

6. There were 32 confirmed cases in humans, but only one with pig exposure.

7. How did JEV get into Australia? Currently, the possibilities discussed include that it arrived via water bird migration, bats, wind-dispersed mosquitoes, or mosquitoes hitching a ride on a plane or in shipping.

8. Finding JEV, ASF, or FMD in a pig in the U.S. would make quick identification extremely important due to potential adverse effects on our export markets. Interestingly, it is thought that JEV could have been circulating in the pig population for over a year before detection in the Australian outbreak.

9. The sooner this or any other FAD is identified, controlled, and contained, the faster and easier it will be to stamp out and regain negative status for the country. This would enable a return to
regular export operations if any limitations were enacted.

10. Preparation for responding to any FAD, including JEV, is an ongoing activity at the local, state, and national levels. Biosecurity, first and foremost, starts at the farm level using every feasible method to keep bugs out.

Keeping FADs out of the U.S. is a top priority. It requires a team effort from all parties involved, including each producer and their herd veterinarian, state government officials, and federal entities. Prevention and preparation are paramount. In vet school, the saying was, “when you hear hoofbeats think horses, not zebras.”

Being familiar with those “zebra diseases” is useful in our modern world. With a global economy and environment, when “something just doesn’t feel quite right in the barn,” I would highly encourage a producer to ask questions and get your herd vet involved.

Stevenson, G. W., Schwartz, K. J., Karriker, L. A., & Zimmerman, J. J. (2012). Diseases of swine. John Wiley & Sons.
Sundberg P. JEV Symposium: Australian experience informs US preparedness. AASV.
November 2, 2022. Accessed January 3rd, 2023.