Although Japanese encephalitis virus (JEV) is not currently established in the mainland U.S., its potential introduction poses a severe dual threat to swine production and public health. The virus gained global urgency in 2022 when an outbreak swept through Australian commercial herds, underscoring North America’s geographic vulnerability.
While humans and horses are dead-end hosts, pigs are primary amplifying hosts in the mosquito transmission cycle. Infected swine develop high levels of virus in their blood, fueling rapid transmission back to mosquito vectors and threatening breeding herds with severe reproductive losses, including abortions, mummified fetuses and stillbirths. With no approved swine vaccine currently available in the U.S., finding a rapid-response tool has become an industry priority.
Breaking the Amplification Cycle
To assess off-the-shelf options, a study funded by the Swine Health Information Center (SHIC) and led by Dr. Thomas Monath evaluated ImoJev—a recombinant live-attenuated vaccine widely utilized for human immunization in parts of Asia and Australia.
Researchers administered the vaccine to 6- to 8-week-old crossbred pigs before challenging them with JEV Genotype IV. Because growing pigs rarely exhibit overt clinical signs when infected peripherally, the trial measured viremia, or the concentration of virus in the bloodstream, as the primary indicator of protection.
The trial revealed that two intramuscular doses provided robust protection:
- Suppressed Viral Load: All pigs on the two-dose program were protected from detectable viremia, with only a single animal exhibiting a low-level breakthrough infection.
- Halted Transmission: Follow-up dose trials confirmed that vaccinated pigs maintained peak viremia levels far below the threshold necessary for feeding mosquitoes to acquire and transmit the virus.
- Broad Antibody Response: Neutralizing antibody levels against four JEV genotypes exceeded the 1:10 titer threshold recognized as the international correlate of protection in humans.
- Clean Safety Profile: The live-attenuated candidate produced no adverse safety signals and no detectable viral shedding in vaccinated animals.
The trial highlighted key physiological differences between species. Unlike human immunization programs that rely on a single subcutaneous shot, pigs required a higher antigen dose and a two-dose intramuscular schedule to achieve consistent, robust immunogenicity.
Next Steps for U.S. Preparedness
While the findings confirm ImoJev’s potential as an emergency intervention, researchers emphasize that critical hurdles remain before the vaccine could be deployed in U.S. herds.
Because wild viral introductions are unpredictable, further studies must verify cross-protection against all five JEV genotypes, as well as test efficacy in pregnant sow challenge models where clinical reproductive failure can be measured directly.
“While the vaccine proved to be less immunogenic in pigs than in nonhuman primates and humans, it could potentially be utilized as a veterinary vaccine for emergency preparedness against a JEV introduction into the U.S. swine population,” SHIC reports.
