Despite such low endemicity, the existence of JEV in Japan
can be clearly demonstrated by annual surveillance of
hemagglutinin inhibition antibody-positive domestic pigs;
these surveys reveal that a large proportion (up to 100%), of
the pigs become seropositive for the virus [2]. Although JE is
no longer endemic in Japan (due mainly to mosquito control
measures and immunization programs), the disease has the
potential to re-emerge if herd immunity thresholds decrease.
Circumstances where this could happen include, for example,
increases in the proportion of unvaccinated individuals within
a population, and/or increases in the frequency of infectious
mosquito bites, perhaps due to environmental factors such as
global warming or destruction of infrastructures. Since there
are no effective drugs against JEV, nor any alternate specific
treatment for the disease, a routine vaccination program for
disease endemic areas is currently the only control strategy
available.