Japanese encephalitis in horses in Japan.

The research article provides a detailed account of the historical and current status of Japanese Encephalitis (JE) in horses in Japan, including the development and utilization of vaccines to manage the disease.

Historical Context and Disease Status

• The article references that a disease resembling JE was already known to affect horses in Japan in the 19th century. Regular outbreaks of this disease were noted every summer along the banks of the River Chikugo in the north-western part of Kyushu.
• The causal virus of JE was first isolated from the human brain during a significant epidemic in 1935, affecting both humans and horses. Soon after, the same virus was identified in equine brains, affirming its linkage with the human case.
• In terms of disease manifestation, it is highlighted that seven horses were found to have JE without showing any symptoms. However, when taken to a different environment (Tokyo), all of them had subclinical infection over the summer and antibodies were detected in their bodies. In another instance, 40 horses sent to the River Chikugo area yielded two cases of clear disease while the rest remained healthy, despite 37 of them developing antibodies.
• Nationally, major JE epidemics in horses took place in 1948 and 1949, with deaths numbering 1,216 and 3,679 respectively. Since then, no large outbreaks have been recorded, although the incidence per year varies between 0 to 9 cases. Alongside this, it is also noted that the horse population in Japan has been on a steady decline.

Vaccination Development and Application

• Kii and his colleagues developed a formalin-inactivated vaccine using a 10 per cent suspension of infected mouse brain. Horses injected with 10-20 ml of this suspension were able to resist a strong challenge with an active JE virus administered intracerebrally.
• The first field application of the inactivated JE vaccine in horses was officially authorized in 1948. These vaccinations continued annually and enjoyed a wider application in horses six years before the vaccines were approved for use in humans.
• Early authorized vaccines were formalin-inactivated preparations made from 1% infected mouse brain or 10% infected chick embryo tissue or a combination of the two. Later in 1957, a “high potency vaccine” made from a 10% suspension of infected mouse brain became prevalent.
• The effectiveness of the vaccination was evaluated in 1948 with a survey conducted by the Ministry of Agriculture and Forestry. The survey found a significant discrepancy between the morbidity rates of vaccinated and unvaccinated horses, highlighting the effectiveness of the vaccination.

Summary

• The research provides a comprehensive history of the JE epidemics in Japanese horses and emphasizes the critical role of vaccines in controlling the disease. It underscores the importance of continued vaccination and monitoring for the ongoing management of JE in horses in Japan.