Risk assessment for Japanese encephalitis vaccination

Abstract

Japanese encephalitis (JE) is the most commonly diagnosed viral encephalitis in Asia. JE is caused by a virus called JE virus (JEV), a member of the genus Flavivirus, family Flaviviridae, and is transmitted by Culex mosquitoes. Neutralising antibody to JEV protects against JE, and can be induced by vaccination. JE is a potential threat to travellers to endemic areas, which are most of South and Southeast Asia and some Pacific Islands. The risk of JE can be expected to increase with increasing mosquito exposure and time spent in regions and seasons of active transmission. JE is very rare in travellers, but mortality is high, around 1 in 3, and there is a high rate of lasting neurological damage. JE can therefore be a profoundly life changing event for a traveller. Travellers and their healthcare providers need to balance the low risk of disease against the very high severity of disease if it does occur. In order to make an informed decision, the severity of JE disease should be carefully explained to travellers to Asia.

Keywords: Japanese encephalitis; risk assessment; travellers; vaccination.

Figure 1.

JE endemic areas. The geographical area endemic for JE is shown. Within this area, the potential for transmission can vary widely, for example with distance from the equator, altitude, and season. As a general rule, further south, the more the tendency for low level, year-round, transmission (and hence risk to travellers). Further North, the risk becomes more seasonal. JE outbreaks, however, are recognised to occur at all latitudes. Image courtesy of CDC.

Figure 2.

The enzootic cycle of JEV. The natural hosts of JEV in nature are wading birds (ardeids). JEV circulates in bird populations, transmitted by mosquitoes of the genus Culex. Mosquitoes infected with JEV after feeding on birds can bite, and infect, other birds, or other species such as pigs and humans. In some parts of Asia horses can also become infected. Pigs act as amplifying hosts because they exhibit sufficient viraemia to be onwardly infectious to the mosquito vectors and they live in close proximity to humans, resulting in a reservoir of JEV and increasing the risk of transmission to humans. Several studies have shown a wave of JEV infections in pigs before the emergence of human cases. Adapted with permission from: Tsai T, Yu Y. Japanese encephalitis vaccines. In: Plotkin S, Orenstein W, eds. Vaccines. 3rd Ed. Philadelphia, PA: WB Saunders, 1999: 672–710.