JEV is maintained in an enzootic cycle in nature. The cycle is maintained between pig, bird and mosquito. Wild and domestic birds and pigs act as the primary host and play an important role in maintenance and amplification of the virus.

More specifically birds such as pond herons, egrets and other ardeid birds act primarily as maintenance host and pig acts as amplifying host.

Humans get the infection accidentally by the bite of the infected mosquito. Human to human transmission through mosquito is not observed in JEV infection. Hence, humans are considered as the dead end host.

Vector: Culex tritaeniorhynchus is the main vector for JEV transmission. Other species of Culex are Culex vishnui, C. pseudovishnui, C. gelidus, C. whitmorei, C. pipiens also act as the vector mainly in Southeast Asia. Other mosquito species of Mansonia, Anopheles and Aedes also act as the secondary vector. In Northern Australia, Culex annulirostris is the main vector. These vectors mainly breed in stagnant water, paddy field, ditches and pools.

Amplifying host: Pig plays the role of amplifying host in JEV transmission. The titer of virus in infected pig becomes very high which lasts for 2–3 days. This provides ample time for the mosquito to get the infection from the pig. Infected pigs remain asymptomatic with rare instance of fetal loss. The close proximity of pig to human dwelling makes the amplifying host even more important for human transmission. High birth rate in swine also provides a large number of susceptible populations every year.

Maintenance host: Both bird and pig play the role of maintenance and amplifying hosts. However, pig is more known as amplifying host and bird as maintenance host. The titer of virus in bird becomes sufficient enough to infect mosquito.

Blocking host: The vector mosquito Culex tritaeniorhynchus prefers to feed on cattle than pig’s blood. However, the titer of viremia in cattle is not sufficient enough to facilitate transmission of virus. Therefore, cattle play a negative or blocking role in JEV transmission.

Culex vectors are, however, opportunistic feeder and feed on pigs when the pig density is high in the locality.

Overwintering: This is thought to be the mechanisms of JE virus survival during the cold season in the temperate and subtropical endemic areas. The overwintering of JEV is evident by:

• Presence of sustained low level viremia in bats, snakes, frogs and lizards under experimentally simulated hibernation.

• Vertical transmission of JE virus in mosquito from one generation to next generation. This has been shown in experimentally infected mosquito through eggs at the time of fertilization and subsequently it passes onto larva, pupa and adult stages of mosquito. Virus can survive in the developmental stages during the adverse condition.

However, robust evidence of overwintering is lacking under natural condition.

 Role of ecology and environment in JE transmission: JE is primarily a disease of rural area. In most of the southeast Asian countries, rice cultivation is the main agriculture practice, presence of water logged paddy fields is a common scenario in rural agricultural settings which provides a favorable breeding place for the Culex mosquito vector and attracts the ardeid birds where farmer has to work in the field for hours together. This ecological setting brings all the required components for transmission together, where mosquito gets the infection from the host ardeid bird and transmits to human. Time taken for mosquito to become infectious to other host is around 14 days and known as extrinsic incubation period.

In a poor rural domestic set-up, presence of open drainage (which provides favorable vector-breeding site) and pig rearing near the human dwelling (amplifying host) are favorable for JEV transmission. The presence of infected mosquito in the vicinity, when it bites the amplifying host pig, the virus gets multiplied and the pig develops viremia. When mosquito bites the viremic pig it gets the infection and transmits to humans present in the locality (after the extrinsic incubation period) (Fig. 1).

Fig1. Japanese encephalitis virus transmission cycle

Epidemiological pattern: Two epidemiological patterns are observed.

 1. Epidemic pattern: This is observed in temperate areas with distinct summer seasonality. This is seen in northern Vietnam, northern Thailand, and northern India, Nepal, Korea, China and Japan.

2. Endemic pattern: This is observed in southern tropical areas, such as southern Thailand, southern Vietnam, southern India, Sri Lanka, Malaysia, Indonesia and Philippines, where JE virus transmission is endemic. Sporadic JE cases occur throughout the year in these parts with a peak during the rainy season.

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