Causative Agent of Japanese Encephalitis
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The Japanese Encephalitis virus has been recovered from about 30 species of mosquitoes from 5 genera. C. Tritaeniorhynchus thrive in rice fields. They inhabit elevations below 1500 meters and usually bite at dusk, dawn and extreme hours of night. The important factors which ensure spillover of the disease to humans are density and longevity of infected mosquito populations, existence of amplifying hosts like pigs and human exposure to infected vectors. Mosquitoes are zoophilic (Feed on animals – vertebrate hosts). Female mosquitoes get infected after feeding on viraemia hosts.
Non Human Vertebrate Hosts
Pig to pig transmission ensures virus multiplication. Bovines largely attract mosquitoes but do not amplify the virus and hardly demonstrate viremia. Horses may manifest encephalitis. Ardeid birds including herons and egrets are also involved in transmission. Transovarial transmission may occur within mosquito population.
The period of viremia in the pig is 1–2 weeks. It has been shown that porcine sero conversion precedes human infections by 2–3 weeks and that vector density enhancement correlated with increased sero conversion in pigs and virus amplification in the pig occurs in the last quarter of the year. Serological studies in several vertebrates in the endemic areas using HI tests for antibody demonstrated that Pigs exhibit highest sero conversion rate and titres. This is substantiated by the fact that outbreaks are evident during and after the monsoons, in the North from June to November and from September to January in the south.
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