Mosquitoes are zoophilic (feed on animals – vertebrate hosts) – female mosquitoes get infected after feeding on viremia hosts.
After 9–12 days, incubation period – mosquitoes transmit disease to other vertebrate hosts. The average life of Culex mosquito is 21 days. Culex can fly over long distances of 4–5 kms. Epidemics occur during monsoon and post monsoon period because the vector density is high. However, in endemic areas, sporadic cases may occur throughout the year.
Japanese Encephalitis in man
Incubation period: 5 – 15 days.
All individuals bitten by infected mosquitoes do not suffer from the disease.
Ration of overt disease to in apparent infection = 1: 200 to 1: 1000.
Thus Japanese Encephalitis follows the pattern of iceberg phenomenon of disease i.e. the cases seen represent only a few individuals while those suffering from in apparent infections are many and are not approached by health functionaries.