Japanese encephalitis (JE) – so called as the deadly disease broke out first in the 1940s in Japan but has been contained since – is again claiming lives in Uttar Pradesh, Bihar and Assam in pockets in northern and eastern India.
Grey herons were found to be the avian reservoirs, rice field breeding mosquitoes the vector and pigs the amplifiers. The Japanese produced a vaccine and vaccinated all pigs. The infection ended. It never appeared again in Japan.
The first big JE epidemic in India occurred in North Arcot district of Tamil Nadu in 1957-58, in Burdwan and Bankura in West Bengal (1976) and in Tirunelveli in Tamil Nadu (1978). One can expect JE epidemics in India year after year in regions subjected to prolonged drought-like conditions followed by heavy rains. This causes heavy mosquitogenic conditions.
In India, there are ‘pond herons’ (Ardeola grayii) and ‘cattle egrets’ (Bubulcus ibis) which share the similar niche. Pigs circulate high concentration of the virus and can infect a large number of mosquitoes.
The mosquitoes which pick up the infection are mainly zoophilic (i.e. they prefer to feed on animals including birds). The cattle can circulate virus but do not play any significant role in the transmission.
When the mosquito population increases enormously following heavy rains and floods, an odd mosquito — out of thousands — bites man.
Unfortunately, an impending JE epidemic is recognized by the authorities only when the first human case is repoted. It is too late.
It is now accepted that once JE infection is detected in man, there is no use carrying out any vector control measure.
This is because while in the case of malaria or filariasis a mosquito can pick up the infection from one man and transmit to another, this does not happen in JE.
The chances are very remote for the vectors of JE to transfer infection from man to man.
After getting a blood meal, the JE mosquito must wait till its egg-laying is complete – which takes 7-10 days – before it can bite another man. So the time to launch vector control measures is when the conditions are ripe for vectors to start biting humans, not after detecting the first case.
In a country like India, where drought and floods occur periodically or with regular frequency, the occurrence of JE can be forecast. Large water pools ideal for breeding of vectors of JE are conducive for nesting and breeding of Ardeid birds such as egrets and herons.
The mosquitoes are there, the avian hosts are there and in the adjacent villages cattle, poultry and pigs are present. JE will strike humans when all these factors co-exist.
One can therefore predict with reasonable accuracy an impending epidemic. In fact, research on methods of predicting an epidemic should be a continuous process. Instead the authorities prefer to carry out fire-fighting operations during epidemics.