The Japanese encephalitis is a mosquito-born flavivirus infection that causes encephalitis in human and equines and abortion in pigs, caused by Japanese encephalitis flavivirus (JEV) of the genus Flavivirus, a member of Flaviridae family. JE infection is prevalent in human and also in equines, as confirmed by serology and viral demonstration.
Epidemiology: JE is primarily an enzootic disease of rural areas and is endemic in mo st c o untr ie s o f Asia . Vir us multiplie s in the ve c to r mo sq uito e s (Cule x tritaeniorhynchus) and then it cycles with mosquito bites in vertebrate hosts, viz. swine, water birds (egrets and herons), as well as starlings and chicks. Asymptomatic or clinical infection in human and horses develop after biting of infected mosquitoes. Swine and water birds are natural amplifier of the virus after mosquito bites. Human and horses are considered dead end hosts.
Clinical signs: Three types of clinical syndromes have been described.In transient type of JE infection in horses, fever rises up to 40oC for 2-3 days, loss of appetite and most infected horses recovers within a few days.In lethargic type of JE infection horses may show fluctuating fever up to 40oC with disturbance of vision, paralysis of facial and ocular muscles with drooping of lips and ears. Petechial haemorrhages in visible mucous membranes are seen. In some infected horses, there are difficulties in mastication, swallowing of feed and paralysis of hind quarter.
In hypersensitive type of JE infection, high fever, aimless wandering, violent behavior, blindness, profuse sweating, muscle trembling, collapse and death have been reported. The mortality rate in clinically infected horses is about 5%, but may be as high as 40-70% in severe outbreaks. Death may occur in 3-7 days after the onset of fever.
Diagnosis: Isolation of JE virus from heparitinized blood, blood clots are done from affected horses. Brain tissues are collected from dead horses that showed clinical signs of encephalitis. Isolation procedures include the inoculation of brain tissues in unweaned mice and cell culture. The JE virus isolation is confirmed by fluorescent antibody test (FAT), VNT and RT-PCR. The serological assays include VNT, haemagglutination inhibition assay, complement fixation test, ELISA IgM capture ELISA (MAC-ELISA), plaque reduction neutralization test. Demonstration of four- fold rise in JE specific antibody by serology helps to demonstrate recent infection.
Prevention and control: The mosquito is best controlled through integrated management programmes involving a variety of measures including source reduction, chemical control and biological control. Inactivated vaccine prepared from mice brain is available for human in India. A formalin inactivated tissue culture vaccine is commercially available for horses and widely being used in some Asian countries (Japan, Hong Kong and Singapore).