Introduction:

Japanese encephalitis (JE) is a mosquito borne viral disease causing encephalitis in human as well as in horse, and abortion in pigs. The virus is maintained in nature by a complex cycle that involves pig as the amplifying host, ardeid birds as reservoirs and mosquitoes as vector. Human and horse are accidental hosts for JE virus but infection can lead to illness and death. Japanese encephalitis virus (JEV) is the most common cause of epidemic encephalitis and childhood mortality globally with an estimated 35,000 to 50,000 cases and 10,000 deaths annually. It was first isolated in 1935 from human brain in Tokyo, Japan and subsequently the virus was recovered from the brain of sick horse in 1937. In India, JE is a major pediatric problem and epidemics are reported from many parts of the country. The first clinical case of JE in India was observed in 1955 at Vellore, Tamil Nadu.

Japanese encephalitis is a major problem in Assam and is reported from almost all the districts of Assam. Outbreak of JE has been reported first from Lakhimpur district of Assam between July-August, 1989. Since then, the disease is constantly present in almost all the districts of Assam and every year many people die of Japanese encephalitis virus infection.

Structure of the Japanese encephalitis virus

JEV is a 40-50 nm enveloped virus possessing a positive sense single-stranded RNA (11 kb) genome. The virus belongs to the genus Flavivirus, family Flaviviriade. The structural genes are three in number and are involved in antigenicity, they are expressed on the virus coded by capsid protein and involved in capsid formation designated as nucleocapsid (C), pre Membrane (prM) and Envelope (E). Among the three structural genes, E gene is the most important and studied one. There are seven non-structural genes: NS1, NS2a, NS2b, NS3, NS4a, NS4b, NS5 and these are involved in virus replication.

Transmission cycle of Japanese Encephalitis virus

JEV is maintained in nature by a complex cycle that involves pig as the amplifying host, birds as reservoirs and mosquitoes as vector. Pig-mosquito-pig and bird-mosquito-bird are two basic cycles of JEV in nature. JEV is mainly transmitted by Culex tritaeniorhyncinus. Other important vectors include Cx. Vishnui, Cx. pseudovishnui, Cx. gelidus and Cx. Fuscocephala. Pigs play an important role in disease transmission to human.

Clinical symptoms in animals and human

Most human infections with JEV are asymptomatic and less than 1% of people infected with the virus develop clinical disease. In persons showing clinical symptoms, the incubation period is 5-15 days. Acute encephalitis is the most common clinical manifestation of JEVinfection which begins with sudden onset of fever, headache, and vomiting. Mental status changes, focal neurologic deficits, generalized weakness and movement disorders of those who survive. The case-fatality rate can be as high as 30% among those with disease symptoms.

The virus infects a number of animals such as swine, horse, chicken, duck, guinea fowl and reptiles in nature. Horses infected with JEV may develop fatal encephalitis with anorexia, lethargy and fever. In swine, infection is generally inapparent except stillbirth, abortion in infected sows and aspermia in boars.

Factors responsible for causes of Japanese encephalitis in Human and animals

Widespread distribution of Japanese encephalitis in pigs increases the risk of JEV infection in human population. Moreover, other causal factors like mosquito density and presence of water bodies that are breeding sites for mosquitoes also paly a role. When JEV infected birds come searching for food in standing water bodies, they are usually bitten by mosquitoes and get infected with the virus. When these infected mosquitoes bite to human beings, the virus gets transferred to human. Wild, water-living birds belonging to the family of Ardeidae are considered to be the main maintenance reservoirs, other wild birds as well as domestic birds such as chickens and ducks can also be infected with the virus. As a result of infection they can develop a viraemia and spread the JEV among the large geographical areas. In Assam the major agricultural product is rice and most of the farmers were using their land for rice cultivation. In most of the areas, the rice fields are close to human habitation and pig farms. Rice fields are the preferred sites for breeding of Cx. tritaeniorhynchus mosquitoes and during the rainy season, plenty of water logging in the paddy field and temperature facilitates the larval growth resulting in an increase in the density of the mosquitoes and ultimately transfer of JEV to pigs. The practice of paddy cultivation, proximity of houses to water bodies and suitable climatic factors were the most important environmental factors associated with several outbreaks in Northeast India.

Diagnosis:

Various serological techniques used for assay of antibodies to JEV in pigs are Neutralization test (NT), compliment fixation test (CFT) and Haemagglution inhibition (HAI). ELISA is a more promising immuno-diagnostic tool detecting antigen as well as antibodies.It is simple, rapid, sensitive, economic and is a suitable assay for routine laboratory work, and sero-epidemiological surveillance of human and animals. A quantitative ELISA is also used to detect antibodies against JEV in pigs. Molecular diagnostic techniques like polymerase chain reaction provides additional diagnostic option in comparison to the conventional methods. The RT-PCR has been successfully used for detection of JEV from clinical samples of human, swine and horse. Other molecular techniques like reverse transcription loop-mediated isothermal amplification (RT-LAMP) and real-time RT-PCR are very rapid and sensitive assays for detection of JEV in swine blood samples.

Treatment

There is no specific antiviral treatment for JE; therapy consists of supportive care and management of complications.

Prevention and control of Japanese encephalitis virus

• The best way to prevent occurrence of JE in endemic population is to vaccinate human population of all ages. Safe and effective JE vaccines are available to prevent the disease. WHO recommends to having strong JE prevention and control activities, including JE immunization in all regions where the disease is a recognized public health priority.

• Mosquito control strategies should also be applied on the pig farms, especially before the monsoon season.

• Awareness programmes will be the key for success against JE.

• Pig houses should be built away from the human habitation.

• Wider issues, including current agricultural practices, water management system and human behavior pattern need to be investigated.

• One health approach is needed for JE control in the endemic areas; all veterinary, medical, forest, municipality, fishery and public health institutions need to join hands together to effectively implement the control policy adopted.

Uttam Rajkhowa
JRF, Dept. of Animal Biotechnology
College of Vetreniray Science, AAU,
Khanapara, Guwahati - 781022