Veterinary Epidemiology and Zoonosis

JAPANESE ENCEPHALITIS

Synonym

• Japanese B encephalitis

Type of zoonosis

• Metazoonosis type I 

Definition

• Japanese encephalitis (JE) is one of a group of mosquito-borne virus diseases, affects the central nervous system characterized by rapid onset, hyperpyrexia and cerebral and meningeal manifestations, and even death.

Etiology

• JE is caused by the Japanese encephalitis virus (JEV), an arbovirus (arbovirus is short for arthropod-borne virus).
• JEV is an enveloped virus (Genus Flavivirus, Family Flaviviridae). It is a positive sense single stranded RNA genome is packaged in the capsid, formed by the capsid protein.
• JEV is antigenically associated with West Nile virus and St. Louis encephalitis virus.
• The outer envelope is formed by envelope (E) protein and is the protective antigen. It aids in entry of the virus to the inside of the cell.
• JEV is maintained principally by biological transmission between mosquitoes and vertebrates.

Reservoir and incidence

• Domestic pigs and wild birds (herons and egrets) are reservoirs of the virus.
• Human, cattle and horses are dead-end hosts.
• Swine acts as amplifying host and has very important role in epidemiology of the disease.
• Japanese encephalitis can be a risk to travelers to rural areas where the disease is common.
• The most important mosquito vector is Culex tritaeniorhynchus, which feeds on cattle in preference to humans; it has been proposed that moving swine away from human habitation can divert the mosquito away from humans and swine.
• The mosquitoes reproduce in paddy fields and natural water bodies of fresh water.
• Japanese encephalitis is an endemic disease in tropical areas. Epidemics occur during rainy season and a seasonal disease in temperate regions. 
• JE is most common in Asian countries, with 30,000 - 50,000 cases reported annually.
• Case-fatality rates range from 0.3% to 60% and depends on the population and on age.
• People living in rural areas where the disease is common.
• In India, the earliest evidences of occurrence of JE are from Nagpur (Maharastra) and Chingelpet (Tamil Nadu) in 1952. Over the next decade, several cases of encephalitis presenting at the Christian Medical College, Vellore were identified to be JE. Around the same time, extensive serological surveys in South India revealed widespread Flavivirus activity.
• The first major epidemic of JE from India was reported from the Bankura and Burdwan districts of West Bengal in 1973. Since then, repeated annual outbreaks have occurred especially in the post monsoon, high mosquito season in West Bengal, Bihar, Assam and the North East, Uttar Pradesh and the 3 southern states of Tamil Nadu, Karnataka and Andhra Pradesh.

Transmission

• The extrinsic incubation period is about 14 days.
• JEV is transmitted by a biological vector, Culex tritaeniorhynchus that lives in rural paddy fields and stray pigs roaming regions.
• Breeding of this mosquito is very high in rice fields and stagnated water bodies. However, person to person transmission is not been reported.
• Inoculation of this virus in to the susceptible person leads to invasion in to the central nervous system, including the brain and spinal cord.
• Spread of JE is enhanced by several socio-economic status of rural people: low cot houses with open windows, open drainage, semi-urban and dwellers. 
• Transplacental spread has been reported in human beings.

Disease in animals

• Cattle and horses: Fatal encephalitis
• Pigs: Asymptomatic, except in pregnant sows, it causes abortion and fetal abnormalities.  

Disease in man

• Incubation period ranges from 4 to 14 days.
• The majority of human infections are asymptomatic, only 1 in 500 - 1000 develops JE.
• The onset is rapid and usually starts as a flu-like illness, with fever (between 38 and 41°C), chills, tiredness, headache, nausea and vomiting,  sometimes swelling of the testicles in male. Cerebral and meningeal manifestations are stiff neck, convulsions in children, confusion, disorientation, delirium and finally progressing to coma in humans.
• The illness can progress to a serious infection of the brain (encephalitis) and case fatality is about 30%, mostly in children. Among the survivors, 30% may have serious brain damage, including paralysis.
• JE affects the ear, particularly the cochlea due to neurological involvement.
• Life-long neurological defects such as deafness, psychic and motor deformity as sequealae.  

Diagnosis

• Based on isolation and identification of JEV.
• Demonstration of antibodies in serum and CSF (cerebrospinal fluid) by serological methods (IgM capture ELISA).
• Haemagglutination inhibition test may also be used for diagnosis.
• Paired sera are very useful in diagnosis of JE.

Treatment

• Supportive treatment with clinical management is highly required to the infected person.
• Raised intracranial pressure due to JE may be managed with mannitol.
• Affected person do not need to be isolated, because, there is no person to person transmission.

Prevention and control

• Prompt reporting of JE incidence to the local authority. Recovered patients from JEV confer life-long immunity.
• Evironment and personal hygiene.
• Food and water sanitation.
• Prompt treatment.
• Use of mosquito net and mosquito repellents.
• Vaccination
  • Vaccination of the susceptible population comprised of children.
  • All current vaccines are based on the genotype III virus.
  • A formalin-inactivated mouse-brain derived vaccine was first produced in Japan in the 1930s.
  • The high cost of the vaccine, which is grown in live mice, means that poorer countries have not been able to afford to give it as part of a routine immunization programme.
  • Neutralizing antibody persists in the circulation for at least two to three years and perhaps longer.
  • The total duration of protection is unknown, but because there is no firm evidence for protection beyond three years, boosters are recommended every two years for people who remain at risk.
  • There are a number of new vaccines under development.
  • The mouse-brain derived vaccine is replaced by a cell-culture derived vaccine that is both safer and cheaper to produce. China licensed a live attenuated vaccine in 1988 and more than 200 million doses have been given; this vaccine is available in Nepal, Sri Lanka, South Korea and India.
  • There is also a new chimeric vaccine based on the yellow fever 17D vaccine that is currently under development.