HT Explainer: Japanese Encephalitis

It is a zoonotic viral disease caused by the Japanese Encephalitis virus transmitted from animals, pigs, and birds, particularly the ones belonging to the family Ardeidae to humans

An “isolated” case of Japanese Encephalitis (JE) has been reported in Delhi, prompting public health measures as per the National Centre for Vector Borne Diseases Control guidelines, people aware of the matter said. A 72-year-old man tested positive for the disease at All India Institute of Medical Sciences and was discharged on November 15. Here is all you need to know about the disease:

The infection in human beings is caused due to its spillover from a zoonotic cycle. (Sourced)

What is JE

It is a zoonotic viral disease caused by the JE virus transmitted from animals, pigs, and birds, particularly the ones belonging to the family Ardeidae (cattle egrets, pond herons, etc) to humans by the Vishnui group of Culex mosquito.

Why is it called a zoonotic disease

JE is a disease of animals and the infection in human beings is caused due to its spillover from a zoonotic cycle. At a low vector density level, the virus circulates in an ardeid birds-mosquito ardeid bird cycle. At the commencement of monsoon and/or increased availability of surface area mosquito breeding places (eg rice fields, irrigation canals, etc), the vector population builds up rapidly. The virus from wild birds through vector mosquito species spreads to peridomestic birds and then to mammals such as cattle and pigs, etc, and eventually spills over to humans.

The role of pigs

Pigs play an important role in the natural cycle and serve as an amplifier host since they allow virus multiplication manifold without suffering from disease and maintain prolonged high-level viremia. Due to prolonged viremia, mosquitoes get infected from pigs. Humans are a dead-end host in the transmission cycle due to low and short-lived viremia. Mosquitoes do not get infections from JE patients.

Human-to-human transmission

There is no human-to-human transmission of JE.

Signs and symptoms

The infection presents with symptoms similar to any other virus-causing encephalitis. It may result in febrile illness of variable severity associated with neurological symptoms ranging from headache to meningitis or encephalitis. Symptoms can include headache, fever, signs of meningitis, stupor, disorientation, coma, tremors, paralysis (generalised), hypertonia, loss of coordination, etc.

Vectors of JE in India

JE virus isolation has been made from a variety of mosquito species. Culicine mosquitoes mainly the Vishnui group of Culex (Culex Tritaeniorhynchus, Culex Vishnui Culex Pseudovishnui, etc) are the chief vectors of JE in different parts of India. The Ulex Vishnui subgroup is widespread and breeds in water with luxuriant vegetation mainly in the rice fields. The abundance of vectors is related to rice cultivation, shallow ditches, and pools. These vectors are primarily outdoors in vegetation and other shaded places. In summer, they may also rest indoors.

Diagnosis

Suspected JE cases involve the acute onset of fever of not more than seven-day duration and a change in mental status which may be a new onset of seizures (excluding febrile seizures) or other early clinical findings such as irritability, somnolence, or abnormal behaviour greater than that seen with usual febrile illness

Laboratory confirmed JE

A suspected case with any one of the following markers: Presence of JE virus-specific lgM antibody in a single CSF (cerebrospinal fluid) sample or serum detected by IgM Capture ELISA specific for JE. Detection of JE virus nucleic acid detection in blood, CSF, or tissue by RT PCR. Detection of four-fold or greater difference in lgG antibody titer in paired sera collected 14 days apart. JE Virus isolation from brain tissue, CSF, serum, blood, or plasma JE Antigen detection by immunofluorescence

India cases

The first human case in India was reported in 1955 in Vellore, Tamil Nadu. In 2024, there have been 15,48 cases in 24 states/Union Territories. As many as 925 cases are from Assam alone.

The number of endemic states/territories has increased. Assam, Meghalaya, and Manipur are among the northeastern states included in the endemic zone. Other endemic states include Jharkhand, Bihar, Odisha, Uttar Pradesh, Madhya Pradesh, Chhattisgarh, Andhra Pradesh, Telangana, Karnataka, Tamil Nadu etc.

Almost 30-50% of total JE cases have been detected in Assam in recent years. Outbreaks have been reported from across the country. JE is endemic in 355 districts of 24 states/territories.

Seasonal outbreaks

JE cases in India often follow a seasonal pattern, with increased transmission during the monsoon (July to August) and a post-monsoon period (October to November) when breeding places and mosquito populations increase. Outbreaks typically occur from July to October, peaking during the rainy season.

Prevention and treatment

The preventive methods available are the JE vaccine and vector control measures. There is no specific treatment but case management is advised symptomatically. The disease affects primarily children but can affect individuals of any age. Two doses of the JE vaccine have been a part of the Universal Immunisation Programme since 2013. Adult JE vaccine has been introduced in high-burden states.