Contents
Introduction
Japanese Encephalitis (JE) is an infective illness cause by the mosquito-borne arbovirus Japanese Encephalitis Virus (JEV). It is endemic to the Asia-Pacific region, and recently it has become more widespread in Australia – due to warmer and wetter conditions as a result of climate change.
Many infections are asymptomatic (thought to be >99%) but in those with symptoms there is high morbidity and mortality.
Symptoms start 5-15 days after infection, and are often non-specific in the first few days, induing diarrhoea and headaches. Then significant neurological features appears – including headache, altered mental state, focal neurological symptoms and seizures.
The fatality rate ranges from 5-50% – with an agreed estimated rate of 18%. Half of those who survive have long-term neurological deficits.
- When referring to JE – this typically refers to symptomatic cases
- When referring to JEV this typically refers to all infections
Outbreaks in humans are often closely associated with pigs and pig farming.
The most effective way to reduce the risk of the illness is to take measures to avoid mosquito bites – such as using repellants, wearing long clothing, and reducing mosquito breeding sites – e.g. by removing areas of stagnant water around your home.
A Japanese Encephalitis vaccine is available and recommended for those who travel to high risk areas or have a high risk of exposure (e.g. farm workers).
Epidemiology
- Japanese Encephalitis is of the most important causes of infective encephalitis worldwide
- On average thought that >99% of infections of JEV are asymptomatic but estimates vary widely from 1 in 25 to 1 in 1000 infected individuals who experience symptoms
- It is thought that in populations not previously exposed to the virus the incidence of symptomatic infections is greater
- Thought to be about 100 000 symptomatic cases of JE worldwide each year
- Cases generally occur rurally in areas with rice farming – as this provides excellent conditions for mosquito habitat
- Cases also associated with pig farming – this is the more common scenario is Australia where rice is not typically grown commercially
- Cases peak in the wet season when mosquito populations are highest
Virus transmission
JEV is a single stranded RNA virus of the family Flaviviridae. There are 5 known genotypes. Type IV is most prevalent in Australia.
JEV infects multiple animal hosts including wading birds – but pigs are the most important – especially in regards to infecting humans. Humans and horses are “dead-end hosts” and are not involved in the continuation of the cycle of infection.
Transmission between animals and humans happens via mosquitoes. In Australia the mosquito species most commonly involved is Culex annulirostris which can travel several kilometres a day.
An infected pig has a high viral load for 3-5 days – which enables ongoing transmission to mosquitos. Farmed pigs in close proximity to humans pose a particular problems – as there is often a high turnover – meaning there is little time for immunity to develop. Even in areas where farmed pigs have been removed JEV cases have been known to persist – probably due to feral pig populations.
The incubation period is about 5-15 days.
Presentation
- 5-15 days after infection
- Initially – non-specific viral symptoms, including:
- Fever +/- rigors
- Coryza (runny nose)
- Diarrhoea
- This typically lasts for 3-4 days
- Neurological symptoms being 3-4 days after the other symptoms and can include:
- Headache
- Irritability
- Agitation
- Confusion
- Drowsiness
- Coma
- Parkinsonian featruesmay be present
- Mask-like face
- Pill-rolling tremor
- Cogwheel rigidity
- Dystonia
- Rigidity
- Other focal neurological symptoms
- Seizures
- Especially in children – up to 85% of patients, 10% of adults
- Persistent seizures indicate raised intracranial pressure and is a marker for poor outcome
- Other symptoms
- Hepatomegaly
- Splenomegaly
- Raised liver enzymes
- Thrombocytopenia
- Symptoms of meningitis may also be present
Investigations
Testing for JEV can be difficult. PCR testing can be performed on various sample including CSF, blood, urine, serum – and confirms diagnosis if it is positive. However – viral load levels are often low and transient and this results in a high rate of fall negative results.
- PCR is not always useful
- Positive IgM in CSF is the gold standard diagnostic test
- IgG positive blood results may also be useful – but may not be positive initially
Also – other flaviviruses can cause false positives – such as Dengue, Murray Valley encephalitis and Kunjin. It may also be necessary to specifically test for these viruses to assess which is the true cause of a positive result.
Imaging
- MRI may show parenchymal inflammation – typically affecting the thalami, basal ganglia and brain stem
- This is not diagnostic for JE
Management
Any presentation suspicious for encephalitis and meningitis is a medical emergency. Initial treatment of these patients should involve:
- Basic resuscitation
- Empiric antibiotic therapy – check local guidelines for meningitis
- E.g. Ceftriaxone 2g IV every 12 hours
There are no specific treatments for Japanese Encephalitis. Treatment is supportive and aimed at reducing the risk of neurological damage:
- Control seizures
- Manage raised intracranial pressure
Prevention
Prevention involves measure to avoid mosquito bites, as well as vaccination against Japanese Encephalitis in at risk groups.
Mosquito prevention measures include:
- Limiting time outdoors during peak mosquito activity (dawn and dusk)
- Wear long, loose fitting clothing
- Apply effective mosquito repellant (usually a DEET based product)
- Minimise locations where standing water can accumulate
- Use of fly screens and mosquito nets where appropriate
There are two Japanese Encephalitis vaccines available:
- Imojev – single dose, live attenuated vaccine. Not suitable for use in pregnancy, breastfeeding or if immunocompromised. No booster required.
- JEspect – inactivated virus – required a two dose schedule 28 days apart. Booster recommended at 1-2 years for those with ongoing risk
The following groups are recommended to be vaccinated
- Spending >1 month in endemic areas with activities that put them at risk of mosquito exposure
- Those working in laboratory environment with JEV
- Those who work in close proximity to pigs in endemic countries
References
- Japanese encephalitis clinical update: Changing diseases under a changing climate – AGJP – Volume 52, Issue 5, May 2023
- Japanese Encephalitis fact sheet – NSW Health
- Japanese Encephalitis – Better Health Channel
