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Japanese Encephalitis

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.

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 distribution prior to 2021. Since 2021 – cases and outbreaks have been recored throughout the eastern states of Australia as far south as Victoria.

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

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.

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

Management

Any presentation suspicious for encephalitis and meningitis is a medical emergency. Initial treatment of these patients should involve:

There are no specific treatments for Japanese Encephalitis. Treatment is supportive and aimed at reducing the risk of neurological damage:

Prevention

Prevention involves measure to avoid mosquito bites, as well as vaccination against Japanese Encephalitis in at risk groups.

Mosquito prevention measures include:

There are two Japanese Encephalitis vaccines available:

The following groups are recommended to be vaccinated

References

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