top of page

Japanese Encephalitis

Overview

Japanese encephalitis is a viral brain infection that is transmitted via the bite of infected Culex mosquitos, particularly Culex tritaeniorhynchus. The flavivirus usually occurs in rural agricultural areas throughout South East Asia, the Pacific islands and Far East. The majority of infected people either remain asymptomatic or present with mild, flu-like symptoms. A small percentage of infected people (<1 in every 250 cases) will go on to develop encephalitis. Despite the name, Japanese encephalitis is now uncommon in Japan due to the introduction of mass immunisation programmes. WHO estimates there are 68,000 clinical cases worldwide each year yet <1 in 1million travellers will develop Japanese encephalitis in a given year. There has not been a reported case in the UK in the past 10 years.

 

Signs and symptoms

For individuals presenting with symptoms, the incubation period

typically lasts for 10-15 days.

Initial symptoms include:

  • Headache

  • High fever

  • Vomiting

  • Diarrhoea

  • Muscle pain

 

After a few days, infected individuals develop:

  • Stiff neck

  • Mental state changes

  • Disorientation

  • Neurological symptoms

  • Agititation

  • Movement disorders

  • Incapability of speech

  • Paralysis

  • Tremors and convulsions

  • Coma

 

Up to 1 in 3 cases presenting with the more serious, advanced symptoms will result in fatality. A large proportion of surviving individuals are left with permanent brain damage due to the infection causing scarring of the brain. Long-term complications of this can include tremors, muscle twitches, personality changes, muscle weakness, learning difficulties and paralysis of the limbs.

 

Causes

The flavivirus affects both humans and animals. The main carriers of

Japanese encephalitis are pigs and wading birds. Mosquitos become

infected with the flavivirus during consumption of blood from an infected

animal or bird. The infected mosquito then goes on to bite humans,

ransmitting the flavivirus into the bloodstream when feeding. Japanese

encephalitis cannot be transmitted from person-to-person.

 

Risk factors / at risk groups

Japanese encephalitis occurs primarily in rural agricultural areas, most

commonly nearby rice productions and flooding irrigation where humans

live in close proximity to the vertebrate hosts.

In Asia, transmission is seasonal. Peaks of infection occur in the summer

and autumn. In the tropics and subtropics, transmission can occur all year

round but often peaks throughout the rainy season when the mosquito population is at its highest.

75% of cases are in children aged 15 and under.

 

Diagnosis / microbiology testing

To rule out other causes of encephalitis, serum or cerebrospinal fluid is tested.

 

Treatment

There is no specific treatment for Japanese encephalitis. Patient management focuses on supportive care and management of complications. Individuals are managed in a hospital setting with rest, fluids, oxygen, analgesics and antipyretics.

 

Vaccines / Preventative measures

It is recommended individuals travelling to an endemic for 1 month or more are vaccinated against Japanese encephalitis, particularly if during the virus transmission period. The vaccine is given as a 2 dose series, spaced 28 days apart with the final dose administered at least 1 week before travel. It is effective in 9 out of every 10 people. The vaccine is not recommended for short-term travelling restricted to urban areas or during times outside the virus transmission season.

 

Preventative measures include:

  • Applying insect repellent to bare skin and clothes

  • Wearing adequate clothing to cover the limbs

  • Reducing potential exposure to peak biting hours (dusk to dawn)

  • Sleeping in accommodation with gauze over windows and doors

  • Sleeping in mosquito nets impregnated with an insecticide

  • Spraying accommodation area with insecticide during early evening

bottom of page