Japanese Encephalitis

Japanese encephalitis is a mosquito borne viral encephalitis caused by Japanese encephalitis virus, a flavivirus transmitted by Culex mosquitoes. Pigs and wading birds are amplifying hosts. It is the leading cause of viral encephalitis across large parts of Asia and the Western Pacific.

The large majority of infections are asymptomatic or produce only a mild non specific febrile illness. A small proportion progress to severe encephalitis with substantial mortality and, among survivors, a high rate of permanent neurological sequelae.

Pathophysiology and epidemiology

Transmission occurs predominantly in rural agricultural areas. Rice paddy irrigation supports the mosquito vector, and pig rearing supports the amplifying animal hosts. Transmission is typically seasonal, corresponding to periods of vector activity.

Children in endemic areas bear the highest burden of severe disease. Adults residing long term in these regions are more likely to have pre-existing immunity from prior asymptomatic infection.

The large majority of infections are asymptomatic or produce only a mild, self limiting febrile illness. A small proportion (estimated at less than one percent of infections overall) progress to encephalitis after a several day prodrome of fever, headache and vomiting. Features are altered consciousness, seizures (particularly common and often the presenting feature in children), and focal neurological signs.

A characteristic movement disorder resembles parkinsonism, with rigidity, tremor and a mask like facial expression, together with other extrapyramidal features. It reflects the virus's predilection for the basal ganglia and thalamus, and is a distinctive though not universal feature.

Acute flaccid paralysis, resembling poliomyelitis, can occur as an alternative or additional presentation. It reflects anterior horn cell involvement.

Severe disease can progress to coma and death. Mortality in those developing overt encephalitis has historically ranged from twenty to thirty percent. A substantial proportion of survivors are left with permanent neurological, cognitive or behavioural sequelae.

The standard diagnostic approach is serological testing. Enzyme linked immunosorbent assay detects Japanese encephalitis virus specific immunoglobulin M in serum or, more specifically, cerebrospinal fluid. Cerebrospinal fluid immunoglobulin M generally becomes detectable within the first week of neurological symptom onset.

Cross reactivity with other flaviviruses (including dengue, West Nile virus and Zika virus, all of which circulate in overlapping geographical regions) can complicate serological interpretation. Plaque reduction neutralisation testing is a more specific confirmatory test, used where available and where cross reactivity is a concern. These related infections have differing clinical and public health implications.

Magnetic resonance imaging characteristically shows bilateral thalamic signal change, with basal ganglia and midbrain involvement. This pattern is not entirely specific but is strongly suggestive in the appropriate clinical and epidemiological context.

Cerebrospinal fluid analysis typically shows a lymphocytic pleocytosis with mildly elevated protein, similar to other viral encephalitides. It does not by itself distinguish this cause without specific serological or molecular testing.

There is no specific antiviral treatment with proven efficacy for Japanese encephalitis. Management is entirely supportive. It focuses on raised intracranial pressure, seizure control and general intensive supportive care.

The principles are the same as for viral encephalitis of any cause. This applies once a specific treatable diagnosis such as herpes simplex encephalitis has been excluded, or once empirical aciclovir has been given while awaiting diagnostic clarification in an undifferentiated presentation.

Seizures are managed with standard anticonvulsant therapy, typically levetiracetam or sodium valproate as first line agents, titrated to seizure control. Status epilepticus is managed according to standard escalating protocols.

Raised intracranial pressure and significant cerebral oedema are managed with head elevation and careful fluid balance, avoiding both hypovolaemia and fluid overload. For significant or symptomatic raised pressure, give osmotic therapy with mannitol or hypertonic saline, with intensive care monitoring. Consider intracranial pressure monitoring in the most severe cases.

Ventilatory support is required for patients with significantly reduced consciousness unable to protect their airway, or with respiratory failure from severe brainstem involvement. Manage them in an intensive care setting.

Corticosteroids have not shown consistent evidence of benefit in Japanese encephalitis and are not routinely recommended as specific treatment. This contrasts with their established role in some other central nervous system infections.

Prevention through vaccination is the most effective available strategy. Inactivated and live attenuated vaccines are available and recommended for residents of and long term travellers to endemic areas, particularly those with rural or extended exposure during the local transmission season.

Mosquito avoidance measures (insect repellent, protective clothing, bed nets) are an important adjunct for any traveller to an endemic region, regardless of vaccination status.

Rehabilitation is an essential and often prolonged component of care for survivors. It includes physiotherapy, occupational therapy and, where relevant, management of movement disorder and cognitive or behavioural sequelae. Permanent disability is a substantial burden among those who develop overt encephalitic disease.