Introduction

Japanese encephalitis is a viral infection that can lead to severe inflammation of the brain, known as encephalitis. It is primarily transmitted by mosquitoes. It is a potentially life-threatening condition, which makes it a major public health concern. Understanding the symptoms, modes of transmission, and impact of this disease is important, especially for those living in or visiting regions where the virus is prevalent.¹ This article provides an overview of Japanese encephalitis and the common symptoms of the disease.

Overview of Japanese Encephalitis

Japanese encephalitis is caused by the Japanese encephalitis virus, a member of the Flavivirus family, which is closely related to the West Nile and dengue viruses. It is spread to humans through bites from infected Culex mosquitoes. It is most commonly seen in rural and agricultural areas, where rice paddies and standing water serve as breeding grounds for these mosquitoes. Pigs and wading birds act as reservoirs for the virus, while humans are considered dead-end hosts, meaning they do not spread the infection even though they get infected. This virus is endemic to certain regions of Australia, Asia, and the Pacific. An estimated 50,000 cases of Japanese Encephalitis are reported annually, with the majority occurring in Southeast Asia and the Western Pacific countries.¹

The majority of cases occur in children. The adult population in endemic areas is usually immune to infection through asymptomatic exposure to the virus or vaccination in childhood. Certain populations are more susceptible to developing this infection. These high-risk groups include children in rural areas, older age groups, and travellers (irrespective of age) from non-endemic areas.¹

Common symptoms of Japanese Encephalitis

The incubation period for Japanese Encephalitis typically ranges from 4 to 15 days. This is the time between the bite of an infected mosquito and the onset of symptoms. Most people infected with the Japanese encephalitis virus remain asymptomatic. In such cases, individuals are infected with the Japanese encephalitis virus but do not show any symptoms and remain healthy. These individuals completely recover without any noticeable signs of illness. However, in cases where symptoms develop, they can range from mild to severe and potentially life-threatening. ¹

Initial Symptoms (Prodromal Period)

The initial symptoms of Japanese Encephalitis are typically non-specific and resemble those of other viral infections. Infected individuals present with mild flu-like symptoms that last for a few days and often resolve without medical intervention. These flu-like symptoms make it challenging to differentiate Japanese Encephalitis from other viral conditions in the early stages. Therefore, many cases go undiagnosed until more severe symptoms manifest. Initial symptoms include:¹

• Fever
• Headache
• Nausea
• Vomiting
• Diarrhoea
• Myalgias (muscle pains)

Progression of Symptoms

Although most cases are asymptomatic, others may develop severe, potentially life-threatening neurological complications. Approximately 1 in 250 cases progresses to severe disease.¹ As the infection progresses, the virus affects the central nervous system, causing encephalitis—a severe inflammation of the brain. This leads to a range of neurological symptoms, which can evolve rapidly, some of which are life-threatening. These symptoms include:¹

• High fever: The patient presents with a sudden onset of high fever. This is the body’s immune response to the viral infection
• Altered mental status: It is a serious symptom that reflects the impact of the virus on the central nervous system. It can range from mild confusion to severe impairment of consciousness leading to coma. Patients often present with confusion, disorientation, and agitation
• Tremors: It is the uncontrolled shaking of the hands, arms, or legs. These tremors usually occur at rest. It results from the virus's effects on motor control areas in the brain
• Psychosis: It is a mental condition in which the patient loses contact with reality. It is characterised by hallucinations and delusions
• Severe headaches: They are typically intense. It occurs as a result of increased intracranial pressure caused by the inflammation in the brain
• Movement disorders: The patient can present with different movement disorders, such as dystonia (involuntary muscle contractions)
• Meningismus: It is a condition characterised by a set of symptoms that include neck stiffness, headache, and photophobia
• Seizures: It is a sudden, temporary, and uncontrolled body movement. It is usually seen in children
• Mutism: The virus can impair the ability to speak
• Flaccid paralysis: It is muscle weakness or paralysis and reduced muscle tone

Complications and Long-term Effects

In severe cases, Japanese encephalitis can result in death. The mortality rate for severe Japanese Encephalitis is approximately 20% to 30%. Even for those who survive, 30% to 50% of patients have significant neurologic and psychiatric sequelae.¹ These sequelae can include permanent neurologic deficits or psychiatric issues. Long-term neurological consequences include:² 

• Post-Encephalitic Epilepsy: Epilepsy is a neurological condition characterised by recurrent seizures. Many patients who survive Japanese encephalitis develop epilepsy as a long-term complication. This form of epilepsy, referred to as post-encephalitic epilepsy, can develop in weeks to months after the initial infection
• Polyneuropathy and mononeuropathy multiplex: These are neurological complications that affect the nerves
• Stroke: A stroke is a medical emergency that occurs when blood flow to a part of the brain is interrupted, leading to brain cell damage
• Parkinsonism: It is a syndrome characterised by symptoms similar to those of Parkinson's disease, such as tremors, rigidity, bradykinesia(slowness of movement), and postural instability
• Encephalopathy: It is a term used for brain dysfunction. It can present as altered mental status, confusion, memory loss, etc. 

Diagnosis of Japanese Encephalitis

Diagnosis is typically based on a combination of clinical valuation, patient history, laboratory testing and neuroimaging. Various diagnostic approaches include:¹

• A thorough medical and travel history of the patient is taken by the clinician, which includes asking patients about recent travel to endemic areas, exposure to mosquitoes, and symptom onset
• Proper clinical evaluation, especially neurological examination, i.e. assessing cognitive function, motor skills, and reflexes., is done to recognise neurological symptoms associated with this condition
• An enzyme-linked immunoassay (ELISA) test is done on serum or cerebrospinal fluid to detect Japanese encephalitis virus-specific antibody, i.e. immunoglobulin M (IgM)
• Magnetic resonance imaging (MRI) and computed tomography (CT) scans of the brain are done to identify inflammation or other changes in the brain that are characteristic of encephalitis

Treatment and Management

There is currently no effective treatment for the Japanese encephalitis virus. Management is limited to supportive care, which involves treating symptoms such as fever, seizures, and fluid imbalance. Intravenous fluids, anticonvulsant medications, and antipyretics are administered to alleviate these symptoms.¹

Preventive Measures

Since there is no particular treatment for Japanese encephalitis, prevention is the most effective strategy for controlling this disease. The primary methods of prevention include:¹ 

Vaccination

Vaccination is the most reliable method of preventing Japanese encephalitis. The Centre for Disease Control and Prevention (CDC) recommends the use of the Japanese encephalitis vaccine for individuals travelling to these areas, especially if they plan to spend extended periods in rural areas. Individuals living in endemic areas are also strongly encouraged to get vaccinated. 

Vector control

• Wear long sleeves, long pants, socks, and closed-toe shoes
• Applying permethrin, diethyltoluamide containing or any other mosquito repellents to clothing
• Travelers are encouraged to stay in places with air conditioning or to use mosquito nets or screens while sleeping to avoid being bitten

Community and Public Health Measures 

• Public health authorities should run vaccination programs in endemic regions to immunise high-risk populations
• Patients should be educated about recognising the common symptoms of Japanese encephalitis, as early detection can be critical for managing the disease
• Mosquito Control Programs should be considered, such as spraying insecticides in high-risk areas to control mosquito growth
• Patients should be educated about the preventive measures and ensure they are well-prepared before travelling to areas where this virus is prevalent

Summary                    

Japanese encephalitis is a serious neurological condition, but it is largely preventable with the right precautions. It is caused by the Japanese encephalitis virus, which is primarily transmitted by Culex mosquitoes. It is mostly prevalent in rural and agricultural regions of Asia and the Western Pacific. While most people infected with this virus are asymptomatic, a small percentage develop severe clinical symptoms, which can lead to long-term neurological complications. The most common symptoms of the disease include fever, headache, and altered mental status, which can progress to seizures, movement disorders, and even coma. There is no specific antiviral treatment for this condition. Disease management is largely supportive care aimed at managing symptoms and preventing complications. Vaccination is the most effective preventive measure, particularly for individuals living in or travelling to high-risk areas. Understanding the common symptoms of Japanese encephalitis is important for early diagnosis and timely medical intervention.