Overview

Japanese encephalitis (JE) infections are caused by the Japanese encephalitis virus (JEV). It is transmitted by infected mosquitoes which can bite humans, pigs, and birds to induce an inflammatory disease of the brain. Since this virus is prevalent across Asia and the Western Pacific, over 4 billion people are at risk of contracting JEV. Children under the age of 15 are most vulnerable to JE infections. However, older adults are also increasingly affected due to age-related decline in immune function. Given that 20-30% of JE cases result in death, and that 30-50% of survivors develop long-term cognitive, mental, or neurological issues years later, this article will provide you with comprehensive advice on how to avoid JEV contraction.¹

Understanding the epidemiology of JEV

Understanding the epidemiology of JEV is important for public health officials to recognise high-risk areas so they can undertake effective prevention, control, and management of the disease to reduce the impact of JEV on public health.

The epidemiology of JE continues to evolve, since its transmission can depend upon agricultural practices, climate change, tourism, animal vectors and mosquito vector-free transmission. Infected mosquitoes can bite humans, pigs or birds, which allows the virus to be maintained within their respective host but allowing transmission between each host. 

To reduce the risk of JEV transmission, preventing mosquito bites remains a key strategy. Using mosquito nets, candles, or repellant sprays can help reduce the risk of JEV transmission. Additionally, wearing long-sleeved tops and trousers can also reduce the likelihood of being bitten. 

Pigs can also pose as JEV vectors, since it has been shown to contaminate pig excreta, raising concerns about environmental transmission. Therefore, hygienic conditions of pig farms or other agricultural practices must be tightly regulated by authorities.

Epidemiological patterns

There are between 30,000 and 50,000 cases of JE across the world each year, but the disease manifests severely in approximately 1 in 250 infections. The two epidemiological patterns of JE are categorised across 24 countries. One of these is the epidemic pattern, involving seasonal outbreaks, with a high number of cases occurring between May and November. Countries that exhibit this epidemic transmission pattern include:

• China
• Bangladesh
• Bhutan
• Nepal
• Taiwan
• Japan
• South Korea
• North Korea
• Pakistan
• Russia
• Northen India

The second epidemiological pattern is endemic, where outbreaks occur sporadically throughout the year.¹ Countries exhibiting this pattern include:

• Australia
• Burma
• Cambodia
• Indonesia
• Laos
• Malaysia
• Papua New Guinea
• Philippines
• Singapore
• Southern India

Vaccines

Fortunately, many endemic countries have implemented vaccination programmes for children, andis also recommended for tourists travelling to these areas. Vaccination allows the recipient’s immune cells to create an immune response that recognises JEV upon re-exposure to prevent infection.

The first licensed JE vaccine was the inactivated mouse brain-derived (MBDV) JE vaccine. Currently, two types of MBDV vaccines are used in endemic countries such as Japan, South Korea, Taiwan, and India. This type of vaccine has demonstrated durable protection and is more affordable compared to other types of vaccines, making them cost-efficient in endemic regions. A problem with these vaccines, however, is that they cause high reactogenicity and more doses are required compared to newer JE vaccines, which can put a strain on health infrastructures in endemic countries.²

Another widely used vaccine is the PHK cell-derived, live attenuated vaccine which has been used in China since the late 1980s. It has exhibited excellent attenuation for a variety of JE susceptible animals and is highly safe and effective. One study showed that when vaccinated, there was no difference in the incidences of encephalitis, meningitis or other serious conditions compared to a control group.³

Overall, vaccination is vital in reducing the burden of JEV and is the primary preventative measure for people living in endemic countries or are visiting them.

Assessing your personal health risks

While JEV can affect people of all ages, certain groups of people are more vulnerable to infection and hence more severe outcomes. Understanding your personal risk factors is crucial for making informed decisions that protect not only your health but also the well-being of your community.

The most vulnerable groups of people to JEV are:⁴

• Children under the age of 15
• Residents of rural areas in endemic regions
• Travellers to areas where irrigation flooding is employed
• Immunocompromised people
• Unvaccinated people

Knowing the symptoms of JEV

While understanding the symptoms of JEV is an important preventative measure, it is not directly related to avoiding infections. It is important for early detection and rapid medical intervention which can significantly impact the course or outcome of infection. Recognising symptoms early allows individuals to seek medical care promptly, reducing the risk of complications and improving the chances of recovery. Furthermore, this would lead to further public health investigations that would encourage increased vaccination efforts in these endemic areas. 

Symptoms of JEV include:

• Vomiting
• Headache
• Nausea
• Diarrhea
• Myalgia
• fever

A long list of differential diagnoses such as malaria, dengue fever, and typhoid fever, as the symptoms of JEV are non-specific. This reinforces the importance of undertaking the preventative measures recommended in this article.⁵

FAQs

How is JEV diagnosed?

Since the symptoms are non-specific, this makes diagnosis particularly difficult in endemic regions with limited medical resources. However, infected patients may present with specific antibodies against JEV (IgM and IgG), which are usually detectable in the serum between 7-30 days after infection. Patients that fail to generate early these JEV-specific antibodies are at a higher risk of severe or fatal outcomes.⁶

How is JEV treated?

Currently, there is no cure for JEV which means its management is interprofessional. This also reiterates the importance of avoiding mosquito bites entirely. Therore, the aforementioned preventative measures should be performed at all times in endemic regions. In some cases, some drugs may be administered to reduce headaches and nausea.⁵

Summary

JEV is a mosquito-borne virus prevalent across 24 countries in Asia and the West Pacific. One of the key challenges in controlling the disease is that its symptoms are often non-specific, making it difficult to diagnose in the early stages. This delay can hinder the ability of health authorities to implement timely control measures in endemic regions. Because mosquito bites are the primary mode of transmission, preventing mosquito exposure is essential. This can be done by wearing long sleeved clothes and trousers and using mosquito nets. While there is no cure for JEV, there are several types of vaccines available that are safe and effective. It is important that you seek medical advice if you plan to travel to endemic countries. If you already live there, it is advised that you seek vaccination to minimise the adverse effects of JEV if contracted.