Japanese encephalitis, abbreviated as JE, is a brain infection inflicted by the Japanese encephalitis virus (JEV). JEV is of the Flavivirus genus, which means it is related to dengue fever, yellow fever, and the West Nile virus. Much like these three illnesses, JEV is mosquito-borne, and is spread by mosquitoes of the Culex genus. As of April 2022, the infection is only notably present in Southeast Asia and the Western Pacific, with extremely rare cases appearing elsewhere in the world. The virus earnt its name by being first documented in Japan in 1871, but thanks to a vaccination program run by the Japanese government, the virus is now relatively rare in the country.

A 2017 map showing the then-current spread of the Japanese Encephalitis Virus - notably, since then, the virus has gained a significant foothold in Australia

Symptoms

Most humans infected with the Japanese encephalitis virus do not experience any symptoms, with only roughly 1 in 250 cases resulting in the symptomatic development of encephalitis. The virus has an incubation period of 2 to 26 days, and if symptoms appear, they will do so in this window. First, human patients may experience severe ‘rigors’, which is the sudden onset of feverish temperatures while the patient shivers and feels chills. Otherwise, early symptoms can include headaches and malaise, though these symptoms are non-specific to Japanese encephalitis.

Next, the disease enters the acute encephalitic stage, which results in the patient experiencing cachexia, which is a complex condition resulting in muscle loss; hemiparesis, a weakening of one side of the body; neck rigidity; convulsions; and a raised body temperature resting somewhere between 38 and 41 degrees Celsius. At this stage, it is also possible for the patient to suffer some neurological symptoms, which can impact the patient’s capacity for logic and intelligence.

There are some lifelong effects associated with the disease. In some cases, it can result in ongoing deafness, hemiparesis, and uncontrollable mood swings in patients that have had the disease affect their central nervous system. The virus can be deadly, though mortality rates vary by demographics, with children being the worst affected. Additionally, there is evidence that the virus can be passed between mothers and their unborn children via the placenta, which means that pregnant women should be particularly attentive to the risks of the disease.

The Japanese Encephalitis Virus is spread by mosquitoes

Diagnosis & Treatment

Generally speaking, diagnosing Japanese encephalitis is performed via blood tests, though some anti-body tests do require that the cerebrospinal fluid be tested instead. Once diagnosed, though, there is no specific treatment available for those suffering from Japanese encephalitis. Medical centres are able to provide generalised supportive treatment, helping patients eat their food, breathe properly, and control any seizures that may occur. If the patient is suffering from elevated intracranial pressure, doctors are able to prescribe medications to effectively treat that symptom specifically.

Spread

Once infected, human beings are not able to pass the virus between themselves, which means that patients do not need to be isolated from the general population. Instead, the virus is exclusively spread via mosquitoes. A number of species are able to be infected with the virus, including humans, horses, cattle, and pigs. In the first three cases, the virus typically manifests as fatal encephalitis, which forces the host to either repel the virus or die. Consequently, humans, horses, and cattle are all considered ‘dead-end’ hosts for the virus, as they are not able to remain safely infected with the virus long enough for it to multiply and spread.

Once exposed to the Japanese encephalitis virus, pigs are typically asymptomatic, which means they instead perform the important role of acting as a reservoir for it. If, however, the pig is pregnant, then the virus can cause complications in the pregnancy, ranging from fetal abnormalities to miscarriages.

The virus is spread when a mosquito feeds on an infected host and then feeds on an uninfected victim. This makes the Culex tritaeniorhynchus one of the most notable infection vectors of the virus, as while they preferentially feed on cattle, they will also feed on humans.

While these four species are commonly infected with the Japanese encephalitis virus, its natural hosts are birds of varying species. As the virus naturally exists in wildlife, not humans, the current understanding of the epidemiology community is that it is unlikely that the virus will ever truly disappear, no matter how much effort is put into vaccinating the human population against it. This means that herd immunity cannot be accomplished, and that Japanese encephalitis will always pose a risk to those amongst us who cannot be vaccinated against the virus for whatever reason, such as those who are immunocompromised.

Pigs are a suitable repository for the virus, which means livestock owners should take particular care to avoid JEV

How Did It Reach Australia?

By April 2022, the Japanese encephalitis virus was still a rare occurrence in Australia. However, in February and March of the same year, numerous states reported occurrences of the virus in piggeries across the country.
It is believed that the virus was introduced to the country at the same time as the Culex gelidus species of mosquito, which was an unplanned introduction. While humans are able to vaccinate themselves against the virus, there is no current protection offered for livestock, which means homeowners of rural properties will need to control their mosquito population. If the mosquito population is allowed to spread rampantly, the virus could spread amongst the country, putting both humans and wildlife at risk.